Correcting astigmatism at the filter location

[azzythehillbilly]

Hi all,

I have astigmatism of both eyes . The extent is +0.5 diopters.

I have noted the availability of the dioptrx astig corrector, which is a lens, that is attached to the top of the eyepiece . It has the value of correction equal to correction required by the eye.

I happen to have about 10 pcs of 1.5 inch filters of which I am not using several any more.

My optician has told me that he could replace the filter elements with 0.5 diopter corrective lenses. However these filters would screw into the field end of the eyepiece, unlike the dioptrx which sits astride the eyepiece on the "eye end"

I am wondering if it will work the same way or is there some subtle difference such that some other value of power (diopter) would be needed as the correcting lens will now sit on the opposite side. If the power needed is some other value that i can estimate i can get that too. The optician does not require me to present a prescription, just my word.

Would highly appreciate any guidance.

Azzy

[Stargazer_00]

Im just thinking here. Astigmatism of the eye is in a single plane, right? All im thinking is if you screwed a corrective lense into an eyepiece then you would struggle to keep the orientation correct. It's reliant on how much thread there is to screw, where the leading edge is cut and stuff possibly? You could, i guess, mark the outside of the filter so you know once its a tight fit where the axis is you need to orient your head to align to it. this would be obscured once inside the scope though and you would be limited to keeping your head in that alignment to get the benefit of the correction. In a diagonal this would be easy enough possibly but in a newtonian where your alignment to the scope changes more easily, this may present a greater challenge.

In terms of how this would work optically in a light path, there are so many variable i suspect only someone with previous knowledge of this kind of approach or an actual optical lense manufacturer would be able to tell for sure.

[username]

My first thought is that whilst it may be possible I'm not sure if the correction before the eyepiece would be the same as the correction after the eyepiece. Whilst the angle of the astigmatism may be able to be easily corrected for the degree of abberation would be changed by the eyepiece. So +0.5 before eyepiece may come out as +1 at the eye end for examples sake. Plus the ey

epiece can introduce distortions themselves.

Secondly do you really need correction at that degree of astigmatism? I have some in one of my eyes and I can observe fine with it. I can't remember my perscription off the top of my head but I think it's 0.5 for short sighted astigmatism.

Basically only eyepieces which give low enough magnifcation so that the circle of light coming out of the eyepiece (the exit pupil) is large enough that it gets affected by your astigmatism should need correcting. If the exit pupil is small enough so that the surface of the eye its hits is locally just spherically aberatted then you should be fine. Check this chart out at the bottom of the page. It's quite likely you can get away with it

http://agenaastro.com/televue-dioptrx-astigmatism-corrector-0-25-drx-0025.html

Have you tried observing without your glasses? Plus you can get eyepieces which have enough eye relief for glasses if needed.

[ronin]

Are you sure you have astigmatism ?

Also a 0,5 diopter correction is almost nothing, it is in the region that unless you were told you had it that you most likely wouldn't be aware of it.

Astigmatism is at a certain angle, the correction is the addition of a cylindrical lens component, so any corrector applied at the eyepiece would mean the eyepiece would have to be always orientated at the same position relevent to your viewing eye. This also relies on the insert being marked to know the orientation of that.

I think my astigmatism is at 85 degrees and is I think (long time since I read the prescription) more then 0.5, however I often find it better to take my glasses off and everything is still nice point sources and that is without any astigmatism correction.

[alan potts]

In my opinion if this worked why would Televue not have done it in the first place, all thier eypeiecs have threads and if you can afford Televue you could afford a 2 inch version as well as a smaller one. Taking it a step further why not just screw it into the diagonal. I don't think it works the same hense it hasn't been done this way, in needs to be where the glasses are worn.

Alan.

[azzythehillbilly]

Hi and Thanks Stargazer_00, username , ronin and alan potts. All that was useful.

Yes Alan, that crossed my mind. If It was possible to correct for astigmatism this way why would not teleview do just that. But could be some other reason such as convenience of use etc. Or just plain marketing hype

I guess I will just go ahead and have the filter modified by the opto. In any case he wants only 2/3$ for the job and if it does not work I can try a different diopter power.

I will report back the result in case it might be of use to others. I have checked to find that I have 4 or more fiters I do not need/use so if the idea works can have one for each of my low power eyepieces.

