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PACS IBDR 27/28 Feb 2002 Optical System Design N. Geis MPE Optical System Design 1 PACS IBDR 27/28 Feb 2002 Pacs Optical Telescope System Overview Entrance Optics -- chopper -- calibration optics Bolometer Spectrometer To Slicer Field splitter Bolometer Optics Image Slicer Grating Spectrometer Dichroic Anamorphic System Dichroic Bolometer Optics Bolometer Optics Filter Filter Wheel Filter Filter Wheel Red Bolometer Array Blue Bolometer Array Red Photoconductor Array Blue Photoconductor Array Optical System Design 2 PACS IBDR 27/28 Feb 2002 Definition of Image Scale Subsystem Spectrometer Pixel Pitch on Sky (Physical) 9.4 arcsec (3.6 mm) 3.2 arcsec Photometer (60–130 µm) Photometer (130-210 µm) Optical System Design Field-of-View (0.75 mm) 6.4 arcsec (0.75 mm) 47 x 47 arcsec2 214 x 106 arcsec2 211 x 102 arcsec2 3 PACS IBDR 27/28 Feb 2002 Optical Design – Top Optics Optical design for astronomical optical path • Image inverter (3 flats) at the beginning to compensate for telescope image tilt • Chopper assembly on outer side of FPU (servicing) • Labyrinth configuration for baffling (see straylight analysis) • Chopper throw (on sky) reduced to 1 full array size to allow for larger FOV of bolometers with same entrance fieldstop/mirror sizes as previous design. Optical System Design 4 PACS IBDR 27/28 Feb 2002 Optical Design – Top Optics Optical design for calibration sources • Acceptable image quality of pupil • Köhler-type illumination (pupil on source aperture + a field stop) • Source aperture is projected onto M2/Cold Stop • No physical match in source for “field” stop => excellent uniformity expected • Re-use of existing entrance optics mirrors in reverse • Excellent baffling situation • Sources are outside of Instrument Cold Stop • Initial calibration path & field stop outside of Instrument Cold Stop Optical System Design 5 PACS IBDR 27/28 Feb 2002 Uniformity of Illumination by Calibrators The two sources produce mirrored illumination distributions, as seen from the detectors Maximum (unwanted) modulation of the calibration signal by non-uniformity is ~ 5% Compatible with the goal of having relative signal changes of 10% when chopping. E.g., one could set operating points such that the range of signal is 7.5– 12.5% when chopping. Optical System Design 6 PACS IBDR 27/28 Feb 2002 Telescope TO Fold 1 TO Fold 2 TO Fold 3 TO Active 1 Lyot Stop TO Active 2 TO Active 3 Top Optics Astronomical TO Fold 4 Pupil Field Chopper TO Active 4 TO Active 5 Optical System Design Common Focus, Top Optics 7 PACS IBDR 27/28 Feb 2002 Telescope Cal. Source 1 C2 Active 3 C1 Active 3 Cal. Source 2 TO Fold 1 C1 Active 1 (Lens) TO Fold 2 C2 Active 1 (Lens) C1 Active 2 TO Fold 3 C2 Active 2 TO Active 1 Calibrator 1 Calibrator 2 Lyot Stop TO Active 2 TO Active 3 Top Optics Calibration TO Fold 4 Pupil Field Chopper TO Active 4 TO Active 5 Optical System Design Common Focus, Top Optics 8 PACS IBDR 27/28 Feb 2002 Common Focus Top Optics Spectrometer S Collimator 1 S Collimator 1 S Collimator 2 S Collimator 2 Photometer B Active 1 S Fold 1 Dichroic Beamsplitter S Active 1 S Active 2 Grating S Fold 2 B Fold R1 B Fold B1 S Fold 3 B Active R1 B Active B1 S Fold 4 B Active R2 B Active B2 S Active 3 Filter Filter Wheel Red Bolometer Array Blue Bolometer Array Slicer Optics Optical components after Top Optics S Active 4 S Active 5 S Active 6 Dichroic Beamsplitter Filter S Fold 5 Red Spectrometer Array Optical System Design Filter Wheel Pupil Field Blue Spectrometer Array 9 PACS IBDR 27/28 Feb 2002 Optical Design – Photometers Optical design for bolometer cameras finished • very good image quality • good geometry • excellent baffling situation • fully separate end trains • extra pupil and field stops possible on the way to detectors (use for alignment and baffling purposes) • exit pupil with filter at entrance window to cold (1.8K) detector housing • Bolometer arrays mounted close together on top of cryocooler • Photometers are a self-contained compact unit at FPU external wall Optical System Design 10 PACS IBDR 27/28 Feb 2002 Optical Design – Spectrometers No Changes in