Quote:
Originally Posted by glend
After decades of observation through Newts I have decided to try out the 'dark side' and buy a small refractor. I have a Bresser Messier AR102/f5.9, 600mm fl OTA on the way but I need some advice on that achromatic bug bear - color correction.
Not having to deal with this before, I am lost in a sea of options if I check out the net, what is a good filter solution for achros?
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Different "Minus Violet" filters have cutoffs at different places. The longer the wavelength of the cutoff, the more color tinting appears to happen because more of the violet and blue disappears.
In the Baader line, from weakest to strongest, with approx cutoff:
Moon & Skyglow 400nm (this one has some "notches elsewhere in the spectrum)
Semi-Apo 420nm
Fringe Killer 475nm
Contrast Booster 480nm
Note: 475-480nm is considered deep blue.
400-420nm is considered deep violet.
There are, of course, other "minus violet" filters, but these I am fairly familiar with.
It depends on the scope as to how much taming of the violet is necessary.
An f/6 achromat is likely to need a lot.
I'll attach a chart showing CA versus f/ratio in different apertures.