optician's trial lens for telescope use?

[waz]

Hello!

I have a astigmatism developing in my eyes, -0.50 each eye.
I also use a telescope. I prefer to observe without my glasses on if
possible but this is getting to be a problem.

I do not have any test lenses to try, so I have to ask...

In theory, would it be possible to mount an optician's test lens (let's say
to correct for -0.50 cylinder) BETWEEN the telescope eyepiece and the
objective and have it correct the view for me?? Could this lens be placed
somewhere in the barrel of the eyepiece?

(I am sure it would work between the eyepiece and the eye).

Would the distance between the test lens and the eyepiece lenses be critical
or would a centimeter either way be usable?

If this trial lens (-0.50 cyl) would work for one eyepiece of a given focal
length, would work for several eyepices of different focal lengths?

I was thinking of trying some "quality test lenses" but thought I would ask
my question here before I looked into where and how to obtain them.

Many thanks for your thoughts on this.

Waz

waz...@sympatico.ca

[pmer...@breathe.com]

Sorry, I forgot to reply to group....

----- Original Message -----
From: <pmer...@breathe.com>
To: <waz...@sympatico.ca>
Sent: Sunday, September 11, 2005 12:13 PM
Subject: Re: optician's trial lens for telescope use?

> Hi Waz,
>
> -0.50 is a tiny amount of astigmatism and through the reduced entrance
> pupil
> of a telescope eyepiece it might give no subjective improvement. Perhaps
> the
> addition of 2 air/glass surfaces out weigh the benefit of the Rx . But if
> I'm wrong, perhaps your optometrist could get you a cyl made. (s/he might
> lend you one to try). You should ask for a flat lens (not a normal
> spectacle
> meniscus lens which is usually on a base curve of about -5 dioptres)
> preferably with anti-reflection coating. The cyl means nothing without an
> axis. This will be on your written Rx- clockwise from horizontal as you
> look
> forward (add/subtract 90 degees if it's written in "plus cyl") but you
> could
> manually rotate it to find the best axis.
>
> I'm pretty sure that the only place for the lens is between eye and
> eyepiece. Anywhere else would mess with the pricise lens system design. As
> close as you like is ok, still better than specs getting in the way.
>
> Hope this helps,
>
> Pete (Optometrist, Leominster U.K.)