he Hubble Space Telescope(HST) performs much like aground astronomical obser-

vatory. It has three interactingsystems:• Support Systems Module

(SSM), an outer structure thathouses the other systems andprovides services such as elec-trical power, data communica-tions, pointing control andmaneuvering.

• Optical Telescope Assembly(OTA), which collects andconcentrates incoming light inthe focal plane for use by thescience instruments.

• Eight major science instru-ments, four housed in an aftsection focal plane structure(FPS) and four placed along thecircumference of the spacecraft.The Science Instrument

Control and Data Handling (SI C&DH) unit controls all theinstruments except the FineGuidance Sensors (FGS).

The SSM communicates with theOTA, SI C&DH unit and instru-ments to ready an observation.Light from an observed targetpasses through the Telescope andinto one or more of the scienceinstruments, where the light isrecorded. This information goesto onboard computers forprocessing, then it is eithertemporarily stored or sent toEarth in real time, via the space-craft communication system.

Two Solar Arrays (SA) alsosupport HST operations. Theygenerate electrical power andcharge onboard batteries and

communications antennas toreceive commands and sendtelemetry data from the HST.

Figure 5-1 shows the HST configuration.

The Telescope completes oneorbit every 97 minutes and main-tains its orbital position alongthree axial planes. The primaryaxis, V1, runs through the centerof the Telescope. The other twoaxes parallel the SA masts (V2)and the High Gain Antenna(HGA) masts (V3) (see Fig. 5-2).The Telescope points and maneu-vers to new targets by rotatingabout its body axes. Pointinginstruments use references tothese axes to aim at a target inspace, position the SA or changeTelescope orientation in orbit.

HST SYSTEMS

T

5-1

HST SYSTEMS

5-2

Support Systems Module

Design features of the SSM include:• An outer structure of interlocking shells• Reaction wheels and magnetic torquers to maneuver,

orient and attitude stabilize the Telescope

• Two SAs to generate electrical power• Communication antennas• A ring of Equipment Section bays that contain elec-

tronic components, such as batteries, and communi-cations equipment. (Additional bays are provided onthe +V3 side of the spacecraft to house OTA elec-tronics as described on page 5-19, OTA EquipmentSection.)

• Computers to operate the spacecraft systems andhandle data

• Reflective surfaces and heaters for thermal protection• Outer doors, latches, handrails and footholds designed

for astronaut use during on-orbit maintenance.

Figure 5-3 shows some of these features.

Major component subsystems of the SSM are:• Structures and mechanisms• Instrumentation and communications• Data management• Pointing control• Electrical power• Thermal control• Safing (contingency) system.

Fig. 5-1 Hubble Space Telescope – exploded view

Fig. 5-2 Hubble Space Telescope axes

Magnetic Torquer (4)High Gain Antenna (2)

Main BaffleSolar Array (2)

Light Shield

K1175_501

Aperture DoorSupport Systems Module Forward Shell

Secondary Mirror Baffle

Central Baffle

Optical Telescope AssemblySecondary Mirror Assembly

Optical Telescope AssemblyPrimary Mirror and Main Ring

Fine Guidance Optical Control Sensor (3)

Optical Telescope Assembly Focal Plane Structure

Support Systems Module Aft Shroud

Fixed Head Star Tracker (3) and Rate Sensor Unit (3)

Radial Science Instrument Module (1)

Optical Telescope Assembly Equipment Section

Support Systems Module Equipment Section

Optical Telescope Assembly Metering Truss

Axial ScienceInstrumentModule (4)

+V3

+V1

+V2

-V3

+V2 -V1

+V3

-V2

Solar Array

K1175_502

HST SYSTEMS

Structures and Mechanisms Subsystem

The outer structure of the SSM consists of stackedcylinders, with the aperture door on top and theaft bulkhead at the bottom. Fitting together arethe light shield, the forward shell, the SSMEquipment Section and the aft shroud/bulkhead—all designed and built by Lockheed Martin SpaceSystems Company (see Fig. 5-4).

Aperture Door. A door approximately 10 feet (3 m) in diameter covers the opening to theTelescope’s light shield. The door is made fromhoneycombed aluminum sheets. The outside iscovered with solar-reflecting material, and theinside is painted black to absorb stray light.

The door opens a maximum of 105 degrees fromthe closed position. The Telescope aperture allowsfor a 50-degree field of view (FOV) centered on the+V1 axis. Sun-avoidance sensors provide amplewarning to automatically close the door beforesunlight can damage the Telescope’s optics. Thedoor begins closing when the Sun is within ±35degrees of the +V1 axis and is closed by the time thesun reaches 20 degrees of +V1. This takes no longerthan 60 seconds.

The Space Telescope Operations Control Center(STOCC) can override the protective door-closingmechanism for observations that fall within the 20-

degree limit. An example is observing a bright object,using the dark limb (edge) of the Moon to partiallyblock the light.

Light Shield. Used to block out stray light, the lightshield (see Figs. 5-4 and 5-5) connects to both theaperture door and the forward shell. The outer skinof the Telescope has latches to secure the SAs and

Fig. 5-4 Structural components of Support Systems Module

Fig. 5-3 Design features of Support Systems Module

High Gain Antenna

Crew HandrailsEquipment Bay

Computer

Digital Interface Unit

Reaction Wheel Assembly

Communication System

Sun Sensor (3)

Aft Shroud

Access Door

Solar Array

Light Shield

K1175_503

Low GainAntenna

UmbilicalInterface

ApertureDoor

ForwardShell

Batteries and Charge Controller

EquipmentSection

MagneticTorquers

Latch PinAssembly

Forward Shell

Aft Shroud

High Gain Antenna (2)

K1175_504

ApertureDoor

LightShieldMagnetic

Torquer (4)

EquipmentSection

5-3

HGAs when they are stowed.Near the SA latches are scuffplates—large protective metalplates on struts that extendapproximately 30 inches from thesurface of the spacecraft.Trunnions lock the Telescope intothe Shuttle cargo bay by hookingto latches in the bay. The lightshield supports the forward LowGain Antenna (LGA) and itscommunications waveguide, twomagnetometers and two sunsensors. Handrails encircle thelight shield, and built-in footrestraints support the astronautsworking on the Telescope.

The shield measures 13 feet (4 m)long and 10 feet (3 m) in internaldiameter. It is a stiffened, corru-gated-skin barrel machined frommagnesium and covered by athermal blanket. Internally theshield has 10 light baffles, paintedflat black to suppress stray light.

Forward Shell. The forwardshell, or central section of thestructure, houses the OTA mainbaffle and the secondary mirror(see Fig. 5-6). When stowed, theSAs and HGAs are latched flat

against the forward shell andlight shield. Four magnetictorquers are placed 90 degreesapart around the circumferenceof the forward shell. The outerskin has two grapple fixturesnext to the HGA drives, wherethe Shuttle’s RemoteManipulator System can attachto the Telescope. The forwardshell also has handholds,footholds and a trunnion, whichis used to lock the Telescope intothe Shuttle cargo bay.

Machined from aluminumplating, the forward shell is 13feet (4 m) long and 10 feet (3 m)in diameter. It has internal stiff-ened panels and external rein-forcing rings. These rings are onthe outside to ensure clearancefor the OTA inside. Thermalblankets cover the exterior.

Equipment Section. This sectionis a ring of storage bays encirclingthe SSM. It contains about 90percent of the electronic compo-nents that run the spacecraft,including equipment servicedduring extravehicular activities(EVA) by Space Shuttle astronauts.

The Equipment Section is adoughnut-shaped barrel that fitsbetween the forward shell andaft shroud. It contains 10 baysfor equipment and two bays tosupport aft trunnion pins andscuff plates. As shown in Fig. 5-7,clockwise from the +V3 (top)position, the bays contain:1. Bay 8 – pointing control

hardware2. Bay 9 – Reaction Wheel

Assembly (RWA)3. Bay 10 – SI C&DH unit4. Unnumbered trunnion

support bay5. Bay 1 – data management

hardware

HST SYSTEMS

5-4

Fig. 5-5 Aperture door and light shield

Fig. 5-6 Support Systems Module forward shell

Aperture Door

Light Shield

Solar Array Latches

K1175_505

Aperture Door Hinge Support

Integrally Stiffened Skin

Internal Baffle Rings

Aperture Door Hinge

High Gain Antenna Latches

High Gain Antennaand Solar ArrayLatch Support Ring

MagnesiumMonocoqueSkin

Light ShieldAssembly/Forward ShellAttach Ring

Crew Aids

K1175_506

High GainAntenna Mast

MagneticTorquers

Stowed Solar Arrays

IntegrallyStiffenedSkin Panels

Equipment Section/Forward ShellInterface Ring

Forward ShellReinforcingRings

StowedHigh GainAntennas

6. Bays 2, 3 and 4 – electricalpower equipment

7. Unnumbered trunnionsupport bay

8. Bay 5 – communicationhardware

9. Bay 6 – RWA10.Bay 7 – mechanism

control hardware.

The cross section of the baysis shaped like a trapezoid: theouter diameter (the door) is3.6 feet (1 m) and the innerdiameter is 2.6 feet (0.78 m).The bays are 4 feet (1.2 m)wide and 5 feet (1.5 m) deep.The Equipment Section isconstructed of machined andstiffened aluminum framepanels attached to an inneraluminum barrel. Eight bayshave flat, honeycombedaluminum doors mountedwith equipment. In Bays 6and 9, thermal-stiffened paneldoors cover the reactionwheels. A forward framepanel and aft bulkhead enclosethe SSM Equipment Section.Six mounts on the inside ofthe bulkhead hold the OTA.

