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Hi Steve
By "acceptable level of chromatic aberration" I was referring to the theoretical formulas put about by Sidgwick and others.
As i said, I've never owned a refractor and have only looked through a couple over the years.
Quoting from Phil Harrington's book "residual chromatic aberration will not be a factor as long as an achromatic refractor's focal length is equal to or greater that 2.88 times its aperture in inches".
Another version is that the focal ratio should be 3 times the aperture in inches.
I expect that this is only relevant to high magnification viewing of bright objects which is what I want to use my scope for.
Chromatic aberration is not just an annoying false colour, it degrades the image as well.
My original question related to stopping down for the moon, venus and maybe some of the other bright planets where there is already a lot of light.
For deep sky obviously you would use maximum aperture.
I'd be interested to hear how you get on if you do decide to increase the focal ratio of the vixen for lunar/planetary. You might get away without the filter. Even at 100mm the moon is pretty bright.
Cheers
Robert
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