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Old Saturday 26th August 2017, 09:49   #113
Binastro
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Join Date: Aug 2012
Location: S.England
Posts: 3,799
Thanks Kimmo.

Are you saying that with a 40x to 120x zoom the aperture would be 57mm at 40x?
Would a Barlow work?

Are you also confirming that the clear maximum aperture is 93.8mm as I thought a 95mm filter thread would indicate?
Is this further slightly reduced with off axis rays, which would only be perhaps 0.5 degrees off axis near maximum magnification.

With my 123mm f/5 telescope 16x using a 3 inch fitting Kodak large eyepiece the exit pupil is larger than my eye now has, so the aperture is reduced but the 4.7 degree field is very useful. At medium and high powers up to 145x normally, the full aperture is used. Although 210x is possible I don't use it.

There are many cases with larger astro telescopes that very low powers reduce aperture but give usefully wide fields for finding things. I point out the reduced aperture to users so they can use magnifications giving full aperture when needed.

I have been using Japanese monocular converter 10mm eyepieces on large lenses including zooms for about 40 years. But the field is about 55 degrees.
I have used other eyepieces as well, but the image is inverted, which is O.K. for testing.

P.S.
I remember that the 600mm f/5 Tewe and some large Astro Berlin photographic lenses were particularly good visually as scopes. Almost of astro quality.
I have used a Sigma 300 f/2.8 lens visually, but I can't recall the quality.
I have also used large T.V. zooms but again cannot remember the quality. One was an Angenieux.

The Den Oude Delft military mirror lens about 140mm aperture was fully up to astro quality. The view of Saturn was magnificent.
The Sigma 500mm f/4 mirror lens is poor visually.
Nikon 500mm f/5 mirror lens was good visually.

Last edited by Binastro : Saturday 26th August 2017 at 20:31. Reason: Removed sentence re zoom eyepiece.
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