Aft Shroud and Bulkhead.The aft shroud (see Fig. 5-8)houses the FPS containingthe axial science instru-ments. It is also the loca-tion of the CorrectiveOptics Space TelescopeAxial Replacement(COSTAR) unit.

The three FGSs and theWide Field and PlanetaryCamera 2 (WFPC2) arehoused radially near theconnecting point betweenthe aft shroud and SSMEquipment Section. Doorson the outside of the shroudallow astronauts to remove andchange equipment and instru-ments easily. Handrails and footrestraints for the crew run alongthe length and circumference ofthe shroud. During maintenanceor removal of an instrument,

interior lights illuminate thecompartments containing thescience instruments. The shroudis made of aluminum, with astiffened skin, internal panelsand reinforcing rings, and 16external and internal longeronbars for support. It is 11.5 feet

(3.5 m) long and 14 feet (4.3 m)in diameter.

The aft bulkhead contains theumbilical connections betweenthe Telescope and the Shuttle,used during on-orbit mainte-nance. The rear LGA attaches

HST SYSTEMS

5-5

Fig. 5-7 Support Systems ModuleEquipment Section bays and contents

K1175_507

to the bulkhead, which is madeof 2-in.-thick honeycombedaluminum panels and has threeradial aluminum support beams.

The shroud and bulkheadsupport a gas purge system thatwas used to prevent contamina-tion of the science instrumentsbefore launch. All vents used toexpel gases are light tight toprevent stray light from enteringthe OTA focal plane.

Mechanisms. Along the SSMstructure are mechanisms thatperform various functions,including:• Latches to hold antennas and SAs• Hinge drives to open the aper-

ture door and erect arrays andantennas

• Gimbals to move the HGAdishes

• Motors to power the hingesand latches and to rotate arraysand antennas.

There are nine latches: four forantennas, four for arrays and onefor the aperture door. They latchand release using four-bar link-ages. Stepper motors calledRotary Drive Actuators (RDA)drive the latches.

There are three hinge drives,one for each HGA and one forthe door. The hinges also use anRDA. Both hinges and latcheshave hex-wrench fittings so anastronaut can manually operatethe mechanism to deploy thedoor, antenna or array if amotor fails.

Instrumentation andCommunications Subsystem

This subsystem provides thecommunications loop betweenthe Telescope and the Trackingand Data Relay Satellites(TDRS), receiving commandsand sending data through theHGAs and LGAs. All informa-tion passes through the DataManagement Subsystem(DMS).

S-Band Single AccessTransmitter (SSAT). HST isequipped with two SSATs. “S-band” identifies the frequencyat which the science data istransmitted and “single access”specifies the type of antenna onthe TDRS satellite to which thedata is sent.

High Gain Antennas. Each HGA

is a parabolic reflector (dish)mounted on a mast with a two-axis gimbal mechanism and elec-tronics to rotate it 100 degrees ineither direction (see Fig. 5-9).General Electric designed andmanufactured the antenna dishesusing honeycomb aluminum andgraphite-epoxy facesheets.

The HGAs achieve a much higherRF signal gain than the LGAs.The higher signal gain is required,for example, when transmittinghigh-data-rate scientific data.Because of their characteristicallynarrow beam widths, the HGAsmust be pointed at the TDRSs.Each antenna can be aimed with a1-degree pointing accuracy. Thisaccuracy is consistent with theoverall antenna beam width ofover 4 degrees. The antennas

transmit over two frequencies:2255.5 MHz or 2287.5 MHz (plusor minus 10 MHz).

Low Gain Antennas. The LGAsare spiral cones, manufactured byLockheed Martin, that providespherical coverage (omnidirec-tional). They are set 180 degreesapart on the light shield and aftbulkhead of the spacecraft.

HST SYSTEMS

5-6

Fig. 5-8 Support Systems Module aft shroud and bulkhead

Fig. 5-9 High Gain Antenna

Vents

+V3

+V2 +V1

Radial Science Instrument Doors

Axial Science Instrument Doors

Flight Support System Pin

Honeycomb Laminate Panels

Low Gain Antenna

Reinforcing Rings

Crew Aids

K1175_508

ElectricalUmbilicals

Flight Support SystemPin Support Beam

Integrally StiffenedSkin Panels

Equipment Section/Aft Shroud InterfaceRing

K1175_509

Twin-AxisGimbals

High GainAntenna

Operating over a frequency range from 2100 MHzto 2300 MHz, the LGAs receive ground commandsand transmit engineering data. These antennas areused for all commanding of the Telescope and forlow-data-rate telemetry, particularly duringTelescope deployment or retrieval on orbit andduring safemode operations.

Data Management Subsystem

The DMS receives communications commands fromthe STOCC and data from the SSM systems, OTAand science instruments. It processes, stores andsends the information as requested (see Fig. 5-10).

Subsystem components are:• Advanced Computer • Data Management Unit (DMU)• Four Data Interface Units (DIU)• Three engineering/science data recorders • Two oscillators (clocks).

The components are located in the SSM EquipmentSection, except for one DIU stored in the OTAEquipment Section.

The DMS receives, processes, and transmits fivetypes of signals:1. Ground commands sent to the HST systems2. Onboard computer-generated or computer-stored

commands

3. Scientific data from the SI C&DH unit4. Telescope engineering status data for telemetry5. System outputs, such as clock signals and

safemode signals.

Advanced Computer. The Advanced Computer is ageneral-purpose digital computer for onboard engi-neering computations. It executes stored commands,formats status data (telemetry), generates onboardcommands to orient the SAs toward the Sun, evalu-ates the health status of the Telescope systems andcommands the HGAs. It also performs all PointingControl Subsystem (PCS) computations tomaneuver, point and attitude stabilize the Telescope.

Based on the Intel 80486 microchip, the AdvancedComputer operates 20 times faster and has six timesas much memory as the DF-224 computer, which itreplaced on SM3A. It is configured as three inde-pendent single-board computers (SBC). Each SBChas 2 megabytes of fast static random accessmemory and 1 megabyte of non-volatile memory.

The Advanced Computer communicates with theHST by using the direct memory access capabilityon each SBC through the DMU. Only one SBCcontrols the Telescope at a time. The other SBCs canbe off, in an idle state or performing internal tasks.

Upon power on, each SBC runs a built-in self-testand then copies the operating software from slower

HST SYSTEMS

5-7

Fig. 5-10 Data Management Subsystem functional block diagram

Solar Array

AdvancedComputer

Command, Data Clock

Data, Clock, Mode Control

Keep-Alives

Reg Power

Data

Data

ScienceDataCommands

To/From Other Telescope Subsystems

K1175_510

Low GainAntennas

MultipleAccessTransponder

Pointing andSafemodeElectronicsAssembly

Rate GyroAssemblyElectronicsControl Unit

Electrical PowerTemperatureControlElectronics

Control Unit/Science DataFormatter

OvenControlledCrystalOscillator

Wide Field/PlanetaryCamera

MechanismControlUnit

OpticalControlElectronics

ActuatorControlElectronics

FineGuidanceElectronics

Fixed-HeadStar Tracker

DataInterface Unit

(Science InstrumentControl and DataHandling Unit)

Internal SystemControl Signals

Request, Replay, Clock,Synchronizing Pulses

DataManagementUnit

Solar ArrayTransmitter

Discrete SpecialSerial Commands

Support SystemsModule

Optical TelescopeAssembly

Data InterfaceUnits

Data, Status,Mode Control

DataRecorders

High GainAntennas

Functional Unit

Functional Redundancy

Interface Connection

Reference Equipment

Key

HST SYSTEMS

5-8

non-volatile memory to faster random access memory. The self-test can diagnose any problems with theAdvanced Computer and report them to the ground. The Advanced Computer uses fast static random accessmemory to eliminate wait states and allow it to run at its full-rated speed.

The Advanced Computer measures 18.8 x 18 x 13inches (0.48 x 0.46 x 0.33 m) and weighs 70.5 pounds(32 kg). It is located in Bay 1 of the SSM EquipmentSection (see Fig. 5-11).

Data Management Unit. The DMU links with thecomputer. It encodes data and sends messages toselected Telescope units and all DMS units, powersthe oscillators and serves as the central timing source.The DMU also receives and decodes all incomingcommands, then transmits each processed commandto be executed.

In addition, the DMU receives science data from theSI C&DH unit. Engineering data, consisting of sensorand hardware status readings (such as temperature orvoltages), comes from each Telescope subsystem. Thedata can be stored in the onboard data recorders ifdirect telemetry via a TDRS is unavailable.

The DMU is an assembly of printed-circuit boards,interconnected through a backplate and externalconnectors and attached to the door of Equipment Section Bay 1 (see Fig. 5-12). The unit weighs 83 pounds(37.7 kg) and measures 26 x 30 x 7 inches (60 x 70 x 17 cm).

Data Interface Unit. Four DIUs provide a command and data link between the DMS and other electronicboxes. The DIUs receive commands and data requests from the DMU and pass data or status information backto the DMU. The OTA DIU is located in the OTA Equipment Section; the other units are in Bays 3, 7 and 10 ofthe SSM Equipment Section. As a safeguard, each DIUis two complete units in one: either part can handlethe unit’s functions. Each DIU measures 15 x 16 x 7inches (38 x 41 x 18 cm) and weighs 35 lb (16 kg).