optical design for spectrometer since IIDR • ILB column Slicer output was reconfigured such that one pixel’s worth of space is intentionally left blank between slices at the slit focus and on the detector array • Reduces (diffraction-) cross-talk • helps with assembly of detector filters & alignment gap of 0.75 mm between slit mirrors gap of 3.6 mm between detector blocks for filter holder • Image quality diffraction limited • Excellent baffling situation • end optics for both spectrometers separated on “ground floor” • exit field stop of spectrometer inside a “periscope” • extra pupil and field stops possible in end optics (alignment, baffles) Optical System Design 11 PACS IBDR 27/28 Feb 2002 The Image Slicer Optical System Design 12 PACS IBDR 27/28 Feb 2002 Image Slicer and Grating (in) Slit Mirror Slicer Mirror Capture Mirror Grating Optical System Design 13 PACS IBDR 27/28 Feb 2002 Image Slicer and Grating (in+out) Slit Mirror Periscope Optics Slicer Stack Capture Mirror Grating Optical System Design 14 PACS IBDR 27/28 Feb 2002 Optical Design Summary • Clean separation between optical paths – a result of the incorporation of the bolometers. • Realistic accommodation for mechanical mounts. • Significant savings in number of mirrors from the photoconductor-only design • Excellent image quality in both, photometers, and spectrometers Optical System Design 15 PACS IBDR PACS Envelope -filled Optical System Design 27/28 Feb 2002 16 PACS IBDR 27/28 Feb 2002 PACS Optical Functional Groups Optical System Design 17 PACS IBDR Photometer Optics Blue Bolometer 27/28 Feb 2002 Filter Wheel I Slicer Optics 0.3 K Cooler Red Bolometer Grating Grating Drive Encoder sGeGaDetector Red Spectrometer Spectrometer Optics Chopper Calibrator I and II Calibrator Optics Entrance Optics Optical System Design sGeGa Detector Blue Spectrometer Filter Wheel II 18 PACS IBDR Entrance Optics & Photometer 27/28 Feb 2002 Chopper Lyot Stop Telescope Focus Dichroic Calibrator I+II Filter Wheel Blue Bolometer Cryo cooler Red Bolometer Optical System Design 19 PACS IBDR 27/28 Feb 2002 Chopping Left Optical System Design 20 PACS IBDR 27/28 Feb 2002 Chopping Right Optical System Design 21 PACS IBDR 27/28 Feb 2002 The Spectrometer Section Optical System Design 22 PACS IBDR 27/28 Feb 2002 PACS Filter Scheme Optical System Design 23 PACS IBDR 27/28 Feb 2002 Filter Rejection Requirements (determined from template observation scenarios) The requirements from 3 demanding astronomical scenarios... ...lead to the required filter suppression factors. Solid red line: total required suppression Suppression factor • planet with high albedo • deep imaging (Galactic/extragalactic) • FIR excess around bright star (bolometers only) detector response filter transmission overall response Dashed blue line: model detector responsivity Dotted green line: resulting required filter suppression factor Optical System Design Wavelength [µm] 24 PACS IBDR 27/28 Feb 2002 PACS Filters • Filter Functions – definition of spectral bands • photometric bands • order sorting for spectrometer grating – in-band transmission (high) – out-of-band suppression (thermal background, straylight, astronomical) • Filter implementation – Filter types (low-pass, high-pass, band-pass, dichroic) – Technology: Metal mesh filters developed at QMW • Proven technology • Robust • Excellent Performance – Filter location in optical path chosen for • rejection of thermal radiation from satellite • instrument stray light management Optical System Design 25 PACS IBDR 27/28 Feb 2002 PACS Filtering Scheme Optical System Design 26 PACS IBDR 27/28 Feb 2002 Example: Prototype of Long Pass Edge filter Examples of QMW filters Examples of QMW filters Optical System Design 27 PACS IBDR 27/28 Feb 2002 Example Filter Chain: Long-Wavelength Photometer Dichroic beam splitter 130.µm Long-pass edge filters 52 µm 110 µm 125 µm . . . Short-pass edge filter 210.