Date: Mon, 6 Nov 2006
Subject: New version of SPICE

The Spitzer Science Center has released the latest version of the  Spitzer
IRS Custom Extraction (SPICE) software. This is version 1.4.1.

The new version preserves the functionality of previous versions, 
but adds the capability of performing "Optimal Extraction." This is a
technique by which, at each pixel within the extraction aperture, the
extraction is weighted by the instrumental point-source cross-dispersion
profile and the signal-to-noise squared. Optimal extraction will increase
the signal-to-noise ratio for faint source spectra.

Some things to keep in mind regarding optimal extraction:

1) Optimal extraction works best for POINT SOURCES observed in IRS
Staring mode, at the standard nod positions along the slit (33%, 66%
from the end) or at the center position (50% from the end).

2) For sources observed at non-standard locations along the
slit, optimal extraction will choose the closest stellar template, in
terms of Ridge percentage. However, low-frequency sinusoidal wiggles
may appear in optimally extracted spectra if there is a mismatch with
the stellar template.

3) The user can create his/her own custom rectified stellar template
(rectempl) by doing optimal extraction of a bright star observed at the
same Ridge percentage as the science target. The rectified 2D stellar
spectrum is output as a standard optimal extraction product.

4) Prior to the S15 version of the pipeline (to be released by December
2006), there is a mismatch between the standard aperture for the SL3
bonus order and SL2, when using the default settings in the Extract
module. This is corrected by the Tune module, so that SL2 and SL3
spectra have matching flux levels. In S15, the SL3 standard aperture
will be changed to 5.3333 pixels at 8.0 microns to fix this
discrepancy.

Optimal extraction assumes that SL2 and SL3 have the SAME aperture
where they overlap in wavelength. This will lead to a mismatch of SL3
with respect to SL2 for data processed with pipeline versions earlier
than S15. A solution is to re-download old data from the archive after
they have been reprocessed with the S15 pipeline. All data will have
been reprocessed with the S15 by January 2007.

Some outstanding general issues remain with SPICE:

1) SPICE will freeze when doing large (100 spectra) batch jobs. The
exact limit depends on the machine in which it is running, as for every
triplet the software initiates helper processes which may overload the
system's memory. A workaround is to split large batch jobs in smaller
sub-jobs.

2) Batch mode cannot be used in the extended source mode. The spectra
will be extracted correctly but they will be tuned as point sources.
The user interested in doing large numbers of extended source
extractions will need to do them by hand, or apply the correct tuning
at the end of the batch job.

3) There is a 160 character limit to the character string indicating
the file location (path+filename). If SPICE is on a directory such that
the path+filenames are longer than this, SPICE will produce an error.

More details are available in the SPICE User's Guide
(ssc.spitzer.caltech.edu/postbcd/spice.html) and in the archive of
"IRS: Important Notes" (ssc.spitzer.caltech.edu/irs/imptnotes.html)