Engineering/Science Data Recorders. The DMSincludes three data recorders that store engineering orscience data that cannot be transmitted to the groundin real time. These recorders, located in EquipmentSection Bays 5 and 8, hold up to 12 billion bits ofinformation. Two solid state recorders (SSR) are usedin normal operations; the third, a backup, is a reel-to-reel tape recorder. Each recorder measures 12 x 9 x 7inches (30 x 23 x 18 cm) and weighs 20 pounds (9 kg).

The SSRs have no reels or tape and no moving partsto wear out and limit lifetime (their expected on-orbitlife is at least 8 years). Data is stored digitally incomputer-like memory chips until HST operators atGSFC command the SSR to play it back. Althoughthey are the same size as the reel-to-reel recorders,the SSRs can store over 10 times more data—12 giga-bits versus only 1.2 gigabits for the tape recordersthey replaced.

Fig. 5-11 Advanced computer

Fig. 5-12 Data Management Unit configuration

K1175_511

K1175_512

Top Cover(With Compression Pad)

Coax Connector

Bottom Cover

MatrixConnector

Card Guide

InterfaceConnector

Power Supply No. 2(CDI DC-DCConverter)

PC BoardStandard(5 x 7 in.)

HeatShieldPartition Enclosure

(Dip-BrazedAluminum)

Power SupplyNo. 1(DMU DC-DCConverter)

HST SYSTEMS

5-9

Each SSR can record two data streams simultane-ously, allowing both science and engineering data tobe captured on a single recorder. In addition, datacan be recorded and played back at the same time.

Oscillator. The oscillator provides a highly stablecentral timing pulse required by the Telescope. It hasa cylindrical housing 4 inches (10 cm) in diameterand 9 inches (23 cm) long and weighs 3 pounds (1.4kg). The oscillator and a backup are mounted in Bay2 of the SSM Equipment Section.

Pointing Control Subsystem

A unique PCS maintains Telescope pointing stabilityand aligns the spacecraft to point to and remainlocked on any target. The PCS is designed forpointing within 0.01 arcsec and holding theTelescope in that orientation with 0.007-arcsecstability for up to 24 hours while HST orbits theEarth at 17,500 mph. If the Telescope were in LosAngeles, it could hold a beam of light on a dime inSan Francisco without the beam straying from thecoin’s diameter.

Nominally, the PCS maintains the Telescope’s preci-sion attitude by locating guide stars in two FGSs andkeeping the Telescope in the same position relativeto these stars. When specific target requests requirerepositioning the spacecraft, the PCS selectsdifferent reference guide stars and moves theTelescope into a new attitude.

The PCS encompasses the Advanced Computer,various attitude sensors and two types of devices,called actuators, to move the spacecraft (see Fig. 5-13).It also includes the Pointing/Safemode ElectronicsAssembly (PSEA) and the Retrieval Mode GyroAssembly (RMGA), which are both used by thespacecraft safemode system. See page 5-13, Safing(Contingency) System, for details.

Sensors. The PCS uses five types of sensors: CoarseSun Sensors (CSS), Magnetic Sensing System (MSS),Rate Gyro Assemblies (RGA), Fixed Head StarTrackers (FHST) and FGSs.

Five CSSs, located on the light shield and aftshroud, measure the Telescope’s orientation to theSun. They also are used to calculate the initialdeployment orientation of the Telescope, determinewhen to begin closing the aperture door and pointthe Telescope in special Sun-orientation modesduring contingency operations. In addition, theCSSs provide signals to the PSEA, located in Bay 8of the SSM Equipment Section.

The MSS measures the Telescope’s orientation rela-tive to Earth’s magnetic field. Two systems arelocated on the front end of the light shield. Eachconsists of magnetometers and dedicated electronicunits that send data to the Advanced Computer andthe Safemode Electronics Assembly.

HST has three RGAs, each consisting of a RateSensor Unit (RSU) and an Electronics Control Unit(ECU). An RSU contains two rate-sensing gyro-scopes that measure attitude rate motion abouttheir sensitive axes. Two sets of dedicated elec-tronics in each ECU process this output. Three ofthe six gyroscopes are required to continue theTelescope science mission.

The RSUs are located behind the SSM EquipmentSection, next to three FHSTs in the aft shroud. TheECUs are located inside Bay 10 of the SSMEquipment Section. The RGAs provide input to thePCS to control the orientation of the Telescope’s lineof sight and to give the attitude reference whenmaneuvering the Telescope.

An FHST is an electro-optical detector that locatesand tracks a specific star within its FOV. STOCCuses FHSTs as an attitude calibration device whenthe Telescope maneuvers into its initial orientation.The trackers also calculate attitude informationbefore and after maneuvers to help the FGS lockonto guide stars.

Three FGSs provide angular position with respectto the stars (see page 5-20, Fine Guidance Sensor,for details). Their precise fine-pointing adjust-ments, accurate to within a fraction of anarcsecond, pinpoint the guide stars. Two of theFGSs perform guide-star pointing and the third isavailable for astrometry, the positional measure-ment of specific stars.

Pointing Control Subsystem Software. PCS soft-ware accounts for a large percentage of the flightcode executed by Hubble’s main computer. This soft-ware translates ground targeting commands intoreaction wheel torque profiles that reorient thespacecraft and smooth spacecraft motion to mini-mize jitter during data collection. The software alsodetermines Telescope orientation, or attitude, fromFHST or FGS data and commands the magnetictorquer bars to minimize reaction wheel speeds. Inaddition, the software provides various telemetryformats.

Since the Telescope was launched, the PCS has beenmodified significantly. A digital filtering scheme,

HST SYSTEMS

5-10

known as Solar Array Gain Augmentation (SAGA),now mitigates the effect of any SA vibration or jitteron pointing stability and an FGS Re-CenteringAlgorithm improves FGS performance when theTelescope is subjected to the same disturbances.

Software is used extensively to increase Telescoperobustness during hardware failures. Two additional

software safemodes have been provided. The spin-stabilized mode enables pointing of the Telescope -V1 axis to the Sun with only two of the four RWAsoperating. The other mode allows Sun pointing ofthe Telescope without any input from the RGA.Magnetometer and CSS data is used to derive allreference information needed to maintain Sunpointing (+V3 and -V1 are options).

Fig. 5-13 Location of Pointing Control Subsystem equipment

Fine GuidanceSensor (3)

Rate SensorUnit (3)

Coarse SunSensor (2)

Fixed HeadStar Tracker (3)

Science Instruments

Equipment Section

Computer

Reaction Wheel (4)

MagneticTorquer (4)

Magnetometer (2)Coarse Sun Sensor (2)

Bay 1 Data Management Computer

Bay 6 Reaction WheelReaction Wheel Assembly (2)No. 1 ForwardNo. 2 Aft

Bay 7 Mechanism Control

Bay 8 Pointing Control and InstrumentsRetrieval Mode Gyro AssemblyPointing and Safemode Electronics Assembly

Bay 9 Reaction Wheel (2)No. 3 ForwardNo. 4 Aft

Bay 10 ScienceInstrumentControl andData Handling

Looking ForwardK1175_513

-V2

-V3

+V2

+V3

10

9

87

6

5

4

3 2

1

HST SYSTEMS

5-11

A further software change “refreshes” the FGSconfiguration. Data is maintained in the AdvancedComputer memory so it can be sent periodically tothe FGS electronics, which are subject to single-eventupsets (logic state change) when transitioningthrough the South Atlantic Anomaly.

Actuators. The PCS has two types of actuators: RWAsand magnetic torquers. Actuators move the spacecraftinto commanded attitudes and provide control torquesto stabilize the Telescope’s line of sight.

The reaction wheels rotate a large flywheel up to3000 rpm or brake it to exchange momentum withthe spacecraft. Wheel assemblies are paired, twoeach in Bays 6 and 9 of the SSM Equipment Section.The wheel axes are oriented so that the Telescopecan provide science with only three wheels oper-ating. Each wheel measures 23 inches (59 cm) indiameter and weighs about 100 pounds (45 kg).Figure 5-14 shows the RWA configuration.

Magnetic torquers are primarily used to managereaction wheel speed. The torquers react againstEarth’s magnetic field. The torque reaction occurs inthe direction that reduces the reaction wheel speed,managing the angular momentum.

Located externally on the forward shell of the SSM,the magnetic torquers also provide backup control tostabilize the Telescope’s orbital attitude duringcontingency modes (refer to page 5-6, Instrumentationand Communications Subsystem. Each torquer is 8.3feet (2.5 m) long and 3 inches (8 cm) in circumferenceand weighs 100 lb (45 kg).

Pointing Control Operation. To point precisely, thePCS uses the gyroscopes, reaction wheels, magnetictorquers, star trackers and FGSs. The latter providethe precision reference point from which theTelescope can begin repositioning. Flight software

commands the reaction wheels to spin, acceleratingor decelerating as required to rotate the Telescopetoward a new target. Rate gyroscopes sense theTelescope’s angular motion and provide a short-termattitude reference to assist fine pointing and space-craft maneuvers. The magnetic torquers reduce reac-tion wheel speed.

