Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Beginner thinking of solutions for astigmatism


Recommended Posts

Hello!  I'm very much a beginner in astronomy, and doing a very gentle introduction.  Don't have my own instrument, but I'm fortunate to live near an observatory that has public viewing.

Recently, I bought a small spotting scope to take on trips.  I primarily want to look at the Moon, the larger planets, and whatever is visible.  Maybe take a few pictures through the eyepiece.  It's a 20x50mm scope that costs very little and is light enough to be mounted on a camera tripod.

But now I ran into a problem.  My eyeglasses correct my astigmatism fairly well (to 20-20), but they are a fancy "three-zone" type, and it's very difficult to position the "zone" focused to infinity into the right place in front of the eyepiece.  It's the very tip of the glass.

So I'm thinking to ask my optometrist to order a corrective lens that would be round and small enough to fit into the eyepiece, and I'll somehow fix it in there.

What do you think - will this work?  Are there better solutions?

Thank you so much for any suggestions!

As a way of introduction, here's a story about beginners luck:  The very day it came in the mail (two days ago), the skies were partly clear, so I set up the little spotting scope on a camera tripod right next to our house.  I had hoped to see Saturn, but it was obscured by trees.  So I just started looking around.  Vega was near zenith and amazingly bright, and I could see a lot more stars through the scope than I could with my naked eye.

So I called my wife to check it out.  She comes out the door, looks though the scope, and within a couple of seconds, gets to see a satellite sailing through the field of view!

Link to comment
Share on other sites

I too have astigmatism and I often observe without glasses and I do not have too much of a problem. I do try with glasses on but find it more awkward due to getting decent eye relief.

I would think getting a custom insert lens made would be very expensive and the money better spent towards a good 'scope.

  • Like 1
Link to comment
Share on other sites

Hello Boris9,

I have slight astigmatism, and normally wear glasses of the varifocal type, which might be similar to your three zone type.

I got a pair of glasses made up that are simple 'distance only', but including the astigmatism correction. They are cheap and work well at the telescope.

Link to comment
Share on other sites

I use Telviews dioptrx which fits over the eyepiece to correct your astigmatism, but doesn't correct your long or short sightedness which you correct with the focuser. I don't know if a dioptrx would fit on your eyepiece.

Link to comment
Share on other sites

Thank you all for the information!  Yes, currently I view without glasses.  It's just that little extra bit of sharpness should make a noticeable difference when looking at the planets at small magnification.

I had thought about ordering eyeglasses just for viewing, but my experience with eyeglasses so far has been discouraging.  I always end up just taking them off.

Thank you so much for the info about Dioptrx, Richard!  That sounds like just what I was thinking of rigging myself, but of better build quality.

(Except It won't work with this particular spotting scope because of its short eye relief - I just looked it up, it's just 10.6 mm.  Sigh).

At any rate, now I know what to get when I'm ready to buy a true telescope :)

Still, it answers my question: If Dioptrx can work, and the eyeglasses can work (except for the convenience factor), that means a normal prescription lens will work, as long as I can find a way to squeeze it into the eyepiece.  It looks like Dioptrx requires long eye relief simply to make room for the frame.  So if I figure out how to get a lens that fits into the scope's eyepiece and come up with a way to attach it there, it should work.

Actually, attachment should be no problem - the rubber will hold the lens in place.  All I need is the lens of the right size, which shouldn't be too difficult.  I can just order a normal eyeglass lens, then grind it down to size.  I've worked with polycarbonate before, it's a remarkably forgiving material.  Excellent!

Thank you so much!

Boris

Link to comment
Share on other sites

Hi Boris,

You might find that simply using an astro telescope and astro eyepiece, rather than a spotting scope, will give you the sharpness you are looking for. The prisms inside a spotting scope, depending on their quality, will probably rob the images of the planets and other astro targets of some sharpness and contrast as well as introducing some light scatter.

Link to comment
Share on other sites

Hi Boris,

You might find that simply using an astro telescope and astro eyepiece, rather than a spotting scope, will give you the sharpness you are looking for. The prisms inside a spotting scope, depending on their quality, will probably rob the images of the planets and other astro targets of some sharpness and contrast as well as introducing some light scatter.

