Was looking at dioptrx, until i realised I already had something better! (maybe needs quotations)

[pipnina]

Despite being lucky enough to have easy access to a southern-sea overlooking B4 site, I've not done much observing for a few years and my dob genuinely had gotten to a point where I needed to remove the primary and carefully clean it! It had big tufty dustballs all over it. I always liked observing, but between the hassle and the 3.5 diopters of astigmatism, I found my views at the eyepiece a bit uncomfortable and I think it contributed to my equipment's lack of use.

So with mars about, and me planning a higher power EP for planets following a revival of desire to observe after a session last month, I got to thinking about how my vision will impact my observing in the future. Naturally I'd heard of dioptrx but none of my EPs can have one mounted to them.

However, I sold my Valve Index VR headset earlier this year to finance my astrophotography addiction hobby, and I had bought prescription lenses that sat over the lenses provided with the headset, a bit more than 1.5" diameter... They're thin and designed to be close to the eye... You can see where this is going?

Turns out, after a quick experiment with my binoculars and my EPs (not attached to telescope), as long as I have long eye relief EPs (currently use 18mm and 20mm relief EPs) I can correct my astigmatism quite well AND maintain full field of view of the EP by repurposing my VR headset glasses!

Of course it's loose, and I'll have to hold the lenses when I'm using them, but it's a step up from all the stars I see being long lines or ovals! An unexpected win!

Edited November 26, 2022 by pipnina

[Astrobits]

I have been considering getting some glasses from online suppliers using only my astigmatism prescription ( i.e inserting zero power in the order for the glasses ) and purchasing the biggest, cheapest frames. These only cost a few quid and are plastic so cutting them down to fit into any recess in my eyepieces will not be costly if I make a mistake. If I put the same corrections into both eyes on the glasses order form  then I have two lenses to cut up and there is a possibility that I could get two goes from each lens depending on the sizes.

A second question occurs to me. Could I put the correction lenses into a filter holder and screw them into the barrel of the eyepieces. i.e putting the correction lens before the eyepiece rather then between my eye and the eyepiece. This I will also try sometime.

I will report back when I get around to this project unless anyone else does it before me.

Nigel

[globular]

9 minutes ago, Astrobits said:

Could I put the correction lenses into a filter holder and screw them into the barrel of the eyepieces. i.e putting the correction lens before the eyepiece rather then between my eye and the eyepiece.

Interesting idea. Please do let us know the results of your trial.

@Second Time Around I know you are a dioptrx expert - do you know if they would work earlier in the light path, or do they have to be close to the eye for proper correction?

[pipnina]

6 hours ago, Astrobits said:

I have been considering getting some glasses from online suppliers using only my astigmatism prescription ( i.e inserting zero power in the order for the glasses ) and purchasing the biggest, cheapest frames. These only cost a few quid and are plastic so cutting them down to fit into any recess in my eyepieces will not be costly if I make a mistake. If I put the same corrections into both eyes on the glasses order form  then I have two lenses to cut up and there is a possibility that I could get two goes from each lens depending on the sizes.

A second question occurs to me. Could I put the correction lenses into a filter holder and screw them into the barrel of the eyepieces. i.e putting the correction lens before the eyepiece rather then between my eye and the eyepiece. This I will also try sometime.

I will report back when I get around to this project unless anyone else does it before me.

Nigel

I think it might work, but I also think when you put it before the EP you need a different shaped and stronger lens, plus spacing becomes important as with corrector lenses for fast visual and photographic scopes. It might be a bigger stress going that route sadly.

[Second Time Around]

22 hours ago, globular said:

Interesting idea. Please do let us know the results of your trial.

@Second Time Around I know you are a dioptrx expert - do you know if they would work earlier in the light path, or do they have to be close to the eye for proper correction?

I'm certainly no Dioptrix expert, witness I don't know the answer!

My gut feeling says it won't work but I'm probably wrong.  However, I too would be very interested to find out.

P.S. My first thought was wouldn't Televue have made a Dioptrx that screws onto the filter thread.  But then I realised that would be too easy to copy.  They want to sell eyepieces, so it would be better for their sales if a Dioptrx fits just their products.  Luckily for us a fair few other eyepieces from other makers will accept a Dioptrx. 

Edited November 27, 2022 by Second Time Around

[allworlds]

I can't find where but I have read that it doesn't work in the filter thread. I think the eye astigmatism correction needs to be on collimated light as produced by the EP, not on light that's coming from/to a focus as it does between the objective and EP.

Makes me wonder if theoretically you could correct eye astigmatism before the objective? Impractical for all but the smallest telescopes but for binoculars the correction lenses would be reasonably sized. Has it ever been marketed?

But yeah. Other than general Tele Vue quality (and any patents they might still have) there's nothing magic about it. An eyeglass lens appropriately fixed to the EP will work, though I'm not sure if off-axis segments would behave well.

[pipnina]

3 hours ago, allworlds said:

I can't find where but I have read that it doesn't work in the filter thread. I think the eye astigmatism correction needs to be on collimated light as produced by the EP, not on light that's coming from/to a focus as it does between the objective and EP.

Makes me wonder if theoretically you could correct eye astigmatism before the objective? Impractical for all but the smallest telescopes but for binoculars the correction lenses would be reasonably sized. Has it ever been marketed?

But yeah. Other than general Tele Vue quality (and any patents they might still have) there's nothing magic about it. An eyeglass lens appropriately fixed to the EP will work, though I'm not sure if off-axis segments would behave well.

Adaptive optics mirrors can intentionally create astigmatism, coma etc. So I imagine they could adapt to the prescription of an observer, or that filter thread lenses could be used as a more practical solution. But I think normal eyeglasses won't work due to, as you say, the shape of the light being an exit pupil instead of a cone. I think it would need some alternative shape to traditional prescription lenses.

[Astrobits]

I have been giving this some thought and have concluded that, as allworlds says, it won't work.

My reasoning is this:

A telescope can be considered to be composed of two components:- the objective and the eyepiece. The objective produces an image at the focal plane and the eyepiece examines that image. If that image is not perfect then the eyepiece simply shows us a less than perfect image and I'm sure that we have all seen less than perfect images in our observations. Placing an astigmatic lens before the focal plane will distort the image which we will see in the eyepiece. Therefore, any correction must come after the focal plane. Unfortunately, the focal plane almost exclusively occurs within the barrel of eyepieces and after the position where filters are attached, and sometimes between the glass elements of the eyepiece. It is clearly, then, not practical, or possible to have the correction filter anywhere other than after the eyepiece.

Adaptive optics are correcting an incoming, distorted, wavefront in order to produce as perfect an image as possible at the focal plane.

Nigel