Post on 01-Jun-2020
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
Exposureends• Thechargeinapixelisshiftedonepixeldowneachcolumntoareadoutrow
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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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