LECTURE7

LowEnergyTelescopeDesignandInstrumentation

OpticalAstronomy

• Ultraviolet• Visible• Infrared

• “Visible”and“optical”areofteninterchangeable

• Lightthathumanscansee(andabitmore)• ≈3500Å–≈8000Å

• Oldestbranchofastronomy

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MainTypesofOpticalObservations

• Imaging• Photometry• Spectra• Polarimetry

Atmospheretransparentinthreebands

• Optical• Infrared

withmanyabsorptionbandsduetoH2O,CO2,O2,etc.

• Radio

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Mostastronomyisoptical,althoughthisischanging• Atmosphereistransparent• Manyfacilitiesavailable• Longhistory• Thermalprocesses(blackbodyradiation)tendtopeakinornearoptical

OpticalTelescopeDesign

Twobasicdesigns• Refractingtelescope• Reflectingtelescope

RefractingTelescopes

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• Inventedin1600s• Tubewithasinglelens(theobjective)andaneyepiece

Refractorsarenotusedmuchinprofessionalastronomyanymore• Chromatic&sphericalaberration• Hardtomakelargetelescopes• Lensessufferfromcontamination

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ReflectingTelescopes

TubewithamirroratoneendLightisreflectedtoafocalplaneInventedin1663byJamesGregory• Largeobjectivemirrors,lightgatheringpower• Easytoincorporateinstrumentation

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InstrumentationManytypesofopticalinstrumentation• 0‐d

o photometryo polarimeter

• 1‐do spectrograph

• 2‐do photographicplateo ccdimager

Mostcommondetectorisacharge­coupleddevice(CCD).

Exposurestarts• Nphotonslandonapixel• Pixelaccumulatesacharge

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• Theprocessedistimedinordertoreconstructa2‐dimage.

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InstrumentalEffects

Bias

• CCDsoperatewithapedestalvoltage• Slightlydifferentineachpixel• Needtobesubtractedoff

• Takeseveral0‐secondexposureswithclosedshutter

• Taketheaverage• Subtractthisfromthedata

DarkCurrent

• Causedbyrandomcurrentsinthedetector• Darkcurrentisusuallyverysmall

o Afewelectronsperhour• Canbesignificantforlongexposures

• Takeanimagewiththeshutterclosedwiththesameexposureasthedataimage

• Subtractthisfromthebias‐subtracteddataimage

FlatFieldCorrection

• Correctsforpixel‐to‐pixelsensitivityvariations

• Takeanexposureofauniformfieldo Twilightskyo Blankareaofthenightskyo Illuminatedscreen

• Averagemanyexposures• Normalizetheimage• Dividedark‐subtracteddatabythenormalizedflat

InfraredAstronomyWavelengths:≈8000Åand≈1mmMainInfraredBandsWavelength(μm) Band

0.65–1.0 RandI Allmajoropticaltelescopes1.25 J Mostmajoropticaltelescopesand

mostdedicatedinfraredtelescopes1.65 H Mostmajoropticaltelescopesand

mostdedicatedinfraredtelescopes2.2 K  Mostmajoropticaltelescopesand

mostdedicatedinfraredtelescopes3.45 L  Mostdedicatedinfraredtelescopesand

someopticaltelescopes4.7 M  Mostdedicatedinfraredtelescopesand

someopticaltelescopes10 N  Mostdedicatedinfraredtelescopesand

someopticaltelescopes20 Q  Somededicatedinfraredtelescopesand

someopticaltelescopes450 Sub‐mm Sub‐millimetretelescopes

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• Atmosphereisonlyonlytransparentinsomeinfraredbands

• Needspace‐basedobservatories• Near­Infrared

o 7000Å–5μm• Mid­Infrared

o 5μm–40μm• Far­Infrared

o 40μm–350μmInfraredScience• protostars• coolobjects

o browndwarfso exoplanets

• dust• AGNs• high‐redshiftgalaxies

InfraredTelescopes

Near‐andmid‐infraredtelescopessimilartoopticaltelescopesSpitzerSpaceTelescope

Highthermalbackgroundsocoolingisneeded• AtL2,awayfromEarth• AlwayspointsawayfromtheSun• Activecyrogeniccoolingto4K• CoolantranoutinMay2009• Currentlyrunning“warm”mission

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3. OBSERVATORY DESCRIPTION

The Observatory is the space portion of the SIRTF Facility, and is comprised of five major subsystems. There are three

scientific instruments, the Cryogenic Telescope Assembly (CTA), and the Spacecraft. The observatory configuration is

optimized to minimize heat flow from the spacecraft to the CTA. SIRTF has a mass of about 950 kg and is about 5

meters tall. The observatory is shown in figures 3 and 4.

