Spitzer Space Telescope

Post-Launch updates to IRS - 19 Dec 03


We have made a substantial number of updates to chapter 7 of the  SOM
(v4.0) based upon IOC and SV results.  These include, but are  not limited
to:

(1) Revised slit, vignetted region, and peak-up array dimensions  as
reported in sections 7.1.1.1 and 7.1.2.1.  Most slit lengths  and widths
are close to their pre-launch values, with the  exception of the Long-Low
(LL) sub-slits and the LL vignetted  region, which are slightly longer
(now 168 arcsec each) and  slightly smaller (now 24 arcsec wide),
respectively, than reported  previously. The blue and red peak-up
fields-of-view have also  changed, being slightly longer (now 80 and 82
arcseconds,  respectively) and narrower (now 56 and 54 arcseconds,
respectively)  than pre-launch predictions.

(2) Revised wavelength coverages for all slits as reported in  sections
7.1.1.1 and 7.1.2.1.  This includes a discussion of the  effective
wavelength ranges (i.e., the ranges over which the  spectra can be
calibrated) for the 1st order Short-Low (SL) and  LL slits, which are 7.4
- 14.5 microns and 19.5 - 38.0 microns,  respectively.

(3) Revised sensitivity curves for all modules as reported in  section
7.2.1.  All IRS arrays are  more sensitive than pre-launch estimates. 
Section  7.2.1.2.1 includes a new plot of the Bright Source Limit (BSL) 
vs. ramp duration for all modules.  For targets with flux  densities above
the BSL, photon noise from the target dominates the signal-to-noise
calculations.  All of the sensitivity curves are still shown in the faint
source limit (no source photon  noise), but observers can easily use the
new figure and  accompanying formulae to estimate the expected spectral 
signal-to-noise ratio when observing bright sources.  Please  note that
all signal-to-noise estimates are based upon model calculations using
input parameters measured from in-flight  IOC and SV data.  Higher
fidelity estimates of the  signal-to-noise that observers can expect in
IRS spectra  extracted with the online data processing pipeline will be 
provided after the pipeline is commissioned at the opening of  the Spitzer
Data Archive.

(4) As mentioned in the expanded description of the peak-up  algorithm
(section 7.2.3.1.3), observers should note that the  nominal initial
pointing accuracy of Spitzer has been measured  during IOC/SV, and is
~1.0" (1-sigma radial).  Thus, the use of a peak-up in  situations
requiring Low accuracy (i.e., Quick Point protocol)  is not strictly
necessary if the target coordinates are  well-known and if the observer
does not need the set of  peak-up images.

(5) Revised moving target example in section 7.2.4.4.  This  example now
includes a more accurate description of the process  of planning an IRS
observation of a moving target, and  constructing the AORs in SPOT.  For
additional examples of  IRS AOR preparation, also see sections 7.2.4.1 -
7.2.4.3 of  the SOM, as well as the Spitzer Observation Planning
Cookbook,  which will be available 30 December 2003.

(6) Modified section 7.3.2 to include an up-to-date listing and 
description of the archived IRS data products and their  corresponding
file names in the archive database.