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Douglas Denny asked, about the cap that fits over the eyepieces for my old
Vernier sextant, with an azimuth mark to define which-way-up.-

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"Is there any refractive power in this lens?"

I had thought not, but on closer investigation, I find that it does
displace the image of a line-object, very slightly, as it's moved across
the lens aperture, and this depends on the orientation of the lens. But it
displaces that image MUCH less thandoes a 1-dioptre lens does, so I
conclude that it is indeed very weakly astigmatic..

"It might be a supplementary lens for someone with refractive eye error,
and with a mark on it might be an astigmatic lens - in which case there
should be a definite elongation of a star image through it. I still take my
eye-glasses off for sextant use and have to adjust the telescope eyepiece
for best image, which is not as good as if a proper correction is present:
glasses make it difficult with the exit pupil of the telescope - reducing
field of view."

That might well be so, but it seemed to make no difference to my view of
Jupiter, any way round. However, even my "good eye" isn't up-to-much these
days; it's due to get a session of retinal lasering tomorrow.

"It might just have a mark however, to show which is the best position of
the lens fitted to the telescope, as lenses up to post-WW2 possibly up to
1950s were hand-edged and were never a perfect circle so might fit better
in a particular way in the fitting; or the brass fitting might be slightly
oval and the same applies when fitted to the telescope."

There's a Galilean telescope, two eyepieces (with different gains) to fit
the inverting telescope, and a peep. This cap can be fitted over all four
of these eyepieces, having a thin brass edge, with cuts in it, to make it
springy enough to do so. It will fit in any orientation.

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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