Thank you John!

That little scope has surprisingly decent image quality - I guess those sharpshooters are particular about distinguishing individual bullet holes from 100 yards :)  The prisms are Bak-4 porro-type, the same type they use in astronomy binoculars.  The viewfinder is non-zooming, so the field of view is wide (60 degrees) and quite bright.  I haven't noticed distortions when panning, and there are hardly any chromatic aberrations (to my eye, at least).  It's basically a half of a decent binocular - so I don't get to view with both eyes, but also don't have to deal with all the alignment issues between the two tubes.

I have an idea what a view through a nice telescope is supposed to look like - I live within a short drive from an observatory that has public viewing, and they have a very nice guide scope (a 6" Takashi APO refractor that costs like a brand-new car.  The main instrument is a research-grade Planewave CDK 20", which costs like a brand-new house, but it's camera-only).

I'm not ready for a full-blown telescope yet, just don't spend enough time observing to justify the expense.  Also, I want something that I can take on a backpacking/mountaineering trip.

Link to comment
Share on other sites

Thank you all for the information!  Yes, currently I view without glasses.  It's just that little extra bit of sharpness should make a noticeable difference when looking at the planets at small magnification.

I had thought about ordering eyeglasses just for viewing, but my experience with eyeglasses so far has been discouraging.  I always end up just taking them off.

Thank you so much for the info about Dioptrx, Richard!  That sounds like just what I was thinking of rigging myself, but of better build quality.

(Except It won't work with this particular spotting scope because of its short eye relief - I just looked it up, it's just 10.6 mm.  Sigh).

At any rate, now I know what to get when I'm ready to buy a true telescope :)

Still, it answers my question: If Dioptrx can work, and the eyeglasses can work (except for the convenience factor), that means a normal prescription lens will work, as long as I can find a way to squeeze it into the eyepiece.  It looks like Dioptrx requires long eye relief simply to make room for the frame.  So if I figure out how to get a lens that fits into the scope's eyepiece and come up with a way to attach it there, it should work.

Actually, attachment should be no problem - the rubber will hold the lens in place.  All I need is the lens of the right size, which shouldn't be too difficult.  I can just order a normal eyeglass lens, then grind it down to size.  I've worked with polycarbonate before, it's a remarkably forgiving material.  Excellent!

Thank you so much!

Boris

You will need to be wary of the eye relief. Unless your new lens sits flush with the eyepiece glass it will shorten it, to what degree depends on how innovative you are I suppose :)

I'd be interested to here how you get on when you try it.

Link to comment
Share on other sites

Hi I like yourself wear glasses, varifocal type in my case. I very rarely observe with my glasses on, I tend to use a neck core on my glasses and pop them back on to read star charts. I can use glasses on a couple of my 2" EPs that have good eye relief but still prefer to view without them.

Link to comment
Share on other sites

Hi I too suffer with astigmatism, I don't find any issue with observingplanets or the moon without my glasses, also have verifocals, the only issues I have is with stars.

I would discuss your problem with an optician as I don't think it's as simple as just hacking a lens down to fit. I believe lenses to correct astigmatism have an axis and this has to be in the correct orientation to your eye.

Link to comment
Share on other sites

Hi I too suffer with astigmatism, I don't find any issue with observingplanets or the moon without my glasses, also have verifocals, the only issues I have is with stars.

I would discuss your problem with an optician as I don't think it's as simple as just hacking a lens down to fit. I believe lenses to correct astigmatism have an axis and this has to be in the correct orientation to your eye.

Yes, I had a hint that this may be the case when I looked up the eyeglasses prescription notation - turns out, one of the three parameters is the angle:

http://www.allaboutvision.com/eyeglasses/eyeglass-prescription.htm

So I tried turning my eyeglasses in front of my eye, and sure enough, the sharpness disappears almost immediately.

This should not be a huge problem though.  My latest plan is to buy a 25mm diameter polarizer filter (they have a part that can rotate), break out the filter, and glue in the prescription lens in its place - this way, I'll be able to rotate it independently of the focusing.  However, I want to finish my digiscoping set up first.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.