Step 5 - Flatfielding

The next stage in the instrumental correction of your data is to make a `flatfield'. A flatfield is probably best made from exposures of the twilight sky or from long-exposures of dark sky (these can be made from ``dithered'' target frames, if you don't have many objects). Either way it is quite possible that the data have some corrupted parts (such as stars) which should be removed before combination and normalisation. MAKEFLAT `cleans' the input data by comparing it with a locally smoothed mean, rejecting any deviant values outside of a number of standard deviations, then trying again for a given number of iterations. After this has been done it estimates the mean value in each frame (this is how it copes with different exposures) and using the mean as a weight it then combines the data using a method, such as median stacking (see elsewhere), which rejects even more bad data (in fact any method except the mean will reject some spurious data). To use MAKEFLAT just type something like:

% makeflat in='ffr/*' out=master_flatr

and it's done. One master flatfield should be made for each filter used.

The final process in correcting your CCD data is to divide by the flatfield. The flatfield corrects for such things as vignetting (the optical response) and the pixel-to-pixel variations in the CCD response (these can be up to 10 percent). FLATCOR divides data by a flatfield.

% flatcor in='rdata/*_debias_darkc' out='*|debias_darkc|processed|'
          flat=master_flatr

The specification out=*|debias_darkc|processed| is one we have not seen before, its meaning is; call all the output frames the same as the inputs except remove the string `debias_darkc' from the names and replace it with `processed'.