As the Telescope nears the target area, star trackerslocate preselected reference stars that stand outbrightly in that region of the sky. Once the startrackers reduce the attitude error below 60 arcsec,the two FGSs take over the pointing duties. Workingwith the gyroscopes, the FGSs make it possible topoint the Telescope within 0.01 arcsec of the target.The PCS can maintain this position, wavering nomore than 0.005 arcsec, for up to 24 hours to guar-antee faint-object observation.

Electrical Power Subsystem

Power for the Telescope and science instrumentscomes from the Electrical Power Subsystem (EPS).The major components are two SA wings and theirelectronics, six batteries, six Charge CurrentControllers (CCC), one Power Control Unit (PCU)and four Power Distribution Units (PDU). All exceptthe SAs are located in the bays around the SSMEquipment Section.

During the servicing mission, the Shuttle will providethe electrical power. After deployment, the SAs willbegin converting solar radiation into electricity.Energy will be stored in nickel-hydrogen (NiH2)batteries and distributed by the PCUs and PDUs toall Telescope components as shown in Fig. 5-15.Hubble will not be released until the batteries arefully charged.

Solar Arrays. The SA panels, discussed later in thissection, are the primary source of electrical power.Each array wing has solar panels that convert theSun’s energy into electrical energy. Electricity producedby the panels charges the Telescope batteries.

Each array wing has associated electronics. Theseconsist of (1) a Solar Array Drive Electronics (SADE)unit to transmit positioning commands to the wingassembly, (2) a Deployment Control Electronics Unitto control the drive motors extending and retractingthe wings and (3) diode networks to direct the elec-trical current flow.

Batteries and Charge Current Controllers.Developed for the 1990 deployment mission, theTelescope’s batteries were NASA’s first flight NiH2

Fig. 5-14 Reaction Wheel Assembly

RWARWA

K1175_514

HST SYSTEMS

5-12

batteries. They provide the obser-vatory with a robust, long-lifeelectrical energy storage system.

Six NiH2 batteries support theTelescope’s electrical power needsduring three periods: whendemand exceeds SA capability,when the Telescope is in Earth’sshadow and during safemodeentry. The design, operation andhandling of the batteries—including special nondestructiveinspection of each cell—haveallowed them to be “astronaut-rated” for replacement during aservicing mission. To compensatefor the effects of battery aging,SM3A astronauts installed aVoltage/Temperature ImprovementKit (VIK) on each battery. TheVIK provides thermal stability byprecluding battery overchargewhen the HST enters safemode,effectively lowering the ChargeCurrent Controller (CCC)recharge current.

The batteries reside in SSMEquipment Section Bays 2 and 3.Each battery has 22 cells in seriesalong with heaters, heatercontrollers, pressure measurementtransducers and electronics, andtemperature-measuring devicesand their associated electronics.

Three batteries are packaged intoa module measuring roughly 36 x36 x 10 inches (90 x 90 x 25 cm)and weighing about 475 pounds(214 kg). Each module is equippedwith two large yellow handlesthat astronauts use to maneuverthe module in and out of theTelescope.

The SAs recharge the batteriesevery orbit following eclipse(the time in the Earth’sshadow). Each battery has itsown CCC that uses voltage-temperature measurements tocontrol battery recharge.

Fully charged, each batterycontains more than 75 amp-hours. This is sufficient energy tosustain the Telescope in normalscience operations mode for 7.5hours or five orbits. The batteriesprovide an adequate energyreserve for all possible safemodecontingencies and all enhance-ments programmed into theTelescope since launch.

Power Control and DistributionUnits. The PCU, to be replaced onSM3B, interconnects and switchescurrent flowing among the SAs,batteries and CCCs. Located inBay 4 of the Equipment Section,

the PCU provides the main powerbus to the four PDUs. The PCUweighs 120 pounds (55 kg) andmeasures 43 x 12 x 8 inches (109x 30 x 20 cm).

Four PDUs, located on the insideof the door to Bay 4, contain thepower buses, switches, fuses andmonitoring devices for electricalpower distribution to the rest ofthe Telescope. Two buses are dedi-cated to the OTA, science instru-ments and SI C&DH; two supplythe SSM. Each PDU measures 10 x5 x 18 inches (25 x 12.5 x 45 cm)and weighs 25 pounds (11 kg).

Thermal Control

Multilayer insulation (MLI)covers 80 percent of theTelescope’s exterior. The insula-tion blankets have 15 layers ofaluminized Kapton and an outerlayer of aluminized Teflon flex-ible optical solar reflector(FOSR). Aluminized or silveredflexible reflector tape covers mostof the remaining exterior. Thesecoverings protect against the coldof space and reflect solar heat.Supplemental electric heaters andreflective or absorptive paintsalso are used to keep Hubble’stemperatures safe.

Fig. 5-15 Electrical Power Subsystem functional block diagram

Solar A

rrayS

olar Array

Solar ArrayMechanisms

Power

Power

Power Power

Power

Power

Power

Power

Power

Power

Power

Power

Power

CommandsCommands

Commands

Commands

Solar ArrayElectronics

ControlAssembly

Telemetry

Telemetry

PowerControl

Unit

Battery No. 1

23

4

23

4

56

SSMSASIsOTA

SIC&DH

Telemetry

Orbiter Power(Predeployment)

PowerDistr

Unit 1

K1175_515

HST SYSTEMS

The SSM Thermal ControlSubsystem (TCS) maintainstemperatures within set limits forthe components mounted in theEquipment Section and structuresinterfacing with the OTA andscience instruments. The TCSmaintains safe component temper-atures even for worst-case condi-tions such as environmental fluc-tuations, passage from “cold” Earthshadow to “hot” solar exposureduring each orbit and heat gener-ated from equipment operation.

Specific thermal-protectionfeatures of the SSM include:• MLI thermal blankets for the

light shield and forward shell• Aluminum FOSR tape on the

aperture door surface facingthe Sun

• Specific patterns of FOSR andMLI blankets on the exteriorsof the Equipment Section baydoors, with internal MLI blan-kets on the bulkheads tomaintain thermal balancebetween bays

• Efficient placement of equip-ment and use of equipment bayspace to match temperaturerequirements, such as placingheat-dissipating equipment onthe side of the EquipmentSection mostly exposed to orbitshadow

• Silvered FOSR tape on the aftshroud and aft bulkhead exteriors

• Radiation blankets inside theaft shroud doors and MLI blan-kets on the aft bulkhead andshroud interiors to protect thescience instruments

• More than 200 temperaturesensors and thermistors placedthroughout the SSM, externallyand internally, to monitor indi-vidual components and controlheater operations.

Figure 5-16 shows the locationand type of thermal protectionused on the SSM.

During SM3A astronautsinstalled material to cover andrestore some degraded MLI. Thelayer added to the SSMEquipment Section on SM3A is acomposite-coated (siliconedioxide) stainless steel layer,known as the New Outer BlanketLayer (NOBL). The lightshield/forward shell material isTeflon with a scrim backing fordurability. The additional mate-rials have been life-tested to anequivalent of 10 years.

SM3B astronauts will completethe task of replacing thermalprotection in degraded areas astime permits.

Safing (Contingency) System

Overlapping or redundant equip-

ment safeguards the Telescopeagainst any breakdown. In addi-tion, a contingency or SafingSystem exists for emergencyoperations. Using dedicated PSEAhardware and many pointingcontrol and data managementcomponents, this system main-tains stable Telescope attitude,moves the SAs for maximum Sunexposure and conserves electricalpower by minimizing powerdrain. The Safing System canoperate the spacecraft indefi-nitely with no communicationslink to ground control.

During scientific observations(normal mode), the SafingSystem automatically monitorsTelescope onboard functions. Itsends Advanced Computer-gener-ated “keep-alive” signals to thePSEA that indicate all Telescopesystems are functioning. When afailure is detected, entry into theSafemode is autonomous.

The Safing System is designed tofollow a progression of contin-gency operating modes,depending on the situationaboard the Telescope. If amalfunction occurs and does notthreaten the Telescope’s survival,the Safing System moves into aSoftware Inertial Hold Mode.This mode holds the Telescope inthe last position commanded. If a

Fig. 5-16 Placement of thermal protection on Support Systems Module

BlackChemglaze

Aluminized Flexible Optical Solar Reflector (FOSR)

ApertureDoor

LightShield

ForwardShell

EquipmentSection Aft Shroud

90-degMultilayerInsulation(MLI)

90-deg MLI

+V1

AluminizedFOSR

OpticalBlack

MLI

MLI

MLI MLI MLI

FOSR

Aluminized FOSR

Silverized FOSR

Silverized FOSR

K1175_516

5-13

HST SYSTEMS

5-14

maneuver is in progress, the Safing System completesthe maneuver, then holds the Telescope in that position,suspending all science operations. Only ground controlcan return to science operations from Safemode.

If the system detects a marginal electrical powerproblem, or if an internal PCS safety check fails, theTelescope enters the Software Sun Point Mode. TheSafing System maneuvers the Telescope so the SAspoint toward the Sun to continuously generate solarpower. Telescope equipment is maintained within oper-ating temperatures and above survival temperatures,anticipating a return to normal operations. The STOCCmust intercede to correct the malfunction beforescience operations or normal functions can be resumed.

Since deployment of the Telescope in 1990, the SafingSystem has seen additional improvements to increaseits robustness to survive hardware failures and stillprotect the Telescope (refer to page 5-9, PointingControl Subsystem).

