A. Poglitsch, N. Geis

1
PACS Filtering Scheme

• A. Poglitsch, N. Geis
• C.E. Tucker, P.A.R. Ade

2
PACS Filtering Scheme

• Performance requirements
• definition of spectral bands
• photometric bands
• order sorting for spectrometer grating
• in-band transmission
• out-of-band suppression
• Filter implementation
• filter technology (low-pass, high-pass,
  band-pass, dichroic)
• filter location in optical path
• rejection of thermal radiation from satellite
• instrument straylight management

3
Performance Requirements

• Definition of photometric bands
• long wavelength 130 - 210 µm
• short wavelength 60 - 90 µm 90 - 130 µm
• long/short wavelength bands separated by dichroic
  beam splitter
• Selection of grating orders
• 1st order 105 - 210 µm
• 2nd order 72 - 105 µm
• 3rd order 57 - 72 µm
• 1st order separated from 2nd/3rd order by
  dichroic beam splitter

4
Performance Requirements

• In-band transmission
• requirement 40
• goal 50
• Out-of-band rejection
• defined for wavelengths from 0.1 to 10 000 µm,
  subdivided in 8 bands
• derived from set of template observations
• conservative model of detector responsivity
  included, based on EM measurements for
  photoconductors and modeling for bolometers
• (total rejection) (filter suppression) x
  (detector responsivity)

5
Stressed Detector Responsivity
FTS measurement
Model Responsivity
6
Bolometer Responsivity

• Similar to GeGa responsivity, except for
  extended long-wavelength response
• Average responsivity of 0.5 x in-band
  responsivity assumed out to 10 000 µm
• Power beyond 10 000 µm negligible
  (Rayleigh-Jeans) as well as response

7
Template Observations

• Representative for highest demands on filter
  suppression during astronomical observations
• 3 scenarios identified
• planet with high albedo, observed for absolute
  photometric calibration of PACS
• deep imaging (Galactic/extragalactic)
• FIR excess of dust around young star

8
Filter Rejection RequirementsSummary of 3
Scenarios
The requirements from the 3 scenarios have been
combined by taking the most stringent one in each
wavelength band. Solid red line total required
suppression Dashed blue line model detector
responsivity
(bolometers only)
Suppression factor
detector response filter transmission overall
response
Dotted green line resulting required filter
suppression
Wavelength µm
9
Filter Implementation

• Metal mesh filter technology developed at QMW
  found fully adequate for PACS
• Proven technology
• Robust (thermal cycling, vibration, cosmic rays)
• Exploitation of FPU commonality
• Collaboration with SPIRE consortium set up
• QMW will provide facilities some personnel
• MPIA will support SPIRE BSM development
• MPIA/MPE will balance difference in
  cash/personnel

10
Metal Mesh Technology (QMW)

• Manufacturing process
• Air/vacuum gap filters use annular metal spacers
• Hot pressed polypropylene filters use dielectric
  spacers

11
Metal Mesh Technology (QMW)
Examples of QMW filters
12
PACS Filter Types

• Low-pass edge filters, used in transmission
• blocking filters (UV FIR)
• hot-pressed polypropylene
• Band-pass filters, used in transmission
• define sub-bands of short-wavelength photometer
• select grating orders of s-w spectrometer
• hot-pressed polypropylene
• High-pass filters, used in transmission
• Dichroic beam splitters
• reflect/transmit light into short/long-wavelength
  branches of photometer and spectrometer
• air/vacuum gap technology

13
Examples of QMW Multi-Mesh Filters
dichroic
band-pass
14
PACS Filter List
15
PACS Filter Location
16
Filter Transmission Long-Wavelength Spectrometer
(1st Grating Order)

• Dichroic beam splitter, nominal transition
  wave-length 105 µm
• Long-pass edge filters
• 52 µm
• 83 µm
• 100 µm

17
Filter Transmission Short-Wavelength Spectrometer
(2nd Grating Order)

• Dichroic beam splitter, nominal transition
  wave-length 105 µm
• Long-pass edge filters
• 52 µm
• 54 µm
• Band-pass 72 - 105 µm

18
Filter Transmission Short-Wavelength Spectrometer
(3rd Grating Order)

• Dichroic beam splitter, nominal transition
  wave-length 105 µm
• Long-pass edge filters
• 52 µm
• 54 µm
• Band-pass 55 - 72 µm

19
Filter Transmission Long-Wavelength Photometer

• Dichroic beam splitter, nominal transition
  wave-length 130 µm
• Long-pass edge filters
• 52 µm
• 110 µm
• 125 µm
• Short-pass edge filter 210 µm

20
Filter Transmission Short-Wavelength Photometer 1

• Dichroic beam splitter, nominal transition
  wave-length 130 µm
• Long-pass edge filters
• 52 µm
• 60 µm
• Band-pass 90 - 130 µm
• Short-pass edge filter 135 µm

21
Filter Transmission Short-Wavelength Photometer 2

• Dichroic beam splitter, nominal transition
  wave-length 130 µm
• Long-pass edge filters
• 52 µm
• 60 µm
• Band-pass 60 - 90 µm
• Short-pass edge filter 135 µm

22
Conclusions

• Filter scheme with 4 or 5 filters in series in
  each instrument channel provides sufficient
  out-of-band suppression
• Measured/expected in-band transmission
• gt 80 for long-pass and dichroic filters
• 80 for band-pass filters
• gt 40 for filter combination
• 50 possible
• Requirements will be met