Name
flat - create linear gain maps out of twilight data cubes
Synopsis
flat [flags] <twilight> [basename]
Description
flat is used to process twilight data cubes to create linear gain maps
and bad pixel maps. Twilight data cubes are a slowly increas-
ing/decreasing signal that may be used to record pixels’ responses. In
this way, it is mainly possible to see which pixels have a linear
response, in which domain, and if the response is linear, a simple
regression gives the pixel gain, relative to the mean (or median) gain.
Algorithm
flat makes a computation of linear regression factors on every pixel,
outputting the value of the fitted slope. The overall luminosity in
each image is approximated by the median flux. This assumes a majority
of pixels are valid, a valid assumption for standard detectors.
A rejection is applied on the time line to avoid taking into accounts
stellar objects which are seen in twilight frames. This is done by
applying a robust linear fit.
Byproducts of this computation are the y-intercept map, an error map, a
bad pixel map. So far, the only used criterion to declare a pixel as
bad is to tag all pixels outside a [0.5, 2.0] gain interval as invalid.
Some more criteria could be derived from a close study of the slope
fit.
Output file names are all named according to a base name. The gain map
is called base_flat.fits, the intercept map is base_intercept.fits, the
error map is base_errmap.fits, the bad pixel map is base_badpix.fits.
The base name is either given on the command-line after the input file
name, or it can be omitted on the command-line. In that case, the input
file base name (i.e. without .FITS or .fits extension) will be used.
A proportional fit (y=ax) can be requested instead of a full slope
(y=ax+b), in which case no intercept map can be produced. The propor-
tional fit will compute every possible value of y/x, then take the
median one. This has been found to be quite robust.
If you provide a dark frame to subtract from all input twilight frames,
the proportional fit mode will be activated automatically.
Options
-o or --intercepts
Output a y-intercepts map. This is a byproduct of the slope fit
(see above).
-e or --errmap
Output an error map from the linear fit. This image contains for
each pixel the residual sum of squared errors for all fitted
points.
-b or --pixmap
Output a bad pixel map. This is a byproduct of the gain map. All
pixels outside of a fixed [0.5, 2.0] (inclusive) interval are
tagged as bad. The output is a pixel map (i.e. a FITS file). If
you are not satisfied with these threshold settings, use thresh
to set other thresholds on the gain map.
-p or --prop
Instead of trying to fit a full slope (y=ax+b), only a propor-
tional slope (y=ax) will be fitted. This option will be
activated when dark subtraction is requested.
-d filename or --dark filename
Provide a dark frame to subtract from all input twilight frames.
Check that the dark frame you provide uses the same exposure
time as the twilight frames. Proportional fit is activated in
that case.
Files
Input files shall all comply with FITS format.
See Also
thresh, deadpix