Figure 3. Observatory Internal Configuration Figure 4. Observatory External Configuration

Scientific Instruments

The three science instruments are the Infrared Array Camera (IRAC), the Multiband Imaging Photometer for SIRTF

(MIPS), and the Infrared Spectrograph (IRS). Each instrument consists of a cold sensor assembly located in the cryostat

and a redundant warm electronics assembly located in the spacecraft. IRS and MIPS share a common warm electronics

assembly. Cold assemblies are located in the multiple instrument chamber, within the CTA. Instrument descriptions

have been provided in section 2.

Cryogenic Telescope Assembly (CTA)

The Cryogenic Telescope Assembly is comprised of four key parts. The Ritchey-Chretien Telescope, the superfluid

helium cryostat, the outer shell group and the multiple instrument chamber (MIC) which houses the science instruments.

Figure 5 shows the key elements of the Cryogenic Telescope Assembly. The CTA features a light weight 85 cm

aperture, all beryllium telescope, which will be cooled to 5.5 K. It has been figured warm and tested at ~ 10°K at a

cryogenic optics test facility at JPL. Distortions which occur as the telescope is cooled have been reversed figured into

the primary mirror in a process called “cryo figuring”. More details on the performance of SIRTF’s telescope can be

found in Reference 2. The SIRTF telescope exceeds its goal of being diffraction limited at 6.5 µm.

A unique feature of the CTA is the warm launch architecture. Previous infrared space telescope designs have enclosed

both the telescope and instrument payload in the cryostat. In the case of SIRTF, only the science instrument sensor

sections are enclosed in the cryostat. This allows for significant reductions in both observatory mass and required

Solar Panel

Solar Panel Shield

Helium Tank

Nitrogen Tank

High Gain Antenna

Star Tracker and IRU

Spacecraft Shield

CTA Support Truss

Cryostat Vauum Shell

Multiple Instrument Chamber • IRAC Cold Assembly • IRS Cold Assembly • MIPS Cold Assembly • PCRS Cold Assembly

Spacecraft Bus • IRAC Warm Electronics • IRS/MIPS Warm Electronics • PCRS Warm Electronics • CTA Warm Electronics

Cryostat Aperture Door

Outer Shell

Telescope

Battery

Telescope

Solar Panel Shield

Spacecraft Shield

Spacecraft Bus

Low Gain AntennaeHigh Gain Antenna

Outer Shell

Star Trackers andInertial Reference Units

Solar Panel

SpitzerInstrumentsInfraredArrayCamera(IRAC)• Obtainsinfraredimages• 4wavelengthsimultaneousobservations:3.6μm,4.5μm,5.8μm,and8μm

InfraredSpectrograph(IRS)• Infraredspectrographwithfourbands

o 5.3–14µm(lowresolution)o 10–19.5µm(highresolution)o 14–40µm(lowresolution),ando 19–37µm(highresolution).

MultibandImagingPhotometerforSpitzer(MIPS)

• Farinfrared• 24μm,70μm,and160μm

RadioAstronomyRadioastronomycoverseverythinglongerthan≈1cm.Radioastronomystartedin1930withKarlJansky’sinvestigationintoradionoise.

• Discovereda“noise”signalthatcamefromafixeddirectioninthesky.

• Signalmatchedsiderealdaycelestialsource.

• DirectioncorrespondedtothecentreoftheMilkyWay.

RadioTelescopes

• Dishantenna• Parabolicdishtofocusradiowaves• Directional• Canproduceradiomaps

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InterferometricArrays• Linkphysicallyseparatedantennaetoformavirtualtelescope

• Increasesthecollectionarea• Aperturesynthesis

ApertureSynthesis

• Combinesignalsfrommultipleantennaesothattheyareinphase.

• Coherentadditionleadstoatelescopewithaneffectivediameterequaltotheseparationbetweentheantennae.

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VLBIArray

• Combinedatafromseveralradiotelescopesiteso Hawai`io NorthAmericao Caribbean

• Longestbaseline≈5000km• Effectivedishdiameter≈5000km• Resolutiongoesasinverseofdiameter• θ≈milliarcseconds

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