[ATM] 22" F/4 primary, F/6 system

[Dominic-Luc Webb <dlwebb@canit.se>]

I deleted the original post, but have it in memory. I long ago
tried such designs and often had problems getting decent spot
diagrams. I suspect this is why so many of the designs have
higher amplifications. I see some real hardships in producing
anything but a spherical primary much below F/4 (I have some
plate glass F/3 mirrors I am struggling to get spherical, let
alone any more complex conics). Then we are starting to talk
about Maks and Schmidts, although, there are some Schmidt-
Cassegrains and Mak-Cassegrains around F/6. The primaries on
these systems are typically about F/2.5 spherical. The DKs I
know of that have F/4 primaries have system focal ratios
between 12 and 24, consistent with my difficulty in finding
good designs with amplification factors as low as 1.5. There
should be quite good commercial focal expanders for such low
amplification, so maybe others already know what I have
discovered, that low amplification is not the way to go on
these folded systems. I think there are some F/3 primaries
in a DK config, but I even these had system focal ratios of
about F/9.

More than that, it seems like a lot of fuss to produce such a
challenging primary for an amplification of only 1.5, as
compared to an F/6 in a Newt config. Is there a reason you
need such a config?

Dominic-Luc Webb

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