When combining bias frames one needs to reject cosmic rays. A good way to do this is through median averaging, but the end result is digitised. makebias avoids this by median combining small groups and then averaging the results. The small group need to be large enough to ensure good removal of cosmic rays. Another problem with combining bias frames is that they may suffer overall offsets such that one frame hardly contributes to the median (e.g. if it is 20 counts higher than the others but readout noise is only 3 counts). This routine accounts for this using the program 'picstat' to compute the mean level over a region which it subtracts prior to combining the frames, but adds back to the final result before averaging. The result is a mean bias with about the the right level and little digitisation noise. NB the routine takes care to combine the groups with the correct weight. This normally means a slightly larger weight for the final group as it will usually have a few more frames than the others.
makebias list nmedian bias_region output
| list | --- | List of files to be combined. |
| nmedian | --- | The size of the small groups to be medianed. Does not have to divide into the total number of files. |
| bias_region | --- | Region file for picstat to compute mean level of each frame which will be subtracted before they are medianed together. The mean value of the last frame of each frame is added onto the final combined frame of the group |
| output | --- | name of output bias frame |
Author: T.R. Marsh
Created: 10 January 2005
Revised: 24 July 2007