combine - combines a set of frames using median or clipped mean.

combine coadds a set of frames by taking the median or the clipped mean at each pixel. Optionally the frames can be divided through by their mean values before they are combined or have a bias added to keep their means constant. Use this routine for getting mean bias and flat field frames.

NB Be warned that a common problem to encounter is to have used grab to get the frames in the first place, but then to ctrl-C it and end up with the final file corrupted. You need to take care not to include it in the file list you supply to combine.

There is a limit to the number of frames that this routine can handle set by the compiler because all files have to be opened simultaneously. This is of order 1000 to 4000 files depending on the system. The program will collapse if you exceed this limit.


combine list method (sigma careful) adjust output


list---List of file names
method---'m' = median, 'c' = clipped mean. The clipped mean rejects one pixel at a time and then recomputes the mean and rms again before having another go.
sigma---If method = 'c', this is the number of sigmas for rejection
careful---If method = 'c', this controls the mehod of rejection. true means pixels are rejected one at a time, worst first. false means pixels are rejected in groups which can potentially lead to the odd false rejection.
adjust---Whether to adjust the frames before combining. 'i' =ignore, 'n' = normalise the mean level to match the first frame, 'b' = add a bias to give the same mean as the first frame. The adjustment is done on an individual CCD basis. 'b' is useful for combining biases where there may be an overall drift from frame to frame. 'n' is for combining (bias subtracted) sky flats for example where the level varies substantially but the shape is fixed.
output---Output frame


This command is a member of the classes: Arithematic, Programs.

Author: T.R. Marsh
Created: 15 Feb 2002
Revised: 26 May 2010

Page generated Fri Jul 5 12:23:42 2013