µm Optical System Design 28 PACS IBDR 27/28 Feb 2002 Filter Summary • Filter scheme with 4 or 5 filters in series in each instrument channel provides sufficient out-of-band suppression • Measured/expected in-band transmission – > 80 % for long-pass and dichroic filters – ~ 80 % for band-pass filters > 40 % for filter combination – ~ 50 % expected • Requirements will be met Optical System Design 29 PACS IBDR 27/28 Feb 2002 Geometrical Optics Performance Optical System Design 30 PACS IBDR 27/28 Feb 2002 Optical Performance - Blue Bolometer Optical System Design 31 PACS IBDR 27/28 Feb 2002 Optical Performance - Geometry Blue Bolometer 3 1 Optical System Design 2 32 PACS IBDR 27/28 Feb 2002 Optical Performance - Red Bolometer Optical System Design 33 PACS IBDR 27/28 Feb 2002 Optical Performance - Geometry Red Bolometer Optical System Design 34 PACS IBDR 27/28 Feb 2002 Optical Performance - Spectrometer Center of Array, center l Optical System Design Corner of Array, extreme l 35 PACS IBDR 27/28 Feb 2002 Optical Performance - Geometry Spectrometer 174.6 µm 175.0µm 175.4µm 75% Strehl @ 80 µm Optical System Design 90% Strehl @ 80 µm “ILB” 36 PACS IBDR 27/28 Feb 2002 PACS Optical Performance in a System Context New Req? New Goal? Optical System Design 37 PACS IBDR 27/28 Feb 2002 Diffraction Optical System Design 38 PACS IBDR 27/28 Feb 2002 Illumination of Lyot Stop • M2 is system aperture • Image quality of M2 on Lyot stop determined by diffraction from PACS entrance field stop • Diffraction ring ~10% of aperture area • Cannot block “Narcissus effects” from M2 center at Lyot stop without throughput loss 2 Strategies 1 Intensity (arb. units) GLAD 4.5 diffraction analysis l = 175 µm depending on outcome of system straylight analysis 2 Radius [cm] Optical System Design M2 as system stop (baseline): oversize cold stop by ~ 10% area (if only cold sky visible beyond M2, and straylight analysis allows) Lyot stop as system stop (optional): undersize cold stop by ~ 10% area — throughput loss (if diffracted emission/reflection from M2 spider, M2 edge, or straylight is problematic) 39 PACS IBDR 27/28 Feb 2002 Diffraction Analysis - Slicer/Spectrometer Diffraction Analysis of the Spectrometer repeated with final mirror dimensions and focal lengths, and for a larger range of wavelengths. The results were used • as inputs to a detailed grating size specification • for optimizing mirror sizes in the spectrometer path => Diffraction on the image slicer leads to considerable deviations from the geometrical footprint on the grating at all wavelengths Optical System Design 40 PACS IBDR 27/28 Feb 2002 Diffraction Gallery at 175 µm telescope focus, re-imaged “slice” through point spread function entrance slit field mirror capture mirror Detector array pixel grating Optical System Design 41 PACS IBDR 27/28 Feb 2002 Grating: The worst offender at long wavelength • Considerable difference from geometrical optics footprint. • No noticeable spillover problem at short wavelength • Non-uniform illumination profile will lead to change in effective grating resolution => calculate/measure Optical System Design 42 PACS IBDR 27/28 Feb 2002 Grating: The worst offender at long wavelength • Major difference from geometrical optics footprint. • Spillover of ~ 20% energy past grating & collimators at longest wavelength • Non-uniform illumination profile will lead to change in effective grating resolution => calculate/measure Optical System Design 43 PACS IBDR 27/28 Feb 2002 Diffractive Walk-Off Off-axis pixel diffraction throughput For edge pixels, and long wavelength, asymmetric diffraction losses move the PSF peak ~ 0.3 pixel (3’’) from its expected spatial position. Image scale on the sky for the spectrometer depends on wavelength Effect needs to be fully characterized for astrometry/mapping. Optical System Design 44 PACS IBDR 27/28 Feb 2002 Diffraction Summary System stop should be M2 - oversize PACS cold stop accordingly Diffraction lobes introduced by slicer mirrors can still be transferred through most of the spectrometer optics (i.e., image quality is intact) Considerable clipping occurs on collimator mirrors and grating at long wavelength Losses due to “spill-over”: up to 20% (205 µm), 15% (175 µm) other wavelengths tbd. 80% “diffraction transmission” to detector for central pixel Diffraction induced “chromatic aberration” needs further study Optical System Design 45