Baffle and Straylight 1

PACS IIDR 01/02 Mar 2001

Baffle and Straylight

D. KampfKAYSER-THREDE

Baffle and Straylight 2

PACS IIDR 01/02 Mar 2001

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

Baffle and Straylight 3

PACS IIDR 01/02 Mar 2001

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)

Baffle and Straylight 4

PACS IIDR 01/02 Mar 2001

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

Baffle and Straylight 5

PACS IIDR 01/02 Mar 2001

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)

Baffle and Straylight 6

PACS IIDR 01/02 Mar 2001

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)

Baffle and Straylight 7

PACS IIDR 01/02 Mar 2001

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

Baffle and Straylight 8

PACS IIDR 01/02 Mar 2001

TOP OPTICS Straylight Visualisation

Baffle and Straylight 9

PACS IIDR 01/02 Mar 2001

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

Baffle and Straylight 10

PACS IIDR 01/02 Mar 2001

Straylight Visualisation

Centre of secondary mirror reflective

Baffle and Straylight 11

PACS IIDR 01/02 Mar 2001

Straylight Visualisation

Centre of secondary mirror reflective

Baffle and Straylight 12

PACS IIDR 01/02 Mar 2001

Straylight Visualisation

Centre of secondary mirror reflective

Baffle and Straylight 13

PACS IIDR 01/02 Mar 2001

Straylight Visualisation

Centre of secondary mirror blocked (13%)

(realized as hole with absorber behind)

Baffle and Straylight 14

PACS IIDR 01/02 Mar 2001

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)

Baffle and Straylight 15

PACS IIDR 01/02 Mar 2001

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