Re: coma, coma, coma

Bob Luffel (bobl@hpgrla.gr.hp.com)
Tue, 2 May 95 10:01:50 MDT

>
> Coma correctors, and the Pretoria eyepiece have been mentioned for
> helping with the edge of field sharpness lost to coma. However,
> there is probably something else you can do at the eyepiece end of
> your scope which will help. Just get a really good eyepiece.
> In most fast Newtonians, that do not have really great eyepieces,
> most of the blur at the edge of the field is actually caused by
> astigmatism in the eyepiece, and not coma. Most eyepieces have
> a lot of astigmatism when used at f/5 and below. Correcting for
> astigmatism down to about f/4 was one of the great breakthroughs
> by Al Nagler in the design of the Nagler eyepieces and was one of
> the primary reasons the field of view can be so wide. Eyepiece
> astigmatism increases rapidly with increasing angular aparent
> field of view (I think it is quadratic). Coma only increases
> linearly with angular field of view so it is not that much worse
> 40 degrees off axis than it was 30 degrees off axis.
>
> There was a visual comparison review/test published a while back
> (I think it was in ATMJ) between a Nagler and a Pretoria. The
> result was basicly, it looks better through the Nagler ie. it is
> more important to correct the eyepiece astigmatism than the primary
> coma. I think that it stands to reason that if Pretorias were that
> great, then they would have sold a lot more of them, and they would
> still be available...
>
> Assuming your mirror is ok, I would suggest borrowing and trying a
> Nagler eyepiece sometime before regrinding your mirror to really
> evaluate how bad that coma is to you.
>
> Chuck
>

My experiences agree with this completely. I have compared the views through a Pretoria versus a Panoptic (similar focal lengths). Star images at the edge of the field in the 68 degree Panoptic were easily as good as those at the edge of only a 55 degree field in the Pretoria (the Pretoria is not a wide angle eyepiece - one of my gripes about it). This was through a 16" f/5. Naglers extend this performance to 82 degrees. In fact the owner of the Pretoria sold it to purchase the Panoptic.

Still, the Pretoria gave excellent views. If you don't mind the narrower field and can find one cheap, it is worthwhile. Naglers and Panoptics are still the king/queen of eyepieces for fast scopes.

This begs a question - since the Nagler designs there have been no further significant advancements in the eyepiece state of the art. What is next? Aspherics? (I don't think just adding more elements for wider and better correction is the answer)

I have an old WWII Zeiss 90 degree field binocular objective that uses only four elements (supposedly, the eye lens has an aspheric surface). It performs about like an Erfle at the edge of the field (but is 90 degrees vs the 70 degree Erfle I am comparing it to). Why can't someone mass-produce an aspheric eyepiece?

Bob