PACS IHDR 12/13 Nov 2003 Filters1 N. Geis MPE

PACS IHDR 12/13 Nov 2003 Filters2 PACS Filter Scheme Filter scheme with 5 or 6 filters in series in each instrument channel to provide sufficient out-of-band suppression Specified in-band transmission – > 80 % for long-pass and dichroic filters – ~ 80 % for band-pass filters  > 40 % for each filter stack – ~ 50 % expected Out-of band suppression ~ 99% per filter

PACS IHDR 12/13 Nov 2003 Filters3 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

PACS IHDR 12/13 Nov 2003 Filters4 Filter Types Examples of QMW filters This style used in Bolometers and Ge:Ga detectors This style used everywhere else

PACS IHDR 12/13 Nov 2003 Filters5 Filter Curves - Typical Blocking Filters

PACS IHDR 12/13 Nov 2003 Filters6 Filter Curves - Dichroic Spectrometer Shortest wavelength band close to edge of workable dichroic range

PACS IHDR 12/13 Nov 2003 Filters7 Filter Curves - Dichroic Photometer Shortest wavelength band badly affected by resonant effect --> fundamental problem

PACS IHDR 12/13 Nov 2003 Filters8 PACS Filter Status 18 of 26 (single) filters for CQM received Most filters exceed in-band design specs initial surface cleanliness problem solved 2 bolometer filters rejected for FM/FS but to be “used as is” for CQM –Embedded contamination (large items) shadow bolometers significantly (see pictures next page)

PACS IHDR 12/13 Nov 2003 Filters9 Vignetting from Contamination in 2 Bolometer Filters “insect” placed at center red bolometer 1 “streak” Blue bolometer array “Fibre blob” Defects not at real position acceptable: few % unacceptable: 35 %

PACS IHDR 12/13 Nov 2003 Filters10 PACS Filter Status Dichroics are worse than expected: –reflected wavelength range less extended than prediction (resonant damping at 1st overtone of edge wavelength) –suppression of long wavelength in reflection only 95% on average ==> probably a principal problem of QMW dichroic technology, i.e., can not be fixed for FM

PACS IHDR 12/13 Nov 2003 Filters11 OGSE Filters

PACS IHDR 12/13 Nov 2003 Filters12 OGSE Filter Status Room temperature radiation from outside has to be attenuated to ~ telescope levels when using external sources. Attenuation of a few 100 to be achieved by metal-on- mylar filters. Needed attenuation values achieved on prototype filters But...

PACS IHDR 12/13 Nov 2003 Filters13 OGSE Filter Problems Room temperature radiation from outside likely prevents proper cooldown of filters –Low thermal conduction (mylar, inconel stainless) –high absorptivity ( %) Filter predicted temperatures above LN2 temperature, at emissivity ~ 30% ==> remaining background levels factor ~10 too high Need additional cold 10um-absorbing or reflecting filter: -- cryst. Quartz (+) Availability(-) Redesign, ghost images, factor 3 -- QMC mesh filter (+) factor 1(-) Schedule?