Paul Hyndman wrote:

Non-etalon, rear-mount filter configurations usually require long f/ratios
(f/30-f/32 is not uncommon), making full-disk views less accessible. I
believe the PST works at f/10.. another benefit of an etalon design (the
filter can be mounted in front of or behind the objective).

Most of the sub-angstrom passband rear-mounted H-alpha filtering systems *use*
etalons (internal ones) (I don't know of too many rear-mounted filters which
don't use etalons, except in professional circles, where Lyot-style filters
may be used). The DayStar line of filters does require f/ratios longer than
f/30 for the angles of light hitting the etalon to be within the correct
range. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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