Re: Negative values on the histogram - what do they mean?

Posted by karenacollins on Jan 04, 2022; 9:21pm
URL: http://astroimagej.170.s1.nabble.com/Negative-values-on-the-histogram-what-do-they-mean-tp1713p1717.html

It is possible to have legitimate negative pixel values after calibration, but I think it would be wise to make sure your bias and dark values are not being subtracted twice, or that a pedestal value that is inserted by some camera control software is not somehow getting subtracted twice. If you are running all of your raw calibration and science images through AIJ's DP module to produce the calibrated images, incorrect subtraction shouldn't be happening. However, if you are using a combination of calibration from other tools and AIJ, it can happen. In particular, if you are producing master calibration files from other tools and using then in DP, then one must exercise care that bias, dark, and pedestal subtraction only happen once.

Karen