1. PACS IBDR 27/28 Feb 2002 Optical System Design1 N. Geis MPE

2. PACS IBDR 27/28 Feb 2002 Optical System Design2 Pacs Optical System Overview Anamorphic System Grating Spectrometer Telescope Entrance Optics -- chopper -- calibration optics Field splitter Spectrometer Image Slicer To Slicer Bolometer Optics Dichroic Bolometer Red Bolometer Array Blue Bolometer Array FilterFilter WheelFilterFilter Wheel Red Photoconductor Array Blue Photoconductor Array Bolometer Optics Dichroic

3. PACS IBDR 27/28 Feb 2002 Optical System Design3 Definition of Image Scale

4. PACS IBDR 27/28 Feb 2002 Optical System Design4 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 field- stop/mirror sizes as previous design. Optical Design – Top Optics

5. PACS IBDR 27/28 Feb 2002 Optical System Design5 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 Design – Top Optics

6. PACS IBDR 27/28 Feb 2002 Optical System Design6 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.

7. PACS IBDR 27/28 Feb 2002 Optical System Design7 Top Optics Astronomical Common Focus, Top Optics TO Active 5 TO Active 4 Chopper TO Fold 4 TO Active 3 TO Active 2 Lyot Stop TO Active 1 TO Fold 3 TO Fold 2 TO Fold 1 Telescope Pupil Field

8. PACS IBDR 27/28 Feb 2002 Optical System Design8 Top Optics Calibration TO Fold 1 TO Active 1 TO Fold 3 TO Fold 2 Common Focus, Top Optics TO Active 5 C2 Active 3 C1 Active 3 C1 Active 2 C1 Active 1 (Lens) C2 Active 2 TO Active 4 Chopper TO Fold 4 TO Active 3 Cal. Source 1 TO Active 2 Lyot Stop Telescope C2 Active 1 (Lens) Cal. Source 2 Pupil Field Calibrator 2Calibrator 1

9. PACS IBDR 27/28 Feb 2002 Optical System Design9 Optical components after Top Optics Photometer

10. PACS IBDR 27/28 Feb 2002 Optical System Design10 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 Design – Photometers

11. PACS IBDR 27/28 Feb 2002 Optical System Design11 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 Design – Spectrometers

12. PACS IBDR 27/28 Feb 2002 Optical System Design12 The Image Slicer

13. PACS IBDR 27/28 Feb 2002 Optical System Design13 Image Slicer and Grating (in) Slicer MirrorCapture Mirror Slit Mirror Grating

14. PACS IBDR 27/28 Feb 2002 Optical System Design14 Image Slicer and Grating (in+out) Slicer Stack Capture Mirror Slit Mirror Grating Periscope Optics

15. PACS IBDR 27/28 Feb 2002 Optical System Design15 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 Design Summary

16. PACS IBDR 27/28 Feb 2002 Optical System Design16 PACS Envelope -filled

17. PACS IBDR 27/28 Feb 2002 Optical System Design17 PACS Optical Functional Groups

18. PACS IBDR 27/28 Feb 2002 Optical System Design18 Chopper sGeGaDetector Red Spectrometer Blue Bolometer Red Bolometer Calibrator I and II 0.3 K Cooler Filter Wheel I Filter Wheel II Grating sGeGa Detector Blue Spectrometer Encoder Grating Drive Entrance Optics Photometer Optics Calibrator Optics Slicer Optics Spectrometer Optics

19. PACS IBDR 27/28 Feb 2002 Optical System Design19 Dichroic Filter Wheel Blue Bolometer Cryo cooler Red Bolometer Chopper Telescope Focus Lyot Stop Calibrator I+II Entrance Optics & Photometer

20. PACS IBDR 27/28 Feb 2002 Optical System Design20 Chopping Left

21. PACS IBDR 27/28 Feb 2002 Optical System Design21 Chopping Right

22. PACS IBDR 27/28 Feb 2002 Optical System Design22 The Spectrometer Section

23. PACS IBDR 27/28 Feb 2002 Optical System Design23 PACS Filter Scheme

24. PACS IBDR 27/28 Feb 2002 Optical System Design24 Filter Rejection Requirements (determined from template observation scenarios) The requirements from 3 demanding astronomical scenarios... planet with high albedo deep imaging (Galactic/extragalactic) FIR excess around bright star...lead to the required filter suppression factors. Solid red line: total required suppression Dashed blue line: model detector responsivity Suppression factor Dotted green line: resulting required filter suppression factor Wavelength [µm] detector response filter transmission overall response (bolometers only)

25. PACS IBDR 27/28 Feb 2002 Optical System Design25 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

26. PACS IBDR 27/28 Feb 2002 Optical System Design26 PACS Filtering Scheme

27. PACS IBDR 27/28 Feb 2002 Optical System Design27 Example: Prototype of Long Pass Edge filter Examples of QMW filters

28. PACS IBDR 27/28 Feb 2002 Optical System Design28 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

29. PACS IBDR 27/28 Feb 2002 Optical System Design29 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

30. PACS IBDR 27/28 Feb 2002 Optical System Design30 Geometrical Optics Performance

31. PACS IBDR 27/28 Feb 2002 Optical System Design31 Optical Performance - Blue Bolometer

32. PACS IBDR 27/28 Feb 2002 Optical System Design32 Optical Performance - Geometry Blue Bolometer 1 2 3

33. PACS IBDR 27/28 Feb 2002 Optical System Design33 Optical Performance - Red Bolometer

34. PACS IBDR 27/28 Feb 2002 Optical System Design34 Optical Performance - Geometry Red Bolometer

35. PACS IBDR 27/28 Feb 2002 Optical System Design35 Optical Performance - Spectrometer Center of Array, center Corner of Array, extreme

36. PACS IBDR 27/28 Feb 2002 Optical System Design36 Optical Performance - Geometry Spectrometer “ILB” 175.0µm 175.4µm 174.6 µm 90% Strehl @ 80 µm75% Strehl @ 80 µm

37. PACS IBDR 27/28 Feb 2002 Optical System Design37 PACS Optical Performance in a System Context New Goal? New Req?

38. PACS IBDR 27/28 Feb 2002 Optical System Design38 Diffraction

39. PACS IBDR 27/28 Feb 2002 Optical System Design39 Illumination of Lyot Stop 2 Strategies depending on outcome of system straylight analysis 1M2 as system stop (baseline): oversize cold stop by ~ 10% area ( if only cold sky visible beyond M2, and straylight analysis allows ) 2Lyot 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) GLAD 4.5 diffraction analysis = 175 µm Radius [cm] Intensity (arb. units) 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

40. PACS IBDR 27/28 Feb 2002 Optical System Design40 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

41. PACS IBDR 27/28 Feb 2002 Optical System Design41 Diffraction Gallery at 175 µm telescope focus, re-imaged“slice” through point spread function capture mirror entrance slit field mirror grating pixel Detector array

42. PACS IBDR 27/28 Feb 2002 Optical System Design42 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 Grating: The worst offender at long wavelength

43. PACS IBDR 27/28 Feb 2002 Optical System Design43 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 Grating: The worst offender at long wavelength

44. PACS IBDR 27/28 Feb 2002 Optical System Design44 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.

45. PACS IBDR 27/28 Feb 2002 Optical System Design45 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 Diffraction Summary