Three IRS notes of interest


Three little tidbits have been brought to our attention recently.

1. Updated SPICE Extended Source Calibration Files for High-Res
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Four incorrect calibration files were included in the last
calibration update for SPICE. The incorrect files are
C15.0/b1_aploss_fluxcon.tbl, C15.0/b3_aploss_fluxcon.tbl,
C15.0PRE25/b1_aploss_fluxcon.tbl, C15.0PRE25/b3_aploss_fluxcon.tbl.
These files are ONLY used when applying extended source
calibration to high resolution (SH, LH) data processed with
pipelines from S15 onward.

NOTE: CUBISM does not use these calibration files, and so it
is not affected by this problem.  To download the correct
calibration files into SPICE, users should do the following:

 (1)  press "Update" in the bottom left of the SPICE window
 (2)  press "Refresh List"
 (3)  select the box: "IRS Calibration Files Set 15.0"
 (4)  Press "Update Now"

The correct files will be downloaded to the SPICE calibration
directory.  Users should then re-extract and re-tune their
extended source high-res data.

2. SMART v6.2.4 MJy/sr vs mJy
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Users who try to use the SMART/ISAP tool to fit for line fluxes will
notice that the line fluxes between SMART v5 and v6 differ by a factor of
1E9 ! This is because SMART v6 was tweaked to implement MJy as mJy
(clearly stated in the release notes).

Users should be cautious and not feed MJy/sr spectra into the SMART line
fitting tool. Please convert your spectra to Jy by multiplying by the
appropriate solid angle corresponding to the extraction window before
reading them into SMART.

This doesnt make any difference if equivalent widths are the quantity you
are trying to extract.

3. Identifying saturation in peak-up imaging data
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Even when individual BCDs are clearly saturated in imaging observations of
sources brighter than the saturation limit in the Spitzer Observer's
Manual, the mosaics of these sources, might at least superficially, appear
unsaturated. This is because the pipeline mosaicer tries to interpolate
the pixel values for the saturated pixels from the surrounding unflagged
pixels. This will naturally result in an underestimate of the source flux.

When you are observing bright sources, please first check that your
individual bmask.fits files do not have bit values corresponding to
saturated pixels. These are bit 2 for digital saturation and bit 13 for
fatally affected pixels (Table 9.3 of the IRS Data Handbook).

Alternately, check the uncertainty files that come with the mosaic. The
uncertainty for saturated pixels will be anomalously high.

There is no easy way to correct for this - you will probably have to fit
surface brightness profiles to the unsaturated wings of the source and
extrapolate back to the saturated pixels using the Spitzer PSF at those
wavelengths. Your photometric uncertainties will almost definitely be
larger than normal.