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Originally Posted by gregbradley
APO by definition means 3 colours in focus at the same point. Its a bit of loose definition in practice as you get manufacturers claiming things to sell their scope.
TEC140 for example lacked some blue correction and this did not really show up until the Sony ICX694 came on the scene which has much better blue sensitivity than the Kodak sensors (for example its 60% QE in O111 which si about the highest of any sensor).
Peter had a lot of trouble with this. I then notice Yuri improved the latest model of the TE140 to improve the blue correction. These are design parameters the optical designer/manufacturer can tweak.
I had an FS152 fluorite doublet. It was extremely sharp visually but it would tend to get some blue halos on bright stars. This is blue out of focus compared to red and green.
Perhaps others can say, but I was not aware simply refocusing the blue filter fixed it. I thought it was simply the optics don't handle the blue wavelengths well and may not focus it well at all. Worth finding out.
Dr Rhor I think his name is posts tests of various scopes. His tests reveal all the weaknesses even from high end scopes. A German site.
Greg.
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It appears to be an effect with these incredibly high QE (in blue) and most refractors. I don't think it can be focused out although I will try using FWHM for focusing as opposed to a bahtinov mask and see if that makes a difference.
As with Peters testing, having a UV block makes a difference so it think it is less to do with blue correction and just UV correction. I am going to try and see whether 2x2 binning fixed the problem. Worth giving a shot.