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 increasing/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 pro- duced. 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 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 satis- fied 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 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