Limited only by memory - low values should be avoided. Critical in determining the amount of deblending of objects. 0 means local variances will be treated as separate objects. 1 means no deblending. For forced photometry it should not be used, as the number of detections will vary with different frames due to different background calculations. Mask type Correct Gives better magnitude estimates.
Low values means the background is poorly sampled.
For matching objects, a 3 pixel allowance is ample. The catalogues produced can contain several parameters which can be later used for rejecting edge objects, spurious results and to compare differences between each band. Other parameters include FWHM, eccentricity, background etc. which can be useful when objects have been initially identified. Sextractor is able to compute several different types of magnitude calculation - isophotal, corrected isophotal, aperture, best and auto. Each has its advantages and disadvantages.