Quote:
Originally Posted by Shiraz
Hi
Haven't investigated in any depth .

Thanks folks. Have now thought about it further  looks like I was wrong. The camera can do a little less than 13 bits and 512+ subs takes the possible data range to ~22 bits. 32 bit FP can do 2324 bits of precision, so, as Rick implied, it should cover the whole range of values from the camera with a couple of bits to spare  scratch the theory  32 bits FP is good enough for up to about 2000 subs.
the effect is shown in the attached image showing the histogram of a heavily stretched 32 bitFP raw stacked fits file. It may be that this is what 22 bits look like when heavily stretched  there is no noise to hide the fact that data can only take discrete values and there will be values in the histogram that are not produced by the camera/stacking, even with 500+ subs.The other possibility is that the histo binning algorithm in PI does not take full advantage of the high internal precision and the effect is an artefact of it's limited resolution. Whatever... the initial suggestion to use 64 bits is not necessary and whatever the effect is, it is not something that can be (or needs to be) "fixed".
Sorry for the initial misinformation. Must keep thought bubbles under control in future  maybe that could be the guiding principle for all posts on IIS
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median stacking is far worse than just a bit of lost SNR with this camera  every sub has the bottom 4 bits padded with zeros, so the median from any stack will also have zeros in the bottom 4 bits. Since there is no way for noise to fill in the bottom bits, there is almost no way for median stacking to improve the SNR at all. Average works fine.