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I am having some difficulties with [i:a4e1593db1]noao.apphot.daofind[/i:a4e1593db1]. As I understand it, the main parameters that determine how many objects I detect are "sigma" and "fwhm". However, I get the same number of objects using sigma=4 as sigma=15, and the number is ridiculously high. The objects I'm looking at are not precisely point sources, but they should be pretty darn close. Any idea what I might be doing wrong? Thanks.-Brad Peterson
Iowa State University

In help for [b:dae54bd521]daofind[/b:dae54bd521] you may read:[u:dae54bd521]After image convolution , DAOFIND steps through starmap searching for density enhancements greater than findpars.threshold * datapars.sigma, and brighter than all other density enhancements within a semi-major axis of 0.42466 findpars.nsigma * datapars.fwhmpsf. As the program selects candidates, it computes three shape characteristics, sharpness and 2 estimates of roundness. [/u:dae54bd521]Not only findpars.nsigma && datapars.fwhmpsf determine
object count. Parameter findpars.threshold is very important.Try again with findpars.threshold=4 and findpars.threshold=30.
You can see object number changing.[i:dae54bd521]Is this help?[/i:dae54bd521]Azamat

I will try that. Thank you for your help.-Brad

This has indeed helped. Part of my problem was that I was not using the datapars.sigma describing the background at all, I was just using findpars.threshold as the sigma threshold for significance.Thanks again.-Brad

[quote:bf2c089d65="azastro"]In help for [b:bf2c089d65]daofind[/b:bf2c089d65] you may read:[u:bf2c089d65]After image convolution , DAOFIND steps through starmap searching for density enhancements greater than findpars.threshold * datapars.sigma, and brighter than all other density enhancements within a semi-major axis of 0.42466 findpars.nsigma * datapars.fwhmpsf.[/u:bf2c089d65][/quote:bf2c089d65]Followup question: what if my background sigma varies considerably across the image? Do I chop it into sections to run [i:bf2c089d65]daofind[/i:bf2c089d65] with different values of datapars.sigma? Or should I just get background stats on a low-background part of the image and use that throughout?-Brad

I'm having pretty much the same problem, but changing the threshold doesn't change anything, it's still finding way too many stars!