Astigmatism Correcting Filter

[westmarch]

I came across this old thread that interested me.   There are not enough responses for me to be sure that it worked but if I could get a filter made with my astigmatism prescription, what is to stop me:

(a) inserting it in my favourite eyepiece(s) at the place where I would normally screw in a filter.

(b) putting it in the rubber eyecup (as I often do when swapping filters to compare contrast).

I realise that the astigmatism correction depends on getting the rotation of the filter/eyepice just right but that cannot be an insuperable obstacle - it surely only needs me to mark the rim of the filter at the correct point.

I am hoping that someone with a  better understanding of optical pathways than me can advise. ?

John

 

[Ruud]

It is an interesting idea!

Could it be that astigmatism correctors only work predictably when used behind the eyepiece, right where the error occurs? The strength of a Barlow, for instance, depends on its distance to the field stop. I imagine that an astigmatism corrector would behave in a similar fashion.

The distance between the bottom of the barrel and the field stop varies from eyepiece to eyepiece. That may be the problem. 

[Louis D]

Yes, that is basically what the TV DIOPTRX does.  It cams down onto eyepieces in place of the removeable eyecup.  It has a slight range of  diameters it can accomodate.  Once fixed in place, the top is rotated to the proper orientation to correct your particular astigmatism axis.  Of course, each DIOPTRX only corrects one strength of astigmatism and must be removed to share the view with others not having identical astigmatism.  It is made of optical glass, so it is better than any polycarbonate eyeglasses.  The eyepiece still needs to have decent eye relief because it eats up about 15mm of it.

If you were talking about cutting a lens and then fitting it into a filter ring and attaching it to the bottom of the eyepiece, that has been done as well with the help of an optician.  I read about one fellow years ago getting his optician to cut a unit power, astigmatism correcting lens to fit into a 2" filter ring.  Once screwed in place, he just rotated the entire eyepiece before tightening it in the focuser to align his astigmatism axis with that of the filter.

[westmarch]

Thanks both for your responses.  I have thought about TV Dioptrx but do not want to restrict myself to only TV EPs.  I was hoping that the filter option would make the solution more generic.

John