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PACS IIDR 01/02 Mar 2001 Optical System Design N. Geis MPE Optical System Design 1 PACS IIDR 01/02 Mar 2001 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 IIDR 01/02 Mar 2001 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 IIDR 01/02 Mar 2001 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) • Reduced chopper throw (sky) to allow for larger FOV of bolometers with same entrance field stop / mirror sizes Optical System Design 4 PACS IIDR 01/02 Mar 2001 Optical Design – Top Optics Optical design for calibration sources • Acceptable image quality of pupil • Köhler-type illumination (pupil on source aperture + 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 IIDR 01/02 Mar 2001 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 6 PACS IIDR 01/02 Mar 2001 Telescope Cal. Source 1 C2 Active 3 C1 Active 3 Cal. Source 2 TO Fold 1 C1 Active 1 TO Fold 2 C2 Active 1 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 7 PACS IIDR 01/02 Mar 2001 Overall Optical Design Overall optical arrangement has favorable mechanical layout • clean separation between optical paths (no interpenetrating beams) • better accommodation for mechanical mounts • most mechanisms and sub-units can be mounted close to FPU outer walls for modularity Optical System Design 8 PACS IIDR 01/02 Mar 2001 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 S Fold 3 Slicer Optics S Fold 4 S Active 3 Optical components after Top Optics S Active 4 S Active 5 B Fold B1 B Active R1 B Active B1 B Active R2 B Active B2 Filter Filter Wheel Red Bolometer Array Blue Bolometer Array S Active 6 Dichroic Beamsplitter Filter Filter Wheel S Fold 5 Red Spectrometer Array Optical System Design B Fold R1 Pupil Field Blue Spectrometer Array 9 PACS IIDR 01/02 Mar 2001 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 • 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 unit at FPU external wall Optical System Design 10 PACS IIDR 01/02 Mar 2001 Optical Design – Spectrometers Changes in optical design for spectrometer since ISVR • 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 & alignment gap of 0.75 mm between slit mirrors gap of 3.6 mm between detector blocks for filter holder • Better image quality • Excellent baffling situation • end optics for both spectrometers separated on “ground floor” • exit field stop of spectrometer inside “periscope” • extra pupil and field stops possible in end optics Optical System Design 11 PACS IIDR 01/02 Mar 2001 The Image Slicer Optical System Design 12 PACS IIDR 01/02 Mar 2001 Image Slicer and Grating (in) Slit Mirror Slicer Mirror Capture Mirror Grating Optical System Design 13 PACS IIDR 01/02 Mar 2001 Image Slicer and Grating (in+out) Slit Mirror Periscope Optics Slicer Stack Capture Mirror Grating Optical System Design 14 PACS IIDR 01/02 Mar 2001 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 • Improved image quality in both, photometers, and spectrometers Optical System Design 15 PACS IIDR 01/02 Mar 2001 A Walk Through PACS Optical System Design 16 PACS IIDR 01/02 Mar 2001 PACS Envelope -filled Optical System Design 17 PACS IIDR 01/02 Mar 2001 PACS Functional Groups Optical System Design 18 PACS IIDR 01/02 Mar 2001 PACS Envelope Optical System Design 19 PACS IIDR 01/02 Mar 2001 PACS Envelope + Top Optics Optical System Design 20 PACS IIDR Top Optics Optical System Design 01/02 Mar 2001 Chopper Lyot Stop Telescope Focus 21 PACS IIDR Calibrators Optical System Design 01/02 Mar 2001 Calibrator I+II 22 PACS IIDR 01/02 Mar 2001 Chopping Left Optical System Design 23 PACS IIDR 01/02 Mar 2001 Chopping Right Optical System Design 24 PACS IIDR 01/02 Mar 2001 Entrance Optics + Blue Photometer Dichroic Filter Wheel Blue Bolometer Cryo cooler