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Eyeglasses, Astigmatism, and EPs

Started by dwalton, 07/28/2004 01:11PM
Posted 07/28/2004 01:11PM Opening Post
I'd like to get some clarification on the assertion that short focal length (i.e. high power) EPs allow you to not use your glasses, even if you have an astigmatism.

At what point does this principle take effect? Assume a 14.5" f/4.3 dob for this example. Can I expect to be able to use a 3-6 Nagler zoom without my glasses, and not have my astigmatism affect the view? (That would be about 317-528x without a Paracorr, with exit pupils going from 1.2 down to .7)

Is there a formula relative to exit pupil that gives an answer? Is it variable based on the amount of astigmatism one has?

Thanks, and if I need to provide more info, let me know. Obviously, I'm wondering if I should buy a 3-6 Nagler zoom - it doesn't have enough eye relief for use with glasses, but it has plenty without.

Doug

Posted 07/28/2004 02:24PM #1
That gets me to thinking. Given all the discussion on the quality of optics and the field results therein, what is the effect of eyeglasses on the optical train?

I would guess the wavefront error isn't very great on eyeglasses.
Posted 07/28/2004 02:50PM | Edited 07/28/2004 02:51PM #2
Doug,
I have been cursed with severe astigmatism to the degree that I can’t wear contacts and am forced to wear glasses for all of my viewing. I have tried not wearing my glasses at high magnifications and still get a much more resolved image when I put my glasses back on. The single most positive experience was switching from single EP viewing to a binoviewer. I know you did not want to hear that, but using both eyes, even through glasses lets me see more detail in all objects. Your ability to view without glasses at different magnifications is totally dependent on your degree of astigmatism.
Posted 07/28/2004 04:06PM #3

I'd suggest trying some other eyepieces, barlows, etc. you
already have, and see if 1.2mm exit pupil (317x) is OK for you.
Then decide about the Nagler 3-6 Zoom. It only depends on exit
pupil, not the eyepiece type.

> Is there a formula relative to exit pupil that gives an answer?
I expect the error decreases very rapidly as exit pupil
gets smaller; probably goes something like the square of
the exit pupil.

> Is it variable based on the amount of astigmatism one has?
Yes.
Posted 07/29/2004 06:25AM #4
I guess I'll do some experimenting with EPs and see how it goes!

Doug
Posted 07/29/2004 10:55PM | Edited 07/29/2004 10:58PM #5
I encountered this problem 12 years ago. So I tested my astigmatism at various exit pupils. In reasonable quality eyepieces, it isn't detectable at exit pupils of less than 2.5mm, is just starting to get pronounced at 3mm, and is pretty bad at 4mm or larger exit pupils.

If there's some low power (large exit pupil) eyepiece that you use a lot, you can always stick an old eyeglass lens over it. Just get the guy at the eyeglass shop to grind down your old prescription lenses to a circle, mark where the top is with a notch for right eye, two notches for left, and then stick one over your eyepiece with reuseable adhesive (Blutack).

I've got old lenses stuck onto my 7X50 Fujinons and 11X70 Nikon binoculars else the view be too disgusting. And one stuck on my 40mm eyepiece.

I have the misfortune of not being able to wear eyeglasses at the telescope. When I do, I can't achieve focus on my star atlases.

The good thing about astigmatism is that it saves you money - there little point buying very expensive, premium low to moderate power eyepieces. You can instead just concentrate on the high power ones.
Renato