What is the best method to combine subframes with different exposure times? A year ago I took a fair number of 5 minute frames of M33. Over the last couple of weeks, I have taken more at 10 minutes. I'd like to combine them but am unsure of the best method.

Here's what I've tried so far. I've calibrated and aligned all the frames. I sigma-rejected each group of similarly exposed frames, then averaged them. I then summed the two averaged frames (one from each exposure length).

Is this the best (or at least a good) method? I figure I need to do the sigma reject on frames of the same exposure. Should I just sum the all lengths after that?

I apologize if I am looking for some magic bullet that doesn't exist. But I thought it was work asking.

Thanks in advance.

--Andy

Hi Andy,

Sorry to be so late in reply.

CCDStack automatically weights frames with different exp time or different transparency or even taken thru different scopes. This is done in the "Normalize" step, which should be performed prior to "Data Rejection".

Weights are reported during Normalization and are visible in “Image Manager”. When the frames are combined, the weights determine the impact of each frame. For example, a 10 minute frame will have about 2x more impact than a 5 min frame.

Stan

Stan,

I thought that might be the case, but wasn't sure.

It is good news, since it gives an even larger statistical basis for the sigma reject algorithm.

Thanks,

--Andy