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?