The Infrared Spectrograph IRS

1
The Infrared Spectrograph (IRS)

• Jeffrey Van Cleve
• Cornell_at_Ball
• Lee Armus
• Sirtf Science Center

http//astrosun.tn.cornell.edu/SIRTF/irshome.htm
2
IRS Science Team
Other places Dan Watson Bill Forrest Dan
Weedman Tom Soifer Lee Armus Charles Lawrence Tom
Roellig Harold Reitsema Keith Matthews
Cornell University Jim Houck, PI Terry
Herter Jeffrey Van Cleve Bernhard Brandl Keven
Uchida
particular interest in Solar System studies
The IRS is being built by Ball Aerospace in
Boulder, CO and will be delivered in April 2000
3
IRS Architecture and Design

• Modular Architecture
• Four separate spectrograph modules
• Two low resolution (R60-120) modules
• Two high resolution (R600) modules
• A peak-up camera to enable source acquisition.
• The modules are mounted onto a common baseplate
• Modular architecture decouples designs,
  fabrication, assembly, and testing of the
  spectrographs
• Design Concept
• All aluminum construction of the housing and
  optics to maintain the optical alignment and
  focus from room temperature assembly to cryogenic
  operation.
• Hollow box with internal baffles, and the optics
  bolted on from the outside.
• No moving parts or adjustments.

4
Basic IRS Capabilities
Sensitivity Levels (point source) Low Res 1
mJy _at_10 ?m (5?, 500 seconds) High Res
3x10-18 W m-2 _at_15 ?m (5?, 500 seconds)
Peakup 0.5 mJy
Saturation Limits (point extended sources)
Low Res 5 Jy _at_10 ?m (8 seconds) or
0.4 Jy arcsec-2 High Res 50 Jy _at_15 ?m (8
seconds) or 2.1 Jy arcsec-2
Peakup 0.5 Jy (4 seconds) or 40
mJy arcsec-2
5
IRS Slits (details)
54.6
54.6
23
Short-Low
3.6
2nd Order
1st Order
(5.38.5 ?m)
(7.514.5 ?m)
151.3
151.3
43
9.7
2nd Order
1st Order
Long-Low
(22 40 ?m)
(14 22 ?m)
22.40
11.8
Long-High
Short-High
5.3
11.10
(1937 ?m)
(1019.5 ?m)
6
IRS Spectral Properties

• Resolution
• Low Res 60 to 120 (constant Dl in an order)
• High Res 600 1.0 cm-1 at 16.7 mm
• Peakup 13-18.5 mm and 18.5-26 mm bands
• Fringing Limits the Minimum Line to Continuum
  Ratio
• A variation in instrument response, which is
  periodic in wavenumber, caused by internal
  reflections in the detector. If this periodicity
  is not resolved by the spectrometer module, it
  cannot be accurately calibrated for line fluxes.
• Low Res (5.3 mm-10 mm) 15
• High Res 2

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Unresolved Fringes Limit Line Flux Accuracy
Both line sources wind up on the same pixel, but
source A is at the maximum response and source B
is at the minimum response. Hence the observed
flux is not known to better than max/min, a ratio
called the contrast.
A
B
one pixel 1/2 spectral resolution element
8
Peak Up

• The IRS slits are about one PSF wide
• Point and shoot will not work to place a source
  on the slit, by dead reckoning
• Need an autonomous target acquisition system
  Peak Up
• Peak-Up imaging fields
• Blue, 13-18.5 mm
• Red, 18.5-26 mm
• both about 1 arcmin x 1.5 arcmin
• Peak-Up algorithm
• rejects numerous proton hits
• finds centroid of source and calculates size and
  shape (2nd moments)
• reliable, accurate, and sensitive for point
  sources (0.5 mJy lower limit)
• designed to accommodate extended sources but
  performance still TBD
• no provision made for Peak-Up on saturating
  sources (gt0.5 Jy)

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SIZE matters

• Peak-Up computes, using pixels significantly
  brighter than the noise
• total flux (zero moment)
• centroid (first moments)
• SIZE and shape (2nd moments in diagonalized
  co-ordinates)
• SIZE is a parameter passed to the Peak-Up command
  to select targets of a given SIZE or larger
• 2nd moments calculated around centroid and
  normalized by total source flux
• 2nd moments independent of outer integration
  bound only for source intensities which fall off
  faster than 1/r4

L0 PSF SIZE normalization constant
SIZE L1L2-L02
Shape (L1-L0)/(L2-L0)
10
IRS Peakup
72
72
37
Peak-Up Apertures
Vignetted Field
Blue
Red
60
(18-26 ?m)
(13-18 ?m)
1st Order
Blue
1st Order spectrum 7.5 ? 15?m
Short-Low Array 128x128 pixels (1.8 arcsec/pxl)
2nd Order 5.3 ? 7.5?m
Red
11
First Light in the Short-Low Module
The 1st order spectrum (7.5-15 ?m, top
-to-bottom) is at the left. The 2nd order
spectrum (5.3-7.5 ?m, middle -to- bottom) is at
the middle. The dots at the right are point
sources in the blue (top) and red (bottom) peakup
fields. The spectrum at middle top is 1st order
from 7.5-8.5 ?m.
?
12
Short-Low Module
Filters
Collimator Mirrors
Pickoff Mirrors
Schmidt Plates
Flat Mirror
Grating
Camera Mirrors
SiAs FPMA
Foreground Short Low Background Peak-up
13
The 4 IRS Modules(view from above)
Short-High
Short-Low Peakup
Long-High
Long-Low
14
The Short-High Module Captures Many Lines at Once
This cross-dispersed echelle spectrum was
synthesized from NH3, and CO2 spectra. It covers
10-19.5 ?m at a spectral resolution of 600. 10
?m is at the right of the image. Lines in the
rightmost third of the image are NH3 at the
center is CO2 and its sidebands. The image is
scaled nonlinearly to show weak features.
15
First Light in the Long-High Module
This cross-dispersed echelle spectrum of the
difference between 55 C and room temperature
blackbodies covers 19-37 ?m at a spectral
resolution of 600. 19 ?m is at the left of the
image. Note periodic modulation of spectrum due
to detector fringing, which places a lower limit
on the line-to-continuum ratio observable with
the instrument. Gaps in spectrum are due to
water absorption outside of test dewar