For the modes described above, the Safing System oper-ates through computer software. If conditions worsen,the system turns over control to the PSEA in HardwareSun Point Mode. Problems that could provoke thisaction include:• Computer malfunction• Batteries losing more than 50 percent of their charge• Two of the three RGAs failing• DMS failing.

If these conditions occur, the Advanced Computer stopssending keep-alive signals. This is the “handshake”mechanism between the flight software and the PSEA.

In Hardware Sun Point Mode, the PSEA computercommands the Telescope and turns off selected equip-ment to conserve power. Components shut downinclude the Advanced Computer and, within 2 hours,the SI C&DH. Before this happens, a payload (instru-ments) safing sequence begins and, if it has not alreadydone so, the Telescope turns the SAs toward the Sun,guided by the CSSs. The PSEA removes operatingpower from equipment not required for Telescopesurvival.

Once ground control is alerted to a problem, NASAmanagement of the STOCC convenes a failure analysisteam to evaluate the problem and seek the best andsafest corrective action while the Safing System main-tains control of the Telescope. The failure analysis teamis led by a senior management representative fromNASA/GSFC with the authority not only to call on theexpertise of engineers and scientists employed by NASAor its support contractors, but also to draft supportfrom any organization previously affiliated with the

Telescope Project. The team is chartered to identify thenature of the anomaly and to recommend correctiveaction. This recommendation is reviewed at a highermanagement level of NASA/GSFC. All changes to theTelescope’s hardware and all software configurationsrequire NASA Level I concurrence as specified in theHST Level I Operations Requirements Document.

Pointing/Safemode Electronics and Retrieval ModeGyro Assemblies. These assemblies are installed in Bay 8. The PSEA consists of 40 electronic printed-boardcircuits with redundant functions to run the Telescope,even in the case of internal circuit failure. It weighs 86 lb(39 kg). A backup gyroscope package, the RMGA, isdedicated for the PSEA. The RMGA consists of threegyroscopes. These are lower quality rate sensors thanthe RGAs because they are not intended for use duringobservations.

Optical Telescope Assembly

Perkin-Elmer Corporation (now Goodrich Corporation)designed and built the OTA. Although the OTA ismodest in size by ground-based observatory standardsand has a straightforward optical design, its accuracy—coupled with its place above the Earth’s atmosphere—renders its performance superior.

The OTA uses a “folded” design, common to large tele-scopes, which enables a long focal length of 189 feet(57.6 m) to be packaged into a small telescope length of21 feet (6.4 m). (Several smaller mirrors in the scienceinstruments are designed similarly to lengthen the lightpath within them.) This form of telescope is called aCassegrain. Its compactness is an essential componentof an observatory designed to fit inside the Shuttlecargo bay.

Conventional in design, the OTA is unconventional inother aspects. Large telescopes at ground-based sites arelimited in their performance by the resolution attain-able while operating under the Earth’s atmosphere, butthe HST orbits high above the atmosphere and providesan unobstructed view of the universe. For this reasonthe OTA was designed and built with exacting toler-ances to provide near-perfect image quality over thebroadest possible region of the spectrum.

Hubble’s OTA is a variant of the Cassegrain, called aRitchey-Chretien, in which both mirrors are hyper-boloidal in shape (having a deeper curvature than aparabolic mirror). This form is completely corrected forcoma (an image observation with a “tail”) and sphericalaberrations to provide an aplanatic system in whichaberrations are correct everywhere in the FOV. The onlyresidual aberrations are field curvature and astigmatism.

HST SYSTEMS

5-15

Both of these are zero exactly in the center of thefield and increase toward the edge of the field. Theseaberrations are easily corrected within the instru-ment optics.

Figure 5-17 shows the path of a light ray from adistant star as it travels through the Telescope to thefocus. Light travels down the tube, past baffles that

attenuate reflected light from unwanted brightsources, to the 94.5-inch (2.4-m) primary mirror.Reflecting off the front surface of the concave mirror,the light bounces back up the tube to the 12-inch(0.3-m)-diameter convex secondary mirror. The lightis now reflected and converged through a 23.5-inch(60-cm) hole in the primary mirror to the Telescopefocus, 3.3 feet (1.5 m) behind the primary mirror.

Four science instruments and three FGSs share thefocal plane by a system of mirrors. A small “folding”mirror in the center of the FOV directs light into theWFPC2. The remaining “science” field is dividedamong three axial science instruments, eachreceiving a quadrant of the circular FOV. Around theoutside of the science field, a “guidance” field isdivided among the three FGSs by their own foldingmirrors. Each FGS receives 60 arcmin2 of field in a90-degree sector. Figure 5-18 shows instrument/sensorfields of view.

The OTA hosts the science instruments and FGSs inthat it maintains the structural support and optical-image stability required for these instruments to fulfilltheir functions (see Fig. 5-19). Components of theOTA are the primary mirror, the secondary mirror, theFPS and the OTA Equipment Section. Perkin-Elmer

Corporation designed and built all the optical assemblies.Lockheed Martin built the OTA equipment section.

Primary Mirror Assembly and SphericalAberration

As the Telescope was first put through its paces onorbit in 1990, scientists discovered its primary mirror

had a spherical aberration. The outer edge of the 8-foot(2.4-m) primary mirror was ground too flat by awidth equal to 1/50 the thickness of a sheet of paper(about 2 microns). After the discovery, Ball Aerospacescientists and engineers built the Corrective OpticsSpace Telescope Axial Replacement (COSTAR). Itwas installed during the First Servicing Mission inDecember 1993 and brought the Telescope back to itsoriginal specifications.

The primary mirror assembly consists of the mirrorsupported inside the main ring, which is the struc-tural backbone of the Telescope, and the main andcentral baffles (see Fig. 5-20). This assembly providesthe structural coupling to the rest of the spacecraftthrough a set of kinematic brackets linking the mainring to the SSM. The assembly also supports theOTA baffles. Its major parts are:• Primary mirror• Main ring structure• Reaction plate and actuators• Main and central baffles.

Primary Mirror. The primary mirror blank, aproduct of Corning Glass Works, is known asultralow-expansion (ULE) glass. It was chosen for itsvery low-expansion coefficient, which ensures theTelescope minimum sensitivity to temperature

Fig. 5-17 Light path for the main Telescope

Aperture Door Secondary MirrorPrimaryMirror

Fine GuidanceSensor (3)

Axial ScienceInstrument (4)

Incoming Light

Stray-LightBaffles

SupportSystemsModule

Radial ScienceInstruments

Focal Plane(Image Formed Here)

K1175_517

HST SYSTEMS

5-16

Fig. 5-18 Instrument/sensor field of view after SM3B

Fig. 5-19 Optical Telescope Assembly components

SecondaryMirrorAssembly

Graphite EpoxyMetering Truss

Central Baffle

Support

Find GuidanceSensor (3)

Focal PlaneStructure

AxialScienceInstrument (4)

Fixed-HeadStar Tracker

Radial ScienceInstrument (1)

Main Ring

Primary Mirror

Electronic Boxes

AluminumMain Baffle

K1175_519

Space TelescopeImaging Spectrograph

+V3 Axis Fine Guidance Sensor 2

Corrective OpticsSpace TelescopeAxial Replacement

Fine GuidanceSensor 1

Advanced Camerafor Surveys

Wide Field and Planetary Camera 2

Incoming Image(View Looking Forward

+V1 Axis into Page)

Two Detectors,Separate Apertures

Near-InfraredCamera and Multi-ObjectSpectrometer

FineGuidanceSensor 3

Optical ControlSensor (3)

WFC

#3#2#1 14.1 arcmin

K1175_518

HST SYSTEMS

5-17

changes. The mirror is of a “sandwich” construction:two lightweight facesheets separated by a core, orfilling, of glass honeycomb ribs in a rectangular grid(see Fig. 5-21). This construction results in an 1800-pound (818-kg) mirror instead of an 8000-poundsolid-glass mirror.

Perkin-Elmer ground the mirror blank, 8 feet (2.4 m)

in diameter, to shape in its large optics fabricationfacility. When it was close to its final hyperboloidalshape, the mirror was transferred to the company’scomputer-controlled polishing facility.

After being ground and polished, the mirror wascoated with a reflective layer of aluminum and aprotective layer of magnesium fluoride only 0.1- and0.025-micrometer thick, respectively. The fluoridelayer protects the aluminum from oxidation andenhances reflectance at the important hydrogen emis-sion line known as Lyman-Alpha. The reflectivequality of the mirror is better than 70 percent at 1216angstroms (Lyman-Alpha) in the ultraviolet spectralrange and better than 85 percent for visible light.

The primary mirror is mounted to the main ringthrough a set of kinematic linkages. The linkagesattach to the mirror by three rods that penetrate theglass for axial constraint and by three pads bondedto the back of the glass for lateral support.

Main Ring. The main ring encircles the primarymirror; supports the mirror, the main baffle andcentral baffle, and the metering truss; and integratesthe elements of the Telescope to the spacecraft (seeFig. 5-22). This titanium ring, weighing 1200 pounds(545.5 kg), is a hollow box beam 15 inches (38 cm)thick with an outside diameter of 9.8 ft (2.9 m). It issuspended inside the SSM by a kinematic support.

