Quote:
Originally Posted by AlexN
First off mate, what are your optics? Some refractors will simply not focus blue as tightly as it will focus other colours.. I'd agree with Fred, Deconvolving the blue channel to to get the stars the same size as the rest is the way to go... My setup required a slight focus change L and R were equally focused, G was slightly out, and blue slightly out again. (with my refractor) however refocusing between filters fixed that up no worries.... I use an LPS inline as robin mentioned, however I don't know how that would make much of a difference? unless your LRGB filters are not UV/IR blocking and the LPS is. My LRGB are UV/IR blocked - I saw no difference in star sizes when using the LPS filter...
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I am using a 127mm APO from North Group, the filters are the Astronomics series II and are both IR blocking and parfocal and the CCD is an Opticstar 145M (Cooled Sony EX ICX285AL at 1.4" per pixel). I know focus on these is critical from using it with my 20D but I have made careful adjustments using a Bahtinov mask and using the focus routines in AA4 either way for the same exposure my stars in B are 50% bigger than the R and G frames.
I have done a few more trials since my last post - but even if I run LRG then refocus for B I get still significantly larger blue stars, I have tried several tricks to tackle this but with only limited success, RL deconvolution does reduce the star sizes a little but it causes rings to appear if pushed hard, I guess I may be better off blurring the G and R and then trusting to the L frame to get the details out?
I think the optics are ok - the attached Ha image of NGC3372 was done as a test and I am happy with that - perhaps NB is my best bet anyway due to the light pollution...