User manual
WHIRC Data Reduction Manual Version 2.0, 2014 June 26 5
2. Scaling: If any of the data were taken with Fowler-4 mode, the pixel values must be
normalized by a factor of 4 prior to any linearity correction. The Fowler-4 mode adds all
of the four readouts but does not divide by 4 to avoid possible noise digitization in the
integer output format. So for the Fowler-4 data only:
imarith *.tr.fits / 4. *.tr.fits
will do this in place.
A safer way to do this is to write all of the data to a list, generate a list of the Fowler
samples for all of the data, then divide:
files *.tr.fits > trlist
hselect @trlist fowler yes > flist
imar @trlist / @flist @trlist
3. Linearity correction: As the wells in the array fill up, the gain changes slightly,
resulting in a slightly sublinear response, which is approximately 4% at 38000 ADU. We
have carried out a quadratic fit to the linearity function for 0.7 v bias up to a value of
38000 ADU. By inverting this function, one can derive a linearity correction function so
that the corrected signal S’ is related to the raw signal S by
S′ = S * (A + B*S + C*S
2
), where
A = 1.0000 A′ = 1.0000
B = 1.29 × 10
-7
B′ = 0.004227
C = 2.506 × 10
-11
C′ = 0.02691
The IRAF task
irlincor is specifically designed to carry out this correction; the coefficients
A′, B′ and C′ above are the
irlincor values. It is critical that linearity correction be
performed on the raw data, prior to any sky or dark subtraction, but after any scaling for
multiple Fowler sampling.
Do an epar irlincor and enter the above values for A’, B’, and C’ into the parameter file
(control-D to exit), then
irlincor *.tr.fits *.%tr%lc%.fits
4. At this point you will have three sets of data; you may want to archive the original raw
data somewhere, and the trimmed (.tr.fits) data can be deleted; the
trimmed/scaled/linearized data (.lc.fits) will be used for reduction.
5. The IRAF script wprep.cl, which can be downloaded from the WHIRC web page, will
automatically carry out all of these steps. In addition, it will check the ROTOFF value in
the image header and recalculate the WCS parameters so that the sky coordinates are