Reaction Plate. The reaction plate is a wheel of I-beams forming a bulkhead behind the main ring,spanning its diameter. It radiates from a central ringthat supports the central baffle. Its primary functionis to carry an array of heaters that warm the back ofthe primary mirror, maintaining its temperature at70 degrees Fahrenheit. Made of lightweight, stiffberyllium, the plate also supports 24 figure-controlactuators attached to the primary mirror andarranged around the reaction plate in two concentric

Fig. 5-20 Primary mirror assembly

Fig. 5-21 Primary mirror construction Fig. 5-22 Main ring and reaction plate

Central Baffle Bracket (5)RadialI-Beam (15)

Figure ControlActuator (24)

Main Ring

Mirror Pad(24)Flexure

Mount (15)

AxialMount(15)

IntercostalRib (48)

Central Ring

K1175_522

MirrorConstruction

FrontFacesheet

InnerEdgeband

LightweightCore

OuterEdgeband

RearFacesheet

The mirror is made of Corning Code 7971ultralow expansion (ULE) silica glass

K1175_521

Primary Mirror Assembly

Main Baffle

Primary Mirror

Mirror Mount

Actuator (Typical)

Reaction Plate

Support SystemsModule AttachmentBrackets

Main Ring

Central Baffle

K1175_520

HST SYSTEMS

5-18

circles. These can be commandedfrom the ground, if necessary, tomake small corrections to theshape of the mirror.

Baffles. The OTA’s bafflesprevent stray light from brightobjects—such as the Sun, Moonand Earth—from reflecting downthe Telescope tube to the focalplane. The primary mirrorassembly includes two baffles.Attached to the front face of themain ring, the outer (main) baffleis an aluminum cylinder 9 feet(2.7 m) in diameter and 15.7 feet(4.8 m) long. Internal fins help itattenuate stray light. The centralbaffle is 10 feet (3 m) long, cone-shaped and attached to the reac-tion plate through a hole in thecenter of the primary mirror. Itextends down the centerline ofthe Telescope tube. The baffleinteriors are painted flat black tominimize light reflection.

Secondary Mirror Assembly

The Secondary Mirror Assemblycantilevers off the front face ofthe main ring and supports thesecondary mirror at exactly thecorrect position in front of the

primary mirror. This positionmust be accurate within 1/10,000inch whenever the Telescope isoperating. The assembly consistsof the mirror subassembly, a lightbaffle and an outer graphite-epoxy metering truss supportstructure (see Fig. 5-23).

The Secondary Mirror Assemblycontains the mirror, mounted onthree pairs of alignment actuatorsthat control its position andorientation. All are enclosedwithin the central hub at theforward end of the truss support.

The secondary mirror has amagnification of 10.4X. Itconverts the primary-mirrorconverging rays from f/2.35 to afocal ratio system prime focus off/24 and sends them back towardthe center of the primary mirror,where they pass through thecentral baffle to the focal point.The mirror is a convex hyper-boloid 12 inches (0.3 m) in diam-eter and made of Zerodur glasscoated with aluminum andmagnesium fluoride. Steeplyconvex, it has a surface accuracyeven greater than that of theprimary mirror.

Ground command adjusts theactuators to align the secondarymirror to provide perfect imagequality. The adjustments arecalculated from data picked up bytiny optical control systemsensors located in the FGSs.

The principal structural elementof the Secondary Mirror Assemblyis the metering truss, a cage with48 latticed struts attached to threerings and a central support struc-ture for the secondary mirror. Thetruss, 16 feet (4.8 m) long and 9 feet (2.7 m) in diameter, is agraphite, fiber-reinforced epoxystructure. Graphite was chosenfor its high stiffness, light weightand ability to reduce the struc-ture’s expansiveness to nearlyzero. This is vital because thesecondary mirror must stayperfectly placed relative to theprimary mirror, accurate to within0.0001 inch (2.5 micrometers)when the Telescope operates.

The truss attaches at one end tothe front face of the main ring ofthe Primary Mirror Assembly.The other end has a central hubthat houses the secondary mirrorand baffle along the optical axis.Aluminized mylar MLI in thetruss compensates for tempera-ture variations of up to 30degrees Fahrenheit when theTelescope is in Earth’s shadow sothe primary and secondarymirrors remain aligned.

The conical secondary mirrorsubassembly light baffle extendsalmost to the primary mirror. Itreduces stray bright-object lightfrom sources outside theTelescope FOV.

Fig. 5-23 Secondary mirror assembly

Shroud

Base Plate

Actuator Pair

Secondary Mirror

SecondaryMirror Baffle

Actuator Pair

Clamp

Flexure

K1175_523

HST SYSTEMS

Focal Plane StructureAssembly

The FPS is a large optical benchthat physically supports thescience instruments and FGSs andaligns them with the image focalplane of the Telescope. The -V3side of the structure, away fromthe Sun in space, supports theFHSTs and RSUs (see Fig. 5-24). It also provides facilities for on-orbit replacement of any instru-ments and thermal isolationbetween instruments.

The structure is 7 feet (2.1 m) by10 feet (3.04 m) long and weighsmore than 1200 pounds (545.5 kg).Because it must have extremethermal stability and be stiff,lightweight and strong, the FPS isconstructed of graphite-epoxy,augmented with mechanicalfasteners and metallic joints atstrength-critical locations. It isequipped with metallic mountsand supports for OrbitalReplacement Units (ORU) usedduring maintenance.

The FPS cantilevers off the rearface of the main ring, attached ateight flexible points that adjustto eliminate thermal distortions.The structure provides a fixedalignment for the FGSs. It hasguiderails and latches at eachinstrument mounting location soShuttle crews can easily exchangescience instruments and otherequipment in orbit.

OTA Equipment Section

The OTA Equipment Section is alarge semicircular set of compart-ments mounted outside thespacecraft on the forward shell ofthe SSM (see Fig. 5-25). Itcontains the OTA ElectricalPower and Thermal ControlElectronics (EP/TCE) System,Fine Guidance Electronics (FGE),Actuator Control Electronics(ACE), Optical ControlElectronics (OCE) and the fourthDMS DIU. The OTA EquipmentSection has nine bays: seven forequipment storage and two forsupport. All bays have outward-opening doors for easy astronautaccess, cabling and connectors for

the electronics, and heaters andinsulation for thermal control.

The EP/TCE System distributespower from the SSM EPS and theOTA system. Thermostats regu-late mirror temperatures andprevent mirror distortion fromthe cold of space. The electricaland thermal electronics alsocollect thermal sensor data fortransmission to the ground.

Three FGE units provide power,commands and telemetry to theFGSs. The electronics performcomputations for the sensors andinterface with the spacecraftpointing system for effectiveTelescope line-of-sight pointing

Fig. 5-24 Focal plane structure

Fig. 5-25 Optical Telescope Assembly Equipment Section

Looking Forward-V2

-V3

V2

V3V1

Fine GuidanceElectronics

DIUACE

Data Interface Unit

OpticalControl Electronics

Fine GuidanceElectronics

ActuatorControl Electronics

MultilayerInsulation on Doors

Electrical Power/Thermal ControlElectronics

K1175_525

A

BC

DEF

GH

J

Primary Mirror Main Ring

Focal Plane Structure

Axial Science Instrument

Science InstrumentLatching Mechanism

Fixed-HeadStar Tracker (3)

Crew System Handle

Rate SensingUnit for Rate GyroAssemblies (3)

Science InstrumentConnectorPanel

K1175_524

5-19

HST SYSTEMS

5-20

and stabilization. There is a guidance electronicsassembly for each guidance sensor.

The ACE unit provides the command and telemetryinterface to the 24 actuators attached to the primarymirror and to the six actuators attached to thesecondary mirror. These electronics select whichactuator to move and monitor its response to thecommand. Positioning commands go from theground to the electronics through the DIU.

The OCE unit controls the optical control sensors.These white-light interferometers measure theoptical quality of the OTA and send the data to theground for analysis. There is one optical controlsensor for each FGS, but the OCE unit runs allcontrol sensors. The DIU is an electronic interfacebetween the other OTA electronics units and theTelescope command and telemetry system.

Fine Guidance Sensor

Three FGSs are located at 90-degree intervals aroundthe circumference of the focal plane structure,between the structure frame and the main ring. Eachsensor measures 5.4 feet (1.5 m) long and 3.3 feet (1 m)wide and weighs 485 pounds (220 kg).

Each FGS enclosure houses a guidance sensor and awavefront sensor. The wavefront sensors areelements of the optical control sensor used to alignand optimize the optical system of the Telescope.

The Telescope’s ability to remain pointing at adistant target to within 0.005 arcsec for long periodsof time is due largely to the accuracy of the FGSs.They lock on a star and measure any apparentmotion to an accuracy of 0.0028 arcsec. This is equiv-alent to seeing from New York City the motion of alanding light on an aircraft flying over San Francisco.

When two sensors lock on a target, the third meas-ures the angular position of a star, a process calledastrometry. Sensor astrometric functions arediscussed in Section 4, Science Instruments. DuringSM2 a re-certified FGS (S/N 2001) was installed as areplacement in the HST FGS Bay 1. During SM3A are-certified FGS (S/N 2002) was installed in the HSTFGS Bay 2.

