In my previous post I put up an image of M81, M82 and NGC3077. During the processing of the 24x30s images that I took on the evening of the 1st February, I explored the option in DeepSkyStacker of stacking just a proportion of the sharpest frames.
The above image shows the galaxy M81 in three different runs of processing the data. The view on the left uses all 24 frames, the one in the middle the sharpest 18 frames (75%) and the one on the right the sharpest 12 frames (50%). You can click on the image to get a closer look at the results. To be honest I don't think removing the less sharp frames makes much difference. You can definitely see an improvement in sharpness from left to right, especially from the 24 to the 18 frames. However, noise in the results is increasing from left to right and what you may think are features, say in the image to the right, may just be noise. So 75% may be a good compromise but as long as you reject images that are trailed or just plain out of focus, then it probably doesn't make a huge difference. Of course, if you take lots and lots of frames then you can be more picky.
One issue with an alt-az telescope like the Celestron NexStar, is that the field rotates from frame to frame and unless your object you are viewing is in the centre of the the field of view, some additional loss of sharpness is going to come from alignment problems at the edge of the frame.
All text and images © Duncan Hale-Sutton 2021
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