[ATM] Re: fast lurie-houghtons

[Vladimir Sacek <vladis.2@juno.com>]

>Is it still the consensus of knowledgeable ATMs that a
fast Lurie-Houghton would require highly corrected
(i.e. Nagler-type) eyepieces?<

Don't know about the consensus, but there is no reason why they wouldn't
benefit from high-corrected eyepieces. Their only aberration is
astigmatism, that should be opposite in sign to that characteristic for
eyepieces, but with eyepiece astigmatism being several times stronger it
doesn't make much difference. If you look into Rutten/Venrooij, thier
raytracing shows that astigmatic blur in regular-type eyepieces at f/5 is
approximatelly 5 arc minutes 10 degree off-axis, increasing with the
square of off-axis distance, and in proportion to the F-ratio. A
Nagler-like ep is somewhat better at 10 degrees off-axis, but its main
advantage is that blur doesn't go much over 5 arc minutes all the way to
the edge.

Ep astigmatism is more of a concern at lower magnifications, but has an
effect at high magnifications too. If you take that angular size of the
Airy disc in the eyepiece is 4.6F/f in arc minutes (F=f#, f=ep f.l.), it
will be 5.75 arc minutes with a 4mm ep at f/5. For the diffraction
limited level, astigmatic blur can't be more than 0.6 the Airy disc
diameter. which would put diffraction limited field radius with an
ordinary 4mm eyepiece at f/5 somewhere around 8 degrees off-axis. This is
only about 1.1mm field diameter. A Nagler-type ep would have it
significantly larger, and also much better quality of the outer field.

Vlad
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