FGS Composition and Function

Each FGS consists of a large structure housing acollection of mirrors, lenses, servos to locate animage, prisms to fine-track the image, beam splittersand four photomultiplier tubes (see Fig. 5-26). The

entire mechanism adjusts to move the Telescope intoprecise alignment with a target star. Each FGS has alarge (60 arcmin2) FOV to search for and track stars,and a 5.0 arcsec2 FOV used by the detector prisms topinpoint the star.

The sensors work in pairs to aim the Telescope. TheGuide Star Selection System, developed by theScience Institute, catalogs and charts guide stars neareach observation target to make it easier to find thetarget. One sensor searches for a target guide star.After the first sensor locks onto a guide star, thesecond sensor locates and locks onto another targetguide star. Once designated and located, the guidestars keep the image of the observation target in theaperture of the selected science instrument.

Each FGS uses a 90-degree sector of the Telescope’sFOV outside the central “science” field. This regionof the FOV has the greatest astigmatic and curvaturedistortions. The size of the FGS’s FOV was chosento heighten the probability of finding an appropriateguide star, even in the direction of the lowest starpopulation near the galactic poles.

An FGS “pickoff” mirror intercepts the incomingstellar image and projects it into the sensor’s largeFOV. Each FGS FOV has 60 arcmin2 available. Theguide star of interest can be anywhere within thisfield. After finding the star, the sensor locks onto itand sends error signals to the Telescope, telling ithow to move to keep the star image perfectly still.Using a pair of star selector servos, the FGS can moveits line of sight anywhere within its large FOV. Each

Fig. 5-26 Cutaway view of Fine Guidance Sensor

EnclosureAspheric CollimatingMirror

Pinhole/LensAssembly (4)

DoubletLens (4)

Koesters’Prisms

Filter (5)

Pickoff Mirror

StarSelectorMirrors

Corrector GroupDeviationPrism

Optical Bench

K1175_526

HST SYSTEMS

5-21

can be thought of as an optical gimbal: one servomoves north and south, the other east and west.They steer the small FOV (5 arcsec2) of the FGSdetectors to any position in the sensor field. Encoderswithin each servo system send back the exact coordi-nates of the detector field centers at any point.

Because the exact location of a guide star may beuncertain, the star selector servos also can cause thedetector to search the region around the most prob-able guide star position. It searches in a spiralpattern, starting at the center and moving out untilit finds the guide star it seeks. Then the detectors arecommanded to go into fine-track mode and hold thestar image exactly centered in the FOV while thestar selector servo encoders send information aboutthe position of the star to the spacecraft PCS.

The detectors are a pair of interferometers, calledKoester’s prisms, coupled to photomultiplier tubes(see Fig. 5-27). Each detector operates in one axis, sotwo detectors are needed. Operating on theincoming wavefront from the distant guide star, theinterferometers compare the wave phase at one edgeof the Telescope’s entrance aperture with the phaseat the opposite edge. When the phases are equal, thestar is exactly centered. Any phase difference showsa pointing error that must be corrected.

Along the optical path from Telescope to detector areadditional optical elements that turn or fold thebeam to fit everything inside the FGS enclosure andto correct the Telescope’s astigmatism and fieldcurvature. All optical elements are mounted on atemperature-controlled, graphite-epoxy compositeoptical bench.

Articulated Mirror System

Analysis of the FGS on-orbit data revealed thatminor misalignments of the optical pupil centeringon Koester’s prism interferometer in the presence ofspherical aberration prevented the FGS fromachieving its optimum performance. During therecertification of FGS (S/N 2001), fold flat #3 in theradial bay module optical train was mechanized toallow on-orbit alignment of the pupil.

Implementation of this system utilized existingsignals and commands by rerouting them with aunique interface harness enhancement kit (OCE-EK)interfacing the OCE, the DIU and the Fine GuidanceSystem/Radial Bay Module (FGS/RBM). The OCE-EK was augmented with the Actuator MechanismElectronics (AME) and the fold flat #3 ActuatorMechanism Assembly (AMA) located internal to theFGS/RBM. Ground tests indicate a substantialincrease in FGS performance with this innovativedesign improvement.

Solar Arrays

New rigid solar arrays will be attached to theTelescope during SM3B. The original arrays fitted toHubble—designed by the European Space Agencyand built by British Aerospace, Space Systems—aretwo large rectangular wings of retractable solar cellblankets fixed on a two-stem frame. The blanketunfurls from a cassette in the middle of the wing. Aspreader bar at each end of the wing stretches theblanket and maintains tension.

Following deployment in 1990, engineers discoveredtwo problems: a loss of focus and images thatjittered briefly when the Telescope flew into and outof Earth’s shadow. The jitter problem was traced tothe two large SAs. Abrupt temperature changes,from -150 to 200 degrees Fahrenheit during orbit,caused the panels to distort twice during each orbit.As a temporary fix, software was written thatcommanded the PCS to compensate for the jitterautomatically. The problem was mitigated duringSM1 by the replacement of the old arrays with newones that had been modified to reduce thermalswings of the bi-stems.

The two new solar array wings to be installed onSM3B are assembled from eight panels built atLockheed Martin Space Systems Company inSunnyvale, California, and designed originally for thecommercial Iridium communications satellites. AtGoddard Space Flight Center in Greenbelt,Maryland, four panels were mounted onto eachaluminum-lithium support wing structure.

Fig. 5-27 Optical path of Fine Guidance Sensor

K1175_527

OpticalTelescopeAssembly

CollimatorFirst Star Selector

Refractive GroupSecond Star Selector

BeamSplitter

Pupil

Rotated90 deg

Field Lensand Field Stop

Phototube

First PupilKoesters’ Prism

Doublet

HST SYSTEMS

5-22

The new wing assemblies, whichhave higher efficiency GalliumArsenide solar cells, will giveHubble approximately 20 percentmore power than the currentarrays. In addition, their smallercross section and rigidity willreduce aerodynamic drag andproduce significantly less vibra-tion than the existing wings (seeFig. 5-28).

New Solar Array DriveMechanisms (SADM) also will beinstalled during SM3B. Thesemechanisms will maneuver thenew arrays to keep themconstantly pointed at the Sun.The European Space Agency(ESA) designed, developed andtested the SADMs. ESA used itsworld-class test facility in TheNetherlands to subject the newarrays and drive mechanisms torealistic simulations of theextreme temperature cyclesencountered in Hubble’s orbit—including sunrise and sunset.

Science InstrumentControl and DataHandling Unit

The SI C&DH unit keeps allscience instrument systemssynchronized. It works with theDMU to process, format,temporarily store on the datarecorders or transmit science andengineering data to the ground.

Components

The SI C&DH unit is a collectionof electronic componentsattached to an ORU traymounted on the door of Bay 10 inthe SSM Equipment Section (seeFig. 5-29). Small Remote InterfaceUnits (RIU), also part of thesystem, provide the interface toindividual science instruments.

Components of the SI C&DHunit are:

• NASA Standard SpacecraftComputer (NSCC-I)

• Two standard interface circuitboards for the computer

• Two control units/science dataformatter units (CU/SDF)

• Two CPU modules • A PCU • Two RIUs • Various memory, data and

command communicationslines (buses) connected bycouplers.

These components are redundantso the system can recover fromany single failure.

NASA Computer. The NSSC-Ihas a CPU and eight memorymodules, each holding 8,192eighteen-bit words. One embeddedsoftware program (the “execu-tive”) runs the computer. It movesdata, commands and operationprograms (called applications) forindividual science instruments inand out of the processing unit.The application programs monitorand control specific instruments,and analyze and manipulate thecollected data.

The memory stores operationalcommands for execution whenthe Telescope is not in contact

Item

SA2Flexible Array

4600 W at6 years(actual)

39.67 ft x 9.75 ft(12.1 m x 3.3 m)

339 lbper wing

Actual on-orbit performancemeasured at Winter Solstice,December 2000

Power Size Weight Comments

SA3Rigid Array

5270 W at6 years(predicted)

24.75 ft x 8.0 ft(7.1 m x 2.6 m)

640 lbper wing

Increased capability forplanned science

Less shadowing and blockage

Fig. 5-28 Solar Array wing detail comparison

39.67 ft (12.1 m)

9.75 ft(3.3 m)

8.0 ft(2.6 m)

24.75 ft (7.1 m)

K1175_528

K1175_528A

SA2 Flexible Array

SA3 Rigid Array

HST SYSTEMS

5-23

with the ground. Each memoryunit has five areas reserved forcommands and programs uniqueto each science instrument. Thecomputer can be reprogrammedfrom the ground for futurerequests or for working aroundfailed equipment.

Standard Interface Board. Thecircuit board is the communica-tions bridge between thecomputer and the CU/SDF.

Control Unit/Science DataFormatter. The heart of the SIC&DH unit is the CU/SDF. Itformats and sends all commandsand data to designated destina-tions such as the DMU of theSSM, the NASA computer andthe science instruments. The unithas a microprocessor for controland formatting functions.

The CU/SDF receives groundcommands, data requests, scienceand engineering data, and systemsignals. Two examples of systemsignals are “time tags”—clocksignals that synchronize theentire spacecraft—and “processorinterface tables”—communica-

tions codes.The CU/SDFtransmitscommandsand requestsafter format-ting them sothat thespecific desti-nation unitcan readthem. Forexample,groundcommandsand SSMcommandsare trans-mitted withdifferentformats.Groundcommands

use 27-bit words and SSMcommands use 16-bit words. Theformatter translates eachcommand signal into a commonformat. The CU/SDF also refor-mats and sends engineering andscience data. Onboard analysis ofthe data is an NSSC-I function.

