1. PACS IIDR 01/02 Mar 2001 Optical System Design1 N. Geis MPE

2. PACS IIDR 01/02 Mar 2001 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 IIDR 01/02 Mar 2001 Optical System Design3 Definition of Image Scale

4. PACS IIDR 01/02 Mar 2001 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) Reduced chopper throw (sky) to allow for larger FOV of bolometers with same entrance field stop / mirror sizes Optical Design – Top Optics

5. PACS IIDR 01/02 Mar 2001 Optical System Design5 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 Design – Top Optics

6. PACS IIDR 01/02 Mar 2001 Optical System Design6 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

7. PACS IIDR 01/02 Mar 2001 Optical System Design7 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 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 Cal. Source 2 Pupil Field Calibrator 2Calibrator 1

8. PACS IIDR 01/02 Mar 2001 Optical System Design8 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 Overall Optical Design

9. PACS IIDR 01/02 Mar 2001 Optical System Design9 Optical components after Top Optics Photometer

10. PACS IIDR 01/02 Mar 2001 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 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 Design – Photometers

11. PACS IIDR 01/02 Mar 2001 Optical System Design11 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 Design – Spectrometers

12. PACS IIDR 01/02 Mar 2001 Optical System Design12 The Image Slicer

13. PACS IIDR 01/02 Mar 2001 Optical System Design13 Image Slicer and Grating (in) Slicer MirrorCapture Mirror Slit Mirror Grating

14. PACS IIDR 01/02 Mar 2001 Optical System Design14 Image Slicer and Grating (in+out) Slicer Stack Capture Mirror Slit Mirror Grating Periscope Optics

15. PACS IIDR 01/02 Mar 2001 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 Improved image quality in both, photometers, and spectrometers Optical Design Summary

16. PACS IIDR 01/02 Mar 2001 Optical System Design16 A Walk Through PACS

17. PACS IIDR 01/02 Mar 2001 Optical System Design17 PACS Envelope -filled

18. PACS IIDR 01/02 Mar 2001 Optical System Design18 PACS Functional Groups

19. PACS IIDR 01/02 Mar 2001 Optical System Design19 PACS Envelope

20. PACS IIDR 01/02 Mar 2001 Optical System Design20 PACS Envelope + Top Optics

21. PACS IIDR 01/02 Mar 2001 Optical System Design21 Top Optics Chopper Telescope Focus Lyot Stop

22. PACS IIDR 01/02 Mar 2001 Optical System Design22 Calibrators Calibrator I+II

23. PACS IIDR 01/02 Mar 2001 Optical System Design23 Chopping Left

24. PACS IIDR 01/02 Mar 2001 Optical System Design24 Chopping Right

25. PACS IIDR 01/02 Mar 2001 Optical System Design25 Entrance Optics + Blue Photometer Dichroic Filter Wheel Blue Bolometer Cryo cooler

26. PACS IIDR 01/02 Mar 2001 Optical System Design26 Entrance Optics + Blue Photometer

27. PACS IIDR 01/02 Mar 2001 Optical System Design27 Entrance Optics + Blue Photometer + Red Photometer Dichroic Red Bolometer Filter

28. PACS IIDR 01/02 Mar 2001 Optical System Design28 Entrance Optics + Blue Photometer + Red Photometer

29. PACS IIDR 01/02 Mar 2001 Optical System Design29 Photometer Unit Common Focus Dichroic Red Dichroic Blue Bolometer Common Focus Fold RedBlue Red Blue Bolometer Dichroic

30. PACS IIDR 01/02 Mar 2001 Optical System Design30 The Spectrometer Section

31. PACS IIDR 01/02 Mar 2001 Optical System Design31 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

32. PACS IIDR 01/02 Mar 2001 Optical System Design32 Geometrical Optics Performance

33. PACS IIDR 01/02 Mar 2001 Optical System Design33 Optical Performance - Blue Bolometer

34. PACS IIDR 01/02 Mar 2001 Optical System Design34 Optical Performance - Geometry Blue Bolometer 1 2 3

35. PACS IIDR 01/02 Mar 2001 Optical System Design35 Optical Performance - Red Bolometer

36. PACS IIDR 01/02 Mar 2001 Optical System Design36 Optical Performance - Geometry Red Bolometer

37. PACS IIDR 01/02 Mar 2001 Optical System Design37 Optical Performance - Spectrometer Center of Array, center Corner of Array, extreme

38. PACS IIDR 01/02 Mar 2001 Optical System Design38 Optical Performance - Geometry Spectrometer “ILB” 175.0µm 175.4µm 174.6 µm

39. PACS IIDR 01/02 Mar 2001 Optical System Design39 Diffraction

40. PACS IIDR 01/02 Mar 2001 Optical System Design40 Illumination of Lyot Stop 2 Strategies 1Use of M2 as system stop (baseline): oversize instrument Lyot stop by ~ 10% area (if only cold sky visible beyond M2 ) 2Use of Lyot stop as system stop (optional); suppresses diffracted emission/reflection from M2 spider, but we lose 5–10% throughput 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 Maximum size of entrance field stop is limited by payload accommodation (M3) and thermal/ stray radiation Diffraction ring ~10% of aperture area

41. PACS IIDR 01/02 Mar 2001 Optical System Design41 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

42. PACS IIDR 01/02 Mar 2001 Optical System Design42 Diffraction Gallery at 175 µm telescope focus, re-imaged“slice” through point spread function capture mirror entrance slit field mirror grating pixel Detector array

43. PACS IIDR 01/02 Mar 2001 Optical System Design43 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

44. PACS IIDR 01/02 Mar 2001 Optical System Design44 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

45. PACS IIDR 01/02 Mar 2001 Optical System Design45 Grating: The worst offender at long wavelength Grating 80mm x 320mm X Y

46. PACS IIDR 01/02 Mar 2001 Optical System Design46 57µm Grating Y-Axis has to be scaled by 1/cos(46.6°) Y-Axis has to be scaled by 1/cos(46.6°) Y-Axis has to be scaled by 1/cos(60.5°) Y-Axis has to be scaled by 1/cos(60.5°) Angle of incidence: 46.6° 3.Order Angle of incidence: 60.4° 1.Order Grating 205µm Angle of incidence: 60.4° 1.Order 57µm Grating Angle of incidence: 46.6° 3.Order Losses due to length of grating at 205 µm, 57 µm OK Collimator Vignetting Grating Vignetting Grating: The worst offender at long wavelength

47. PACS IIDR 01/02 Mar 2001 Optical System Design47 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 Diffraction Summary

48. PACS IIDR 01/02 Mar 2001 Optical System Design48 Future Work Uniformity of Illumination by Calibrators

49. PACS IIDR 01/02 Mar 2001 Optical System Design49 Future Work Examine off-axis pixel diffraction throughput –preliminary result: for an edge pixel, diffraction moves PSF peak ~ 0.2 pixel from expected spatial position for long wavelength