CUPID ClumpFind and backgrounds

Jenny Hatchell has been comparing the CUPID implementation of the ClumpFind algorithm with the IDL implementation by Jonathan Williams. The IDL version differs in one or two significant respects from the original algorithm published in ApJ, and so CUPID provide a switch that selects either the published algorithm or the IDL algorithm. If the IDL algorithm is selected, Jenny finds that the IDL and CUPID implementations allocate exactly the same pixels to each clump. Good news. And more good news is that the CUPID implementation is at least an order of magnitude faster than the IDL implementation.

However, Jenny noted that the clump sizes reported by CUPID were not the same as those reported by IDL. Both implementations use the RMS displacement of each pixel centre from the clump centroid as the clump size, where each pixel is weighted by the corresponding pixel data value. So in principle they should produce the same values. The difference turns out to be caused by the fact that CUPID removes a background level from each clump before using the pixel values to weight the displacements. IDL , on the other hand, uses the full pixel values without subtracting any background. Thus, increasing the background level under a clump will produce no change in the clump sizes reported by CUPID. IDL, however, will report larger clump sizes due to the greater relative emphasis put on the outer edges of the clump.

So should a background be subtracted or not? Having the reported clump size depend on the background level seems an undesirable feature to me. But if you want to compare CUPID results with other systems (e.g. the IDL ClumpFind in this case) that do not subtract a background, you CUPID also needs to retain the bacground level to get a meaningful comparison. Consequently, I've added a parameter to CUPID:FINDCLUMPS to select whether or not to subtract the background before calculating clump sizes. The default is for the background to be subtracted unless CUPID is emulating the IDL algorithm (as indicated by the ClumpFind.IDLAlg configuration parameter).

If the background is retained in CUPID, Jenny found that the CUPID and IDL clump sizes match to within half a percent. So things look OK.

David