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Transcript of Filters
Filters 1
PACS IHDR 12/13 Nov 2003
Filters
N. GeisMPE
Filters 2
PACS IHDR 12/13 Nov 2003
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
Filters 3
PACS IHDR 12/13 Nov 2003
Example Filter Chain: Long-Wavelength Photometer
Dichroic beam splitter 130.µm
Long-pass edge filters
52.µm
110.µm125.µm
Short-pass edge filter 210.µm
Filters 4
PACS IHDR 12/13 Nov 2003
Filter Types
Examples of QMW filters
Examples of QMW filters
This style used in Bolometers and Ge:Ga
detectors
This style used This style used everywhere elseeverywhere elseThis style used This style used
everywhere elseeverywhere else
Filters 5
PACS IHDR 12/13 Nov 2003
Filter Curves - Typical Blocking Filters
Filters 6
PACS IHDR 12/13 Nov 2003
Filter Curves - Dichroic SpectrometerShortest wavelength band close to edge of workable dichroic range
Filters 7
PACS IHDR 12/13 Nov 2003
Filter Curves - Dichroic PhotometerShortest wavelength band badly affected by resonant effect --> fundamental problem
Filters 8
PACS IHDR 12/13 Nov 2003
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)
Filters 9
PACS IHDR 12/13 Nov 2003
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 %
Filters 10
PACS IHDR 12/13 Nov 2003
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
Filters 11
PACS IHDR 12/13 Nov 2003
OGSE Filters
Filters 12
PACS IHDR 12/13 Nov 2003
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...
Filters 13
PACS IHDR 12/13 Nov 2003
OGSE Filter Problems
• Room temperature radiation from outside likely prevents proper cooldown of filters– Low thermal conduction (mylar, inconel stainless)– high absorptivity (10...50%)
• Filter predicted temperatures above LN2 temperature, at emissivity ~ 30%==> remaining background levels factor ~10 too high
Need additional cold 10um-absorbing or reflectingfilter:-- cryst. Quartz (+) Availability(-) Redesign, ghost images,
factor 3
-- QMC mesh filter (+) factor 1 (-) Schedule?