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I had written, about the additional lens supplied with my old sextant-

...so I conclude that it is indeed very weakly astigmatic..

However, even my "good eye" isn't up-to-much these
days; it's due to get a session of retinal lasering tomorrow.
============

Douglas commented-

"Hmmm.   I should have thought it was likely to be an astigmatiser lens,
but very weakly astigmatic will not spread the star image out at all if
placed on the eyepiece; it will however had a definite effect if it is
placed in front of the objective of a telescope.
I took out my lens case to try it, to find the suitable amount of
cylindrical power to use with a sextant telescope.  Even just a half
dioptre lens in front of the objective makes a nice small line with the
laser spot on the wall across my living room. Nothing seen with the
eyepiece of course.

Are you sure it is for the eyepiece or will it fit the objective? Some
older telescopes have the same continuous diameter tube front end and
back."

No, it doesn't fit any of the objectives. It's certainly intended to fit
over the eyepieces.
------
Douglas kindly added-

"Sorry about the eye problem, I sincerely hope all goes well for you."

It's something that many of us diabetics have to get used to, to fend off
worse. I've had retinal laser treatment before, but that was in my left
eye, a "lazy eye" since childhood, so they've been welcome to do what they
liked with it. This time it's my "better" eye, and I'm a bit more cautious.

George

contact George Huxtable, at george{at}hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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