I am not sure is this question goes in this forum but it seems like it could belong to more than one.
Last night, I was doing OIII images of SH2-129 (Giant squid nebula). My exposures were 8 and 10 min to a total of 7h. I noticed that there is a shift between subs and it is more obvious when compare for example frame 1 vs frame 8 (about 40 min later). My stars were round and the total guided RMS about 0.7". Any idea what can cause that?
Thank you so much