Optical System Design

1. Optical System Design N. Geis MPE Optical System Design

2. Telescope Entrance Optics -- chopper -- calibration optics Bolometer Spectrometer Field splitter To Slicer 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 Pacs Optical System Overview Optical System Design

3. Definition of Image Scale Optical System Design

4. 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

5. 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

6. Telescope TO Fold 1 TO Fold 2 TO Fold 3 TO Active 1 Lyot Stop TO Active 2 TO Active 3 Pupil Field TO Fold 4 Chopper TO Active 4 TO Active 5 Common Focus, Top Optics Top OpticsAstronomical Optical System Design

7. Telescope C2 Active 3 C1 Active 3 Cal. Source 1 Cal. Source 2 TO Fold 1 C1 Active 1 C2 Active 1 TO Fold 2 C1 Active 2 C2 Active 2 TO Fold 3 TO Active 1 Calibrator 1 Calibrator 2 Lyot Stop TO Active 2 TO Active 3 Pupil Field TO Fold 4 Chopper TO Active 4 TO Active 5 Common Focus, Top Optics Top OpticsCalibration Optical System Design

8. 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

9. Common Focus Top Optics Spectrometer S Collimator 1 S Collimator 1 B Active 1 S Fold 1 S Collimator 2 S Collimator 2 Dichroic Beamsplitter S Active 1 Grating S Active 2 S Fold 2 B Fold R1 B Fold B1 S Fold 3 Slicer Optics B Active R1 B Active B1 S Fold 4 B Active R2 B Active B2 S Active 3 Filter Filter Wheel S Active 4 Pupil Red Bolometer Array Blue Bolometer Array S Active 5 Field S Active 6 Dichroic Beamsplitter Filter Filter Wheel S Fold 5 Red Spectrometer Array Blue Spectrometer Array Optical componentsafter Top Optics Photometer Optical System Design

10. 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

11. 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

12. The Image Slicer Optical System Design

13. Image Slicer and Grating (in) Slit Mirror Slicer Mirror Capture Mirror Grating Optical System Design

14. Image Slicer and Grating (in+out) Slit Mirror Periscope Optics Capture Mirror Slicer Stack Grating Optical System Design

15. 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

16. A Walk Through PACS Optical System Design

17. PACS Envelope -filled Optical System Design

18. PACS Functional Groups Optical System Design

19. PACS Envelope Optical System Design

20. PACS Envelope + Top Optics Optical System Design

21. Top Optics Chopper Lyot Stop Telescope Focus Optical System Design

22. Calibrators Calibrator I+II Optical System Design

23. Chopping Left Optical System Design

24. Chopping Right Optical System Design

25. Entrance Optics + Blue Photometer Dichroic FilterWheel Blue Bolometer Cryocooler Optical System Design

26. Entrance Optics + Blue Photometer Optical System Design

27. Entrance Optics + Blue Photometer + Red Photometer Dichroic Filter Red Bolometer Optical System Design

28. Entrance Optics + Blue Photometer + Red Photometer Optical System Design

29. Common Focus Photometer Unit Dichroic Dichroic Fold Fold Red Blue Bolometer Red Blue Bolometer Fold Common Focus Dichroic Fold Red Blue Optical System Design

30. The Spectrometer Section Optical System Design

31. 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 Calibrator Optics sGeGa Detector Blue Spectrometer Filter Wheel II Entrance Optics Optical System Design

32. Geometrical Optics Performance Optical System Design

33. Optical Performance - Blue Bolometer Optical System Design

34. Optical Performance - Geometry Blue Bolometer 3 2 1 Optical System Design

35. Optical Performance - Red Bolometer Optical System Design

36. Optical Performance - Geometry Red Bolometer Optical System Design

37. Optical Performance - Spectrometer Center of Array, center l Corner of Array, extreme l Optical System Design

38. Optical Performance - Geometry Spectrometer 174.6 µm 175.0µm 175.4µm “ILB” Optical System Design

39. Diffraction Optical System Design

40. 2 Strategies • Use of M2 as system stop (baseline): oversize instrument Lyot stop by ~ 10% area (if only cold sky visible beyond M2 ) • Use of Lyot stop as system stop (optional); suppresses diffracted emission/reflection from M2 spider, but we lose 5–10% throughput Intensity (arb. units) Radius [cm] 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 Optical System Design

41. 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

42. 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

43. Grating: The worst offenderat 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

44. Grating: The worst offenderat 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

45. Before Grating Angle of incidence: Angle of incidence: 60.4° 46.6° 1.Order 3.Order Grating Grating 205µm 57µm Width of grating sufficient: minimal loss at 205 µm After Grating Angle of incidence: Angle of incidence: 60.4° 46.6° 1.Order 3.Order Grating 205µm Grating 57µm Grating: The worst offenderat long wavelength Grating 80mm x 320mm Y X Optical System Design

46. Before Grating Angle of incidence: Angle of incidence: 60.4° 46.6° 1.Order 3.Order CollimatorVignetting Grating Grating 205µm 57µm Grating Y-Axis has to be Y-Axis has to be 80mm x 320mm scaled by 1/cos(60.5°) scaled by 1/cos(46.6°) Losses due to length of grating at 205 µm, 57 µm OK Y After Grating X Angle of incidence: Angle of incidence: 46.6° 60.4° 3.Order 1.Order GratingVignetting Grating Grating 205µm 57µm Y-Axis has to be Y-Axis has to be scaled by 1/cos(60.5°) scaled by 1/cos(46.6°) Grating: The worst offenderat long wavelength Optical System Design

47. 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