Quote:
Originally Posted by Don Pensack
Correct. As I said, a Chromatic Ratio of 2 is acceptable for FLP-53/FCD100/CaF2 lenses in doublets.
But you can't work miracles--a 2-element solution only crosses the zero point at 2 places, and it's usually the C & F lines.
That makes an 80mm f/6 OK, a 100mm f/8, or a 6" f/12.
Below those f/ratios, though, color will still appear, especially at high powers.
Celestron marketed a 4" CaF2 fluorite refractor back in the '80s, but it was f/9. I asked them about that and they said they had tried f/7 but "it had too much color".
I think that chromatic aberration seems to be less important to astrophotographers (they can tune/filter it out), but a lot of people are buying astrophotographic refractors and using them for visual use in pursuit of shorter f/ratios.
But no glass is going to make an 8" f/8 acceptable in a doublet.
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The subject scope, APO200FL, is an F11 not F8. TEC have replaced their APO200FL F8 (Triplet) with a APO200FL F11 (Doublet). No doubt we will hear about the performance when the first scopes ship and reviews come in.