Bigger problem is the cloudy skies for the last two months, not a single useable day.

Thanks again

Azzythehillbilly

[alan potts]

It will be interesting to know how you get on with it, I am lucky that I don't need glasses for this reason but I do need them for reading and finding my glasses. I am so lucky with the weather here, it's wall to wall blue outside at the moment.

In the words of a famous tennis player from years ago the weather in England is the pitts, that was followed but a number of orange squash bottles being knocked off a table.

Alan

[cs1cjc]

I am pretty sure this would work and will be interested to hear the outcome. However 0.5 is a small correction and Tele Vue suggest that it would only be visible with an exit pupil larger than about 4mm. See: http://www.televue.com/engine/TV3b_page.asp?id=54&Tab=_Choose

[Stargazer_00]

I'm pretty sure i had my astigmatism corrected when i had my lazer surgery.

I haven't been to the opticians for nearly a decade now since i had the upgrade on my eyes. I should go and get an MOT I guess. since I've gotten into this hobby i spend ages looking after my eyepieces and scopes I should really go and ensure the Mk1 eyeball is running tip top too.

[Spookylight]

The astigmatism on my left eye is so bad that the left lens in all my gigs does not fit the frames

I should of got rimless. One lens fell out coz its so oddly shaped.

Anyway, i just remove said gigs when viewing and seem to get on ok, so far, dont have good eyepieces yet tho.

[azzythehillbilly]

Hi guys,

I got the corrective lenses completed and returned on Saturday by the optician. He cut two lenses, one in coated plastic and the other in glass.

I have checked them both. incidentally the optician failed to mark the axis. I therefore tested them by rotating the eyepieces. I tested them with the 20mm and 30 mm eyepieces (with my 650mm Newt i.e.,. Mag. of 30x and 20x). I screwed the lenses on to the bottom of the EP and looked at the nearly full moon. The moon was the only object I could see as the sky was cloudy and has been cloudy here for the last four months, non stop.

It appears to me that the corrective lenses do work. To make a fuller test I would need two eye pieces, with and without the correction, but haven’t.

The weather here has been terrible for the last four months , no day without rain and no clear nights. As soon as I can see a star I will test again with the star, or some nebula.

The lenses are curved (meniscus), like the usual opticians glass blanks so I will ask the opticians if he can arrange flat blanks. I wonder what effect swapping the sides. I have set the convex side towards the reflector.

Till then I will not pass my final judgement. As of now this method seems to work quite well.

Practically speaking, my better half needs her own eye pieces as it is not convenient to repeatedly screw/unscrew the filter each time we swap eyes.

Azzythehillbilly

[azzythehillbilly]

I have uploaded the pictures of the mod to my album <azzythehillbilly's_Album > but the post says "not found".

Can they be seen by clicking on the missing image place keeper?

Someone kindly tell me and/or guide why!

Azzythehillbilly

[cs1cjc]

...

Practically speaking, my better half needs her own eye pieces as it is not convenient to repeatedly screw/unscrew the filter each time we swap eyes.

Yes, viewing with other people is good reason for viewing with spectacles. It simply takes too long if you have to refocus each time you come to the eyepiece.

[Astrobits]

In my opinion if this worked why would Televue not have done it in the first place, all thier eypeiecs have threads and if you can afford Televue you could afford a 2 inch version as well as a smaller one. Taking it a step further why not just screw it into the diagonal. I don't think it works the same hense it hasn't been done this way, in needs to be where the glasses are worn.

Alan.

I don't see why this would not work. Any astigmatism in the mirror/lens comes through the eyepiece so why not put the eye's astigmatism corrector at the filter end instead of in between the eye and eye lens. However, the corrector will have to be removed every time you change eyepiece, or want to show someone else the view.

Alan,

Televue, and any other manufacturer, would not include astigmatism correction in the eyepiece as everyone has a different prescription and fitting a correction element would limit sales to those with that particular prescription. Also the eye's astigmatism can change with time so an eyepiece that corrected my astigmatism today might well not work so well in 6 months time. I would sooner throw away the simple corrector that fits onto the eyepiece than have to find another buyer for my expensive astigmatic eyepiece.

Nigel