[codemonkey (Lee)]

I recently took 3x600 of each R, G and B for NGC 55, figuring that I'd capture small parts of each colour every night and that way I could see it improve as I went, rather than capturing all the R on one or two nights and having to wait until I did everything before I could see the results.

Anyway, when I did this I noticed that the stars were green on one side and red on the other. I thought maybe this was because I only had a small number of subs so they didn't average out slight guiding errors, so I did an experiment last night.

This is NGC 253, obviously, but there's 15-18 subs of each R, G and B here. The subs are short and the moon is obviously about 80% full; I wasn't trying to capture a good image, just test this theory.

I thought that 15 was a good enough sample size that it should average out any small guiding errors. Unfortunately I still have the problem, although it's blue and green this time.

Something to note here is that it seems as though the star shape is different between the colours, not just bigger / smaller or more / less defined, which is why I was thinking it was a guiding error.

Does anyone have some suggestions on what this is? For all the other alternative causes I can think of I would expect to see different size stars, not different shaped stars.

Unless the stars aren't actually a different shape, but a different size and the registration process is getting the center wrong and offsetting them... hmm...