|
Are hot pixels/cosmic rays in my processed data?
If you continue to see what appear to be hot pixels or cosmic rays,
even after processing your data with calstis, there are a
couple of things you should check before contacting the
Help Desk.
-
Check that the data reduction was done using the
best weekly dark files
available for your observation. If you requested your data close to the time
of the observation, you should make sure no new reference files applicable to
your data have been made available since. If new files are available, it might
be better to re-request your data from the archive again, so that they are
automatically recalibrated using the best and current reference files.
-
The standard dark file subtracted from STIS data is based on a
combination of several long dark files taken over an entire week. They
usually don't do a perfect job of subtracting hot pixels, both because
the hot pixels can change on short time scales, and because some of the
hotter pixels may have been saturated in the long dark images. For
these reasons it will often be useful to construct a customized dark
image for that particular day using short darks taken on the same day as
your science image. Procedures for doing this are described at
http://www.stsci.edu/hst/stis/software/scripts/daydark/.
After you create your new daydark file, you should set the
DARKFILE keyword in the [0] extension of the _raw.fits files to reference
this new dark file and then reprocess the data through calstis.
-
Even using a daydark will not give perfect removal of all bad pixels. There
is no routine available that will automatically find and fix the bad pixels.
It will often be necessary to identify them by hand and create a bad pixel
mask. Once such a mask has been made, the IRAF routine fixpix may be useful.
This task interpolates over bad pixels, and allows the bad pixel mask to be
defined with a text file rather than a fits image. It is probably best to
find the bad pixels in the _crj.fits file, use fixpix to adjust the crj
fluxes and errors, and then run subsequent calibration and reduction programs
on the corrected _crj file. For data taken on the same day, most of the bad
pixels will be in the same place in each image.
More information
|
 |
|
