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RE: [ATM] Pretoria eyepiece

Gianfranco, Peter,
You both mentioned Don Dilworth's 90 degree ocular design, patented in the
late 80's.  This came in both coma-correcting and non coma-correcting
variants.  Two prototypes were made of each version around that time.  I
have used the non-CC version on Dilworth's 16" relay telescope (which gives
a prime focus image which is flat and essentially aberration-free) and on a
7" Mak-Newt.  Although they only used MgF2 coatings, they performed
outstandingly well.  Although the 8-element design of the 16mm generated
some veiling glare with those simple coatings, the eye relief was
comfortable (14mm), there was little spherical aberration of the pupil
(kidney bean) or vignetting and the resolution appeared to be limited only
by my visual acuity across the entire field.  I was enormously impressed
with the performance on both instruments.  Anyone seeking more info about
these designs should contact me off-list.

Tony Owens


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