Optical System Design 25 PACS IIDR 01/02 Mar 2001 Entrance Optics + Blue Photometer Optical System Design 26 PACS IIDR 01/02 Mar 2001 Entrance Optics + Blue Photometer + Red Photometer Dichroic Filter Red Bolometer Optical System Design 27 PACS IIDR 01/02 Mar 2001 Entrance Optics + Blue Photometer + Red Photometer Optical System Design 28 PACS IIDR 01/02 Mar 2001 Common Focus Photometer Unit Dichroic Dichroic Fold Fold Red Blue Bolometer Fold Red Bolometer Blue Common Focus Dichroic Fold Optical System Design Red Blue 29 PACS IIDR 01/02 Mar 2001 The Spectrometer Section Optical System Design 30 PACS IIDR Photometer Optics Blue Bolometer 01/02 Mar 2001 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 31 PACS IIDR 01/02 Mar 2001 Geometrical Optics Performance Optical System Design 32 PACS IIDR 01/02 Mar 2001 Optical Performance - Blue Bolometer Optical System Design 33 PACS IIDR 01/02 Mar 2001 Optical Performance - Geometry Blue Bolometer 3 1 Optical System Design 2 34 PACS IIDR 01/02 Mar 2001 Optical Performance - Red Bolometer Optical System Design 35 PACS IIDR 01/02 Mar 2001 Optical Performance - Geometry Red Bolometer Optical System Design 36 PACS IIDR 01/02 Mar 2001 Optical Performance - Spectrometer Center of Array, center l Optical System Design Corner of Array, extreme l 37 PACS IIDR 01/02 Mar 2001 Optical Performance - Geometry Spectrometer 174.6 µm 175.0µm 175.4µm “ILB” Optical System Design 38 PACS IIDR 01/02 Mar 2001 Diffraction Optical System Design 39 PACS IIDR 01/02 Mar 2001 Illumination of Lyot Stop • • • • M2 is system aperture Image quality of M2 on Lyot stop determined by diffraction from PACS entrance field stop Maximum size of entrance field stop is limited by payload accommodation (M3) and thermal/ stray radiation Diffraction ring ~10% of aperture area GLAD 4.5 diffraction analysis l = 175 µm Intensity (arb. units) 2 Strategies Radius [cm] Optical System Design 1 Use of M2 as system stop (baseline): oversize instrument Lyot stop by ~ 10% area (if only cold sky visible beyond M2 ) 2 Use of Lyot stop as system stop (optional); suppresses diffracted emission/reflection from M2 spider, but we lose 5–10% throughput 40 PACS IIDR 01/02 Mar 2001 Diffraction Analysis - Slicer/Spectrometer Diffraction Analysis of the Spectrometer was repeated with current (pre-freezing) 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 41 PACS IIDR 01/02 Mar 2001 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 42 PACS IIDR 01/02 Mar 2001 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 43 PACS IIDR 01/02 Mar 2001 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 44 PACS IIDR 01/02 Mar 2001 Grating: The worst offender at long wavelength Before Grating Angle of incidence: 60.4° 1.Order Angle of incidence: 46.6° 3.Order Grating Grating 205µm 57µm Grating 80mm x 320mm Width of grating sufficient: minimal loss at 205 µm Y After Grating Angle of incidence: 60.4° 1.Order Angle of incidence: 46.6° 3.Order X Grating Grating Optical System Design 57µm 205µm 45 PACS IIDR 01/02 Mar 2001 Grating: The worst offender at long wavelength Before Grating Angle of incidence: 60.4° 1.Order Angle of incidence: 46.6° 3.Order Collimator Vignetting Grating 57µm Grating 80mm x 320mm Grating 205µm Y-Axis has to be scaled by 1/cos(60.5°) Y-Axis has to be scaled by 1/cos(46.6°) Losses due to length of grating at 205 µm, 57 µm OK Y After Grating Angle of incidence: 46.6° 3.Order X Angle of incidence: 60.4° 1.Order Grating Vignetting Grating Grating 205µm 57µm Y-Axis has to be scaled by 1/cos(46.6°) Optical System Design Y-Axis has to be scaled by 1/cos(60.5°) 46 PACS IIDR 01/02 Mar 2001 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 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 Optical System Design 47