Gain and Readout Noise

hawcheng wrote on Apr 14, 2010

I have been using imcombine in order to average frames while rejecting cosmic rays. The frames are of different exposure times, so I have had to scale them by mode in order to be able to sensibly average them.

My question is this : now I want to do photometry on this combined frame, and I need to tell the photometry routine what the gain and readout noise for the image are. I can figure out the answer for this for a simple combined frame or a straight average, but what is it if the frames have been scaled?

---

Francisco Valdes wrote on Apr 14, 2010

In general there is not a simple answer. But if you assume the transparency and sky brightness stayed constant (i.e. the difference in the signal in each exposure is only due to the exposure time) then I think you can imagine scaling the result to the total exposure time. In this case the gain will be the same as for a single exposure and the net readout noise will increase the by number of exposures. To work with an average exposure time is then a simple scaling of the readout noise and gain.

Frank Valdes

---

Last post on Apr 14, 2010