PACS Instrument Design Description and System Performance

1
PACS Instrument Design Description and System
Performance

• A. Poglitsch

2
Science Requirements

• Summarized in PACS Science Requirements Document
• Main scientific drivers
• Investigations of the distant universe galaxy
  formation and evolution - history of star
  formation and nuclear activity
• Studies of star formation and the origin of the
  Initial Mass Function in our own Galaxy
• Physics and chemistry of the interstellar medium,
  Galactic and extragalactic
• Giant planets and the history of the Solar System
• Required observing capabilities
• Imaging photometry in 3 bands in the 60 - 210µm
  range with requirements on sensitivity per
  detector and field of view
• Imaging line spectroscopy in the 60 - 210µm
  (goal 55 - 210µm) range with requirements on
  sensitivity per detector, spectral resolution and
  instantaneous bandwidth, and field of view

3
Instrument Requirements

• Summarized in PACS Instrument Requirements
  Document
• High-level requirements on design, performance,
  and operation of PACS instrument
• Two basic instrument modes/channels
• Imaging photometry
• Imaging line spectroscopy
• Optimization / trade-off for prime science
  objectives
• definition of requirements which enable minimum
  mission
• definition of goals which enhance the performance
  to strengthen the science of the mission
• Minimization of risk and complexity

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Photometer Requirements

• Photometric bands with relative bandwidth Dl/llt2
• Field of view and pixel scale
• note equal f.o.v. in both bands is nearly a
  requirement
• Image quality
• blur telescope limited distortion 1 pixel
  alignment lt1/3 pixel

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Photometer Requirements

• Sensitivity (point source detection)
• requirement 5 mJy (5s), 1h of integration
• goal 3 mJy (5s), 1h of integration
• Dynamic range
• detection from 3 mJy to gt1000 Jy (goal 3000 Jy)
• contrast of up to 1500 in one field
• Post-detection bandwidth
• requirement 0.5 - 5 Hz
• goal 0.05 - 5 Hz
• Observing modes
• pointed observations (fixed, raster) with
  chopping/nodding
• line scans without chopping
• line scans with chopping (fixed throw)

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Photometer Requirements

• Calibration and photometric accuracy

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Spectrometer Requirements

• Spectral coverage
• 57 - 210µm
• goal continuous, no gaps
• Resolving power
• R 1000 - 2000
• Instantaneous bandwidth
• Dv 1000 - 2000 km/s
• Field of view and pixel scale
• 5 x 5 pixels
• 9x9 10 pixel scale
• Image Quality
• blur telescope limited distortion 1 pixel
  alignment lt1/4 pixel

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Spectrometer Requirements

• Sensitivity (point source detection)
• requirement 3x10-18 W/Hz1/2 (5s), 1h of
  integration
• goal 2x10-18 W/Hz1/2 (5s), 1h of
  integration
• Dynamic range
• detection from 1x10-18 W to gt10-13 W
• contrast of up to 1100 (t.b.c.) in one field
• Post-detection bandwidth
• requirement 5 Hz
• goal 10 Hz
• Observing modes
• line spectroscopy (line baseline/continuum)
• chopped
• unchopped, with frequency switching
• range spectroscopy (scan of arbitrary wavelength
  range) chopped

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Spectrometer Requirements

• Calibration and photometric accuracy
• Spectrometer implementation
• integral-field grating spectrometer
• photoconductive detectors
• only concept which can fulfill all requirements

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Instrument Concept

• Imaging photometry
• two bands simultaneously (60-90 or 90-130 µm and
  130-210 µm) with dichroic beam splitter
• two filled bolometer arrays (32x16 and 64x32
  pixels)
• point source detection limit 3 mJy (5s, 1h)
• Integral field line spectroscopy
• range 57 - 210 µm with 5x5 pixels, image slicer,
  and long-slit grating spectrograph (R 1500)
• two 16x25 GeGa photoconductor arrays
  (stressed/unstressed)
• point source detection limit 28 x 10-18 W/m2
  (5s, 1h)

Focal Plane Footprint
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Definition of the FOV for the Photometer
Physical pixel size 0.75 x 0.75 mm2
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Definition of the FOV for the Spectrometer
47
sampling
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PACS Design Focal Plane Footprint
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FPU Functional Groups

• Common input optics
• filter, focal plane splitter
• chopper (observation/calibration)
• calibration sources
• Photometer optical train
• dichroic beam splitter
• separate re-imaging optics for
• short-wavelength bands (60-90/90-130µm)
• long-wavelength band (130-210µm)
• Spectrometer optical train
• image slicer unit for integral-field spectroscopy
• anamorphic collimator
• Littrow-mounted grating with actuator/position
  readout

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FPU Functional Groups (cont.)

• dichroic beam splitter for order separation
• anamorphic re-imaging optics to independently
  match spatial and spectral resolution to pixel
  scale
• Photoconductor arrays (2 long/short wavelength)
• detectors with fore optics
• cryogenic readout electronics
• Bolometer arrays (2 long/short wavelength)
• bolometer assemblies with readout electronics
• common 0.3K cooler
• Band selectors
• filters exchange mechanism (photometer bands)
• filters exchange mechanism (grating orders)

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FPU Layout
Photometer Optics
Filter Wheel I
Slicer Optics
Blue Bolometer
0.3 K Cooler
Red Bolometer
Grating
Grating Drive
Encoder
sGeGaDetector Red Spectrometer
Spectrometer Optics
Chopper
Calibrator I and II
sGeGa Detector Blue Spectrometer
Calibrator Optics
Entrance Optics
Filter Wheel II
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Optical Performance

• Optical design fulfills requirements regarding
• field of view
• spatial sampling
• distortion
• geometrical spot sizes (Strehl ratio)
• alignment
• internal calibration capability
• chopping
• spectral coverage and resolution
• transmission / diffraction losses

