It is well known that few, if any, CCD type detectors are perfect. Normally, it will be found that several pixels are either `hot' or `cold' (contain very high or very low values) no matter what is being imaged. In the case of cheaper, or older, CCDs it may be found that whole columns or rows of pixels are similiarly useless. Efforts should be made to ensure the values in these pixels are disregarded during all stages of image analysis; this may be done by identifying the pixels in question (they remain the same for a given CCD and a list is usually available at the observatory for each CCD they employ) and inserting that information into an ARD file which can be examined by the ESP application MASK. This looks at the ARD file supplied to find out which pixels are suspect and then sets them to bad in the output image thus ensuring they play no role in future analysis.
The application can be used with the following syntax:
This looks at the ARD text file areas.dat (note the use of the `^ ' character) and then sets the pixels defined in p2 to be bad. The image so generated is output as the NDF masked. Other examples may be found in Appendix B.1.% mask in=p2 out=masked ardfil=^areas.dat
ESP --- Extended Surface Photometry