Power Control Unit. The PCUdistributes and switches poweramong components of the SIC&DH unit. It conditions thepower required by each unit. Forexample: The computer memoryboards typically need +5 volts, -5 volts and +12 volts while theCU/SDF requires +28 volts. ThePCU ensures that all voltagerequirements are met.

Remote Interface Unit. RIUstransmit commands, clock andother system signals, and engi-neering data between thescience instruments and the SIC&DH unit. However, the RIUsdo not send science data. Thereare six RIUs in the Telescope:five attached to the scienceinstruments and one dedicatedto the CU/SDF and PCUs in theSI C&DH unit. Each RIU can be

coupled with up to twoexpander units.

Communications Buses. The SIC&DH unit contains data bus linesthat pass signals and data betweenthe unit and the science instru-ments. Each bus is multiplexed:one line sends system messages,commands and engineering datarequests to the module units, and areply line transmits requestedinformation and science data backto the SI C&DH unit. A couplerattaches the bus to each remoteunit. This isolates the module ifthe RIU fails. The SI C&DHcoupler unit is on the ORU tray.

Operation

The SI C&DH unit handlesscience instrument system moni-toring (such as timing and systemchecks), command processing anddata processing.

System Monitoring. Engineeringdata tells the monitoringcomputer whether instrumentsystems are functioning. Atregular intervals, varying fromevery 500 milliseconds to every40 seconds, the SI C&DH unitscans all monitoring devices forengineering data and passes datato the NSCC-I or SSM computer.The computers process or storethe information. Any failureindicated by these constant testscould initiate a “safing hold”situation (refer to page 5-13,Safing (Contingency) System),and thus a suspension of scienceoperations.

Command Processing. Figure 5-30shows the flow of commandswithin the SI C&DH unit.Commands enter the CU/SDF(bottom right in the drawing)through the SSM Command DIU(ground commands) or the DIU(SSM commands). The CU/SDFchecks and reformats thecommands, which then go either

Fig. 5-29 Science Instrument Control and Data Handling unit

K1175_529

Two CentralProcessorModules

Extravehicular Activity Handle

A118A118A118A118

A109

A120 FourMemories

RemoteInterface Unit

A231

A300

A230

A210

A300

A211

Control Unit/Science Data Formatter Power Control Unit

Orbital ReplacementUnit Tray

Bus Coupler Unit A240

Control Unit/Science DataFormatter

RemoteInterface Unit

FourMemoriesTwo Standard

Interfaces

HST SYSTEMS

5-24

to the RIUs or to the NSCC-I for storage. “Time-tagged” commands, stored in the computer’s memory(top right of drawing), also follow this process.

Each command is interpreted as “real time,” as if theSI C&DH just received it. Many commands actuallyare onboard stored commands activated by certainsituations. For example, when the Telescope is posi-tioned for a programmed observation using the SpaceTelescope Imaging Spectrograph, that program is acti-vated. The SI C&DH can issue certain requests to theSSM, such as to execute a limited number of pointingcontrol functions to make small Telescope maneuvers.

Science Data Processing. Science data can comefrom all science instruments at once. The CU/SDFtransfers incoming data through computer memorylocations called packet buffers. It fills each buffer inorder, switching among them as the buffers fill andempty. Each data packet goes from the buffer to theNSCC-I for further processing, or directly to theSSM for storage in the data recorders or transmis-sion to the ground. Data returns to the CU/SDFafter computer processing. When transmitting, theCU/SDF must send a continuous stream of data,either full packet buffers or empty buffers calledfiller packets, to maintain a synchronized link with

the SSM. Special checking codes (Reed-Solomon andpseudo-random noise) can be added to the data asoptions. Figure 5-30 shows the flow of science datain the Telescope.

Space Support Equipment

Hubble was designed to be maintained, repaired andenhanced while in orbit, extending its life and useful-ness. For servicing, the Space Shuttle will captureand position the Telescope vertically in the aft end ofthe cargo bay, then the crew will perform mainte-nance and replacement tasks. The Space SupportEquipment (SSE) provides a maintenance platform tohold the Telescope, electrical support of theTelescope during servicing and storage for replace-ment components known as ORUs.

The major SSE items to be used for SM3B are theFlight Support System (FSS) and and ORU Carriers(ORUC), comprising the Rigid Array Carrier (RAC),the Second Axial Carrier (SAC) and the Multi-UseLightweight Equipment (MULE) carrier. Crew aidsand tools also will be used during servicing. Section 2of this guide describes details specific to SM3B.

Fig. 5-30 Command flow for Science Instrument Control and Data Handling unit K1175_530

RemoteModules

RemoteModules

RemoteModules

RemoteModules

RemoteModules

RemoteModules

To S

cien

ce I

nstr

umen

ts 1

– 5

Com

mands

Supervisory Bus

To PowerControl Unit

StoredCommand Memory

Science Instrument

Unique Memory

NASA Standard Spacecraft ComputerModel-I and Standard Interface

StoredCommand

OutputBuffer

Science Instrument

Unique Memory

Science Instrument

Unique Memory

Science Instrument

Unique Memory

Science Instrument

Unique Memory

Science Instrument

Unique Memory

Data M

anagement

Units 6 and 7

Tim

e Tags and Com

mands

Data M

anagement

Unit 1

Com

puter Com

mands

Data M

anagement

Units 4 and 5

Support SystemsModule ProcessorInterface Table Words

Reformat

Word Order,Time Tag andParity Checks

CU/SDFCommandProcessing

Control

Control Unit/Science Data Formatter (CU/SDF)

ToCU/SDFLogic

Parity and Type Code

Checks

Parity andWord Order

Checks

From CommandData Interface

27-bit Commands

16-bit Commands

From DataInterface Unit

HST SYSTEMS

5-25

Orbital Replacement Unit Carrier

An ORUC is a pallet outfitted with shelves and/orenclosures that is used to carry replacements intoorbit and to return replaced units to Earth. For SM3Bthe Columbia payload bay will contain three ORUCs.All ORUs and scientific instruments are carriedwithin protective enclosures to provide them a benignenvironment throughout the mission. The enclosuresprotect the instruments from contamination andmaintain the temperature of the instruments orORUs within tight limits. Instruments are mountedin the enclosures using the same manually drivenlatch system that holds instruments in the Telescope.

During the change-out process, replaced scienceinstruments are stored temporarily in the ORUC. Atypical change-out begins with an astronautremoving the old instrument from the Telescope andattaching it to a bracket on the ORUC. The astronautthen removes the new instrument from its protectiveenclosure and installs it in the Telescope. Finally, theastronaut places the old instrument in the appro-priate protective enclosure for return to Earth.

The ORUC receives power for its TCS from the FSS.The carrier also provides temperature telemetry datathrough the FSS for readout in the Shuttle and onthe ground during the mission.

Crew Aids

Astronauts perform extravehicular activities usingmany tools to replace instruments and equipment,to move around the Telescope and the cargo bay, andto operate manual override drives. Tools and equip-ment, bolts, connectors and other hardware are stan-dardized not only for the Telescope but also betweenthe Telescope and the Shuttle. For example, grap-pling receptacles share common features.

To move around the Telescope, the crew uses 225feet of handrails encircling the spacecraft. The railsare painted yellow for visibility. In addition, thecrew can hold onto guiderails, trunnion bars andscuff plates fore and aft.

Astronauts can install portable handhold plates wherethere are no permanent holds, such as on the FGS.Another tool is the Portable Foot Restraint (PFR).

While the astronauts work, they use tethers to hooktools to their suits and tie replacement units to theTelescope. Each crew member has a ratchet wrenchto manually crank the antenna and array masts ifpower for the mast drives fails. A power wrench alsois available if hand-cranking is too time consuming.Other hand tools include portable lights and ajettison handle, which attach to sockets on the aper-ture door and to the SA wings so the crew can pushthe equipment away from the Telescope.

Fig. 5-31 Flow of science data in the Hubble Space Telescope

K1175_531

PN

Code

(Only if P

N

Encoding

Selected)

Reed-S

olomon C

heckbit Segm

ents

(Only if R

eed-Solom

onE

ncoding Selected)

Science D

ata Packet S

egments

Science Instrum

ents

Direct M

emory A

ccess 5R

aw S

cience Data

Direct M

emory A

ccess 3

Control W

ordsD

irect Mem

oryA

ccess 8

Direct M

emory

Access 3

Control W

ordsC

ontrol

Direct M

emory A

ccess 14

NA

SA

Model-I C

omputer

Data Logs and P

rocessed Science D

ata

Ancillary D

ata

Science D

ataInput S

election Logic

NASA Standard Spacecraft Computer Model-Iand Standard Interfaces

Control Logic

Control Commands

Control Unit/ScienceData Formatter Logic

Raw Science

Data

FromCentral Unit/Science DataFormatterCommandProcessing

Control Unit/ScienceData Formatter Logic

To Data Management Unit

Packet Buffer 1

Raw or Packetized Data

Time Tag

Format RAM and ROM

Time Tag

Packet Buffer 2

Control Unit/Science Data

FormatterTimer

DestinationSwitch

(1 Per ScienceInstrument)

FillerPackets

ControlSwitch

ControlSwitch

Reed-SolomonEncoder

Science Data Bit Stream

End of Period

PseudorandomNoise CodeGenerator