I am new to this topic and am experimenting with some 5 sec shots off a fixed camera (D80). I am making dark frames and aligning the images using Deep Sky Stacker. It seems to work quite well and I am having fun picking up the Messier objects. However, how do I deal with images taken on different nights and having different darkframes. Is it OK just to stack the already stacked frames. eg night one produces a stack of 10 images and night 2 does the same can I stack the two frames from the two nights' work?

David

There should be no difficulty, provided that there is a significant overlap between the frames, so that most of the brightest stars in one frame are also visible in the others. The scale needs to be similar too - ie, the same focal length lens or optical set-up in the telescope for all frames.
My own software works by looking at the patterns of connecting lines between the brightest stars (the 24 brightest, unless the user changes that number). They do not all have to match.
Stacking is done into an accumulator image having 32 bits per channel, so that adding up brightnesses does not cause them all to saturate (reach the 16-bit limit, which is the depth of the original images). For deep sky work it is important to keep the 32-bit depth for subsequent processing, otherwise you are throwing away the possibility of pulling out the faintest detail from the background. So if the stacking software can take as input files having 32 bits per channel, you will not lose anything by stacking two or more already stacked results. That necessarily means saving the results as FITS files because 32-bit TIFF files (aimed for importing into Photoshop) use 32-bit floating point rather than integer values and so really have only 23 bits per channel resolution (allowing for wasted sign bit).
If you cannot use 32-bit FITS files as input to the stacker you would be better off stacking all of the original 16-bit frames in one sequence, even if from different nights.

Sorry if its stating the obvious, but you should be stacking calibrated frames - ie, frames corrected with darks, flats and biases (from the night the images were made).
Then there should be no problem stacking from different nights.
Good stacking software should align even if there is some misaligment of frames.
I have tended to use IRIS which also works with 32 bit integer data - a point Graham makes well.

Callum

Many thanks for your helpful advice.

David

Just to clarify further;

IRIS does NOT use in any form or fashion 32-bit deep images nor it is capable to read them. It only recognizes 8 and 16-bit deep images and uses 16-bit *signed* FITS files (or PIC) up to 3 planes per image.

IRIS (and all the software I know of) will happily align and stack calibrated and UN-calibrated frames, so it is up to the user decide what to do.

As long as there are 3 stars shared between all the frames IRIS is more than capable to align and stack them. In fact they do not have to have been taken with the same hardware *at all*, as long as there are enough stars to perform trasformation (to a common scale), alignment (with possible morphing) and stacking.

Andrea T.

Hi Andrea,

thanks for clarifying - my memory does not seem to be what it used to be!

Callum