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Flatfielding

Flat-fielding multi-object spectra is a complex procedure, and LEXT has a special routine MAKEFF which simplifies the process. LEXT has no specific facilities for dealing with undispersed flatfields since they may be conveniently reduced with standard image reduction software.

The dispersed flatfields are prepared by fitting the variation in signal level with wavelength separately for each slit and dividing the fit into the actual signal at each column. This is not a simple procedure since the result is undefined for regions which do not receive any illumination - the interslit gaps. The region of validity of the resultant flatfields must be carefully defined by the extent of the slit and the range of wavelengths over which satisfactory levels of illumination have been obtained in the contributing exposures. The resulting flatfields should be applied only in these defined areas. The preparation of the flatfields is time-consuming using standard software (e.g. Figaro) but the LEXT routine MAKEFF makes it a relatively easy operation.

The input data required is a dispersed flatfield obtained with a multiaperture mask illuminated with white light. Care should be taken to ensure that the exposure has high enough counts to make Poisson noise negligible in comparison with the size of the flatfield variations.

MAKEFF generates a flatfield by obtaining for the specified boxes, a smoothed representation of the data in each column and dividing it into the data values for each pixel in that column. The default is for the mean value to be used but the smoothed value can also be obtained from:

• the median over a user-specified filter width (option /MEDIAN)
• a Chebyshev polynomial fit (option /FIT)

The option /PLOT allows the user to observe what is going on. The range of boxes to be processed is specified on the command line, for example MAKEFF/PLOT/FIT 1-3 8-13.