cl> imstat zn2013[745:930,810:950] # IMAGE NPIX MEAN STDDEV MIN MAX zn2013[745:930,810:950] 26226 81.62 10.38 42.46 173.2If you want to be somewhat safer, pick a few sky regions around the image and average the results. And if you want to be safer still, epar imstat and ask it to return the median rather than the mean. The median is less influenced by outlier events such as cosmic rays, hot pixels, etc. Once you have determined the sky level you need only to subtract it from the image:
cl> imexpr expression : a-b output image : zn2013sub operand a : zn2013 operand b : 81.62 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% - done
It pays to think a little about how precisely you need to determine the sky value. In the example, the standard deviation (the root-mean-square deviation from the mean) for all pixels in the region is 10.38. The uncertainty in the mean value is the standard deviation divided by the square root of the number of pixels. Thus the sky level is 81.62 +/- 0.06. This is quite a precise measurement--- IF the errors are dominated by pure counting statistics and not by other systematic effects.
There are at least 3 reasons why your sky may not be flat:
The first thing to do is to combine your blank sky frames. (This does not necessarily mean all of your blank sky frames, e.g., if you have interleaved object and sky exposures. Which you combine depends on what you did at the telescope.) Use the generic 'combine' task to compute the average of the exposures if they all have the same exposure time, or the sum if they do not. Turn off any image scaling by setting "scale" to "none". Recommended rejection algorithms are "crreject" or "minmax"; if you use the latter, start with "nhigh" and "nlow" both set to 1 if you are combining more than 4 frames. Check that the combined image has no sources in it. Next, use 'mkskycor' to smooth the image. (See section 3.11.2 and Fig. 18 of the User's Guide to CCD Reductions with IRAF.) The defaults will probably work, though you may want to increase the clipping sigma parameters.
Now, what you are going to do with this smoothed image depends on whether you think you have an additive contamination (light leak, dark current) or a multiplicative correction (flat-fielding error). Our current state of experience with the ST8 camera suggests that the former is probably the cause of big (say, 20%) variations in the apparent sky level.
To correct an additive contamination, first figure out what the effective exposure time is for the smoothed combined sky image. If you averaged frames of the same exposure time, then the resulting image retains that exposure time; if you summed frames, then you want the sum of the exposure times. Scale this to the exposure time of the image you are correcting, and subtract. For example, suppose you computed the smoothed sum of 4 blank sky exposures of 300, 300, 600, and 600 seconds. The sum has an effective exposure of 1800 s. To use this to correct a 300 s exposure of a galaxy, you would do the following:
cl> imexpr expression : "a-b/6." output image : zn2013corrected operand a : zn2013 operand b : SmoothedSky 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% - done(Note the quotation marks around the expression, necessary if the expression contains a "/".) You may find that the best subtraction is obtained with a scale factor other than that implied by the ratio of the exposure times. This could happen if the sky brightness was changing in time, or if your contamination is a nonlinear dark current. A little trial and error may be necessary to get the best results. The bad news is that it appears that this procedure works poorly for GOT/ST8 data unless the sky and target frames have the same exposure times to begin with.
If, instead, you want to use the blank sky frame to correct for residual flat-fielding errors, then you need only take another pass through 'ccdproc'. Set the input name to the image you want to correct. Turn on the illumination correction, and set the name of the illumination correction image to your smoothed sky image. Run the task. Note that you need to do this before you do any other sky subtraction.