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 lin-
ear gain maps and bad pixel maps. Twilight data cubes are
a slowly increasing/decreasing signal that may be used to
record pixels' responses. In this way, it is mainly possi-
ble 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 crite-
rion to declare a pixel as bad is to tag all pixels out-
side a [0.5, 2.0] gain interval as invalid. Some more cri-
teria 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 proportional 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 acti-
vated 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 proportional 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 pro-
vide 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