Deriving accurate background values

    Probably the most important factor limiting the accuracy of a galaxy profile is the accuracy of the image background value used. If this value is significantly too high (or too low) the profile will be distorted at the faint isophotes. This will modify any scale length value determined for the galaxy by SECTOR or by GRAPHS. Great care should be taken to ensure the most accurate possible value is found.

To this end, two ESP applications are provided: HISTPEAK and LOBACK. HISTPEAK derives a global value for an image by considering all the image pixels apart from those defined as bad by an optional ARD file. This is most useful when the image is well flatfielded. LOBACK, by comparison, determines values for discrete parts of the image centred on image co-ordinates provided in a text file. This is most useful when an image is not perfectly flatfielded.