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Guys,
You all know you can't use a "filter" to turn an achromat into a full blown APO. It's impossible! If it were, these "wonder filters" would be selling like hot cakes.
All filters just reduce the light transmission at nominated wavelengths.
There is one filter, which I can assure you gives 100% APO performance and that's a very very narrow band OIII/HB/Ha filter!!
Doesn't let any other wavelengths through - that's a bit of a problem....
I use Edmund achromats in the Spectra-L200 spectroscope and have to accept that the "best" focus varies about 1.2mm from the Green/blue to the green/ red - such is life.
For "normal" astro work, the UV "halo" seems to be worse problem.
This can be removed, without really affecting the other wavelengths with a UV-IR filter (or the ol' UV blocker filters that Lumicon used to sell)
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