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| Note : Especially when preparing
observations for the IRS, it is very important to read the IRS chapter in
the Spitzer Observer's Manual.
Information not duplicated on the web
pages can be found there! |
The Infrared Spectrograph (IRS) provides the Spitzer Space Telescope with
low and moderate resolution spectroscopic capabilities from 5.2 to 38.0
microns. The IRS is composed of four separate modules, with two modules
providing R ~60-120 spectral resolution over 5.2-38.0 microns and two
modules providing R ~ 600 spectral resolution over 9.9-37.2 microns; see
Table below. The names of these modules are summarized (along with
frequent abbreviations) in the Table below. The IRS has no moving parts
and each module has its own entrance slit in the focal plane. The
low-resolution modules employ long slit designs that allow both spectral
and one-dimensional spatial information to be acquired simultaneously on
the same detector array. Two small imaging sub-arrays ("peak-up arrays")
in the Short-Low (SL) module allow objects to be placed accurately into
any of the IRS entrance slits and provide images of sources for
photometric or structural information. The high- resolution modules use a
cross-dispersed echelle design that allows broad spectral coverage in a
single exposure. The median 1s continuum sensitivity for the IRS
low-resolution modules is about 0.06 mJy from 6 to 15 microns, and 0.4
mJy from 14 to 38 microns in 512 seconds of integration with low
background. The median 1 sigma line sensitivity estimates for the
short- and long-wavelength high- resolution modules are about
2.7x10-19 W/m2 and 8.5x10-19
W/m2, respectively, for 512 seconds of integration with low
background. The IRS was designed to achieve high sensitivity at the
expense of reduced dynamic range. Therefore, observers should take great
care in interpreting weak spectral features.
Table: IRS module operating wavelengths
| Module | Detector | Wavelength Range (microns) |
Resolving Power |
| Short-Low (SL) |
Si:As |
5.2-8.7 (SL 2nd)
7.4-14.5 (SL 1st) |
60-127*
61-120* |
| Blue Peak-Up (SL) |
Si:As |
(13.5-18.7) |
(~3) |
| Red Peak-Up (SL) |
Si:As |
(18.5-26.0) |
(~3) |
| Long-Low (LL) |
Si:Sb |
14.0-21.3 (LL 2nd)
19.5-38.0 (LL 1st) |
57-126*
58-112* |
| Short-High (SH) |
Si:As |
9.9-19.6 |
~600 |
| Long-High (LH) |
Si:Sb |
18.7-37.2 |
~600 |
* delta lambda is approximately constant as a function of lambda
for each module and order.
Table: IRS slit description synonyms
| IRS module formal name | Abbreviation |
|---|
| Hi Res 9.9-19.6 microns | Short High (or SH) |
| Hi Res 18.7-37.2 microns | Long High (or LH) |
| Low Res 5.2-8.7 microns | Short Low 2nd order (or SL 2nd, or
SL2) |
| Low Res 7.4-14.5 microns | Short Low 1st order (or SL 1st, or
SL1) |
| Low Res 14.0-21.3 microns | Long Low 2nd order (or LL 2nd, or
LL2) |
| Low Res 19.5-38.0 microns | Long Low 1st order (or LL 1st, or
LL1) |
Since there are no moving parts in the IRS, multiple uses are made of the
same detector array, and telescope motion is substituted for grating
mechanism motion in the instrument.
Detectors
Each of the four IRS optical modules has a single Focal Plane Mount
Assembly (FPMA) 128x128 BIB (blocked impurity band) focal plane array
detector. Two FPMAs contain arsenic doped silicon (Si:As) arrays, and
two contain antimony doped silicon (Si:Sb) arrays. The Si:As arrays
operate over a 5-26 micron wavelength window, while the Si:Sb arrays
operate over a 14-40 micron window. Each array is electrically connected
via short cables to the Cold Interface Board (CIB) located within the
cold instrument section on the IRS baseplate. As the Combined Electronics
is redundant (Side A and Side B), the CIB provides a reliable means of
signal interconnection and distribution to each of the four modules.
| Note : Especially when preparing
observations for the IRS, it is very important to read the IRS chapter in
the Spitzer Observer's Manual.
Information not duplicated on the web
pages can be found there! |
Go back to IRS page
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