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PACS GeGa Photoconductor Arrays
16 pixel stressed detector module

• 16x25 pixel filled arrays
• 25 linear modules
• integrated cryogenic readout electronics

Feed optics light cone array
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PACS Photoconductor Modules

• GeGa photoconductors
• unstressed 40 - 120µm
• stressed 110 - 210µm
• quantum efficiency gt 30 confirmed through
  independent measurement

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PACS Cryogenic Readout Electronics
In

• Capacitive feedback transimpedance amplifier
  (CTIA) for each pixel, based on AC-coupled
  inverter stage in silicon CMOS technology
• 16 CTIAs multiplexed on each CRE chip for each
  linear detector module
• CRE chips integrated in detector modules
• Amplifier noise compatible with
  background-limited performance in spectroscopy
• Technical feasibility demonstrated

Cf
Out
-A
CAC
CTIA architecture
Sync
Detectors
CA2
CA18
CA1
Reset
Sample
Analog Bus
Switch Control Logic
Clock
18-Bit Shift Register
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PACS Short-Wave Bolometer Array Assembly
2 K buffer amplifiers
(bottom view)
16 x 16 sub-array
Support 300 mK filter
Mechanical interface
Ribbon cable 300 mK
Strap 300 mK
(top view)
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Bolometer Arrays 16x16 Subarray Demonstrator
will be integrated in the interconnection circuit
in next models
I/C 1 Multiplexer
I/C 2 MOS Followers
Pixel
Interconnection circuit with reflectors
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Bolometer Arrays Pixel
pixel architecture
PACS short-wave bolometer pixel efficiency
(calculated)
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Bolometer Readout Performance
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PACS Grating

• Diamond ruled reflection grating
• Optical size 320 x 80 mm
• Used in 1st, 2nd, and 3rd order, angle range 48
  20
• 1st order (red detector) 210 - 105 µm
• 2nd order (blue detector) 105 - 72 µm
• 3rd order (blue detector) 72 - 55 µm
• Groove profile optimized for highest efficiency
  over all 3 orders using PCgrate full EM-code
• Cryogenic torquer motor drive
• Inductosyn angular resolver

Grating efficiency (above) and resolution (below)
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PACS Chopper

• The technical feasibility of the PACS focal
  plane chopper has been shown
• Critical components (pivots, motor, sensor)
  have been predeveloped
• An experimentally verified simulation model
  was established, providing input data for
  optimization and allowing for precise
  chopper control

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Filters/Dichroics

• Used for definition of photometric bands
  (photometer) and for selection of grating orders
  (spectrometer)
• Low-pass, high-pass, band-pass filters and
  dichroic beam splitters in multi-layer mesh
  technology
• Provided through contract with QMW as part of
  SPIRE/PACS technology exchange agreement
• Delivered filters fulfill or exceed transmission
  requirements

• Manufacturing process
• Air/vacuum gap filters use annular metal spacers
• Hot pressed polypropylene filters use dielectric
  spacers

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Instrument Units and Subsystem Responsibilities
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Observing Modes

• Observing modes are combinations of instrument
  modes and satellite pointing modes
• Instrument modes
• dual-band photometry
• single-band photometry
• line spectroscopy
• range spectroscopy
• Pointing modes
• stare/raster/line scan
• with/without nodding

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Dual-Band Photometry

• Both arrays operating
• full spatial sampling in each band
• long-wave array imaging 130-210 µm band
• short-wave array imaging 60-90 or 90-130 µm band
• sub-band selected by filter
• Raw data rate 1.6 Mbit/s (4 times oversampled)
• Standard mode for PACS as prime
  instrument,preferred mode for PACS/SPIRE
  parallel mode
• Observing parameters
• chopper mode (off/on waveform, throw)
• pointing parameters (stare/raster/scannod)
• integration time per pointing

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Single-Band Photometry

• One array operating
• long-wave array imaging 130-210 µm bandor
• short-wave array imaging 60-90 or 90-130 µm band
• Raw data rate 0.3 or 1.3 Mbit/s
• Test mode for PACS as prime instrument
• Fall-back mode for PACS/SPIRE parallel mode
• Observing parameters
• chopper mode (off/on waveform, throw)
• pointing parameters (stare/raster/scannod)
• integration time per pointing

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Line Spectroscopy

• One or two arrays operating
• observation of individual lines
• long-wave array in 105 -210 µm band
• short-wave array in 57-72 or 72-105 µm band
• wavelength in primary band determines wavelength
  in secondary band
• Max. raw data rate 1 or 2 x 1.8Mbit/s
• Observing parameters
• scan width (default 0)
• chopper mode (off/on waveform, throw)
• pointing parameters (stare/raster/scannod)
• integration time per pointing

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Range Spectroscopy

• Two arrays operating
• observation of extended wavelength ranges
• continuous scan (full resolution) or steps (SED
  sampling)
• long-wave array in 105-210 µm band
• short-wave array in 57-72 or 72-105 µm band
• Max. raw data rate 2 x 1.8Mbit/s
• Observing parameters
• start- and end wavelength
• resolution mode
• chopper mode (off/on waveform, throw)
• pointing parameters (stare/raster/scannod)
• integration time per pointing

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Parameters of PACS Instrument Model(Present Best
Estimate)
(a) Values for the photometry modes from 60-90 or
90-130 µm / 130-210 µm, respectively. (b) The
formal transmission of gt1 takes into account the
acceptance solid angle of the
photoconductor light cones / bolometer baffles
which differs from the beam solid angle.
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Background, NEP, Spectroscopic and Photometric
Sensitivity

• Performance requirements (or even goals) met