1. PACS IIDR 01/02 Mar 2001 Baffle and Straylight1 D. Kampf KAYSER-THREDE

2. PACS IIDR 01/02 Mar 2001 Baffle and Straylight2 TOP OPTICS MODEL Housing is modelled for use of ASAP Real mirrors –no scatter characteristics –real optical performance with 99% reflectivity –no mirror thickness Housing made of plates –lambertian scatter characteristic TIS=0.1 on all but optical surfaces –no wall thickness

3. PACS IIDR 01/02 Mar 2001 Baffle and Straylight3 TOP OPTICS MODEL Design –entrance compartement with opening through a small aperture stop only –exit compartement to block out of exit pupil radiation –2 level design to increase straylight rejection –splitting to bolometer and spectrometer at the focal plane of the top optics (field stop)

4. PACS IIDR 01/02 Mar 2001 Baffle and Straylight4 TOP OPTICS MODEL Top Optics bottom view with telescope primary mirror and all optical elements (bottom plate not shown) Optical performance demonstrated by Full Field Spot diagram in focal plane

5. PACS IIDR 01/02 Mar 2001 Baffle and Straylight5 TOP OPTICS Analysis –Forward radiation calculation Optical performance Illuminated objects (only visible for off axis sources) –Backward calculation Source near focal plane Critical objects –Definition of straylight paths Use of scatter random and towards object (aperture stop and walls in clear visibility of the aperture stop baffles near the focal plane (this radiation will be blocked after the field stop, because out of pupil position)

6. PACS IIDR 01/02 Mar 2001 Baffle and Straylight6 TOP OPTICS STRAYLIGHT RESULTS 8 different source positions were taken to create off axis radiation (at 294 mm edge around the center hole of the primary mirror A total of 28 E+6 rays were traced (radiation was directed into the top optics) The straylight hitting the focal plane (all possible angles, worst case assumption) was reduced to better than 1 E-8 These calculations were taken to position the baffles near the last two mirrors (FOLD 4 & 5)

7. PACS IIDR 01/02 Mar 2001 Baffle and Straylight7 TOP OPTICS Straylight Demonstration 28 E+6 scattered rays 8 sources around the central hole (edge 294 mm) 235 rays hitting focal plane (all possible angles) Cutoff at 1E-10 25 max reflections 235 rays with 4E-6 intensity out of 1300

8. PACS IIDR 01/02 Mar 2001 Baffle and Straylight8 TOP OPTICS Straylight Visualisation

9. PACS IIDR 01/02 Mar 2001 Baffle and Straylight9 STRAYLIGHT Backward calculation showed problematic with the central hole at the primary and secondary mirror Assumed hole size of primary: 7.8% from diameter (294 mm) result for blockage of secondary mirror was: 13% central blockage is necessary to block radiation from the rear side of the Primary reflected to secondary and then into the instrument

10. PACS IIDR 01/02 Mar 2001 Baffle and Straylight10 Straylight Visualisation Centre of secondary mirror reflective

11. PACS IIDR 01/02 Mar 2001 Baffle and Straylight11 Straylight Visualisation Centre of secondary mirror reflective

12. PACS IIDR 01/02 Mar 2001 Baffle and Straylight12 Straylight Visualisation Centre of secondary mirror reflective

13. PACS IIDR 01/02 Mar 2001 Baffle and Straylight13 Straylight Visualisation Centre of secondary mirror blocked (13%) (realized as hole with absorber behind)

14. PACS IIDR 01/02 Mar 2001 Baffle and Straylight14 Straylight Future –Tasks for straylight reduction due to telescope design Holes Baffles inside hole Rear side of the primary mirror Baffle design of the cryostat Surface of instruments –Straylight from instrument „crosstalk“ Straylight from edges near FOV Backscatter from „hot“ sources (highly reflective objects near OB)

15. PACS IIDR 01/02 Mar 2001 Baffle and Straylight15 STRAYLIGHT Future –Analysis scatter behaviour of mirrors due to roughness and particulate contamination (Harvey Shack model), necessary to trace scattered rays in FOV (detectable rays) only partly coated instrument (scatter behaviour, weight reduction) internal source positions at illuminated objects (ghost) –ESA request: ASAP model to be integrated into ESA`s APARTH straylight model