sn_sub
sn_sub
is a C-shell script which subtracts the reference
image from the target image and then searches for new stars.
sn_sub
takes the DATIDs of the target and referenc
images and does does the following:
- copies reference and target images to scratch files.
- reads in the offsets derived from
sn_fidmatch or
sn_cormatch
- shifts the target to align the stars with the reference.
- smooths reference image if necessary.
- scales the reference image so that the star brightnesses are the same.
- subtracts the reference from the target.
- makes a search for stars over the difference image (minus the
edges).
The resulting candidate stars are then passed through a series of filters to
remove false candidates:
- Stars with very small FWHMs are detected using the
cosmic
program are rejected
- Stars with a peak intensity less than 20 adu are rejected.
- Stars located within 4+(mag-20)*1.4 pixels are rejected, where mag is the
instrumental magnitude of the star in the reference image.
This ad hoc
formula was adjusted for the thirty inch telescope.
- Stars near defects
in the reference image are rejected.
A statistic called the rejection ratio is determined by doing photometry on
the subtracted image at the location of the fiducial stars and comparing it
to the magnitude in the original
image.
Finally the scratch files containing the target galaxy image, the reference
image and its difference are renamed (to target.fts, ref.fts and diff.fts) so
the procedure
sn_watch
can be used to give you an on-line display of
the progress of the
search.