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Old 07-09-2011, 01:59 PM
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racecar (David)
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Advice please DIY 2-4x Magnification Binoculars

I'd been interested in buying low power binos for viewing the structure of the milky way, but I have found almost nothing. There is a 28 degree 2.3x40 TFOV bino available from Japan http://www.kasai-trading.jp/binoculars.htm, however I think the AFOV is 64.4 degrees (I get this by multiplying 2.3 x 28mm) and I would like more AFOV than this if possible.

What I have been thinking about is making a bino using a Televue Ethos 13mm and a Nikon 35mm F1.8 lens (The Nikon taking the place of a apo scope). Depending on which lens I go with, the Nikon 35mm lens has a > 41 degree full TFOV, although I think 30mm would be optimum.

So as I see it, a 35mm F1.8 lens operates as a 19.44mm (0.7655 inch) F1.8 apochromatic telescope (35 divided by 1.8 and divided by 25.4). With the 13mm Ethos, I should get a 7.2mm Exit pupil and 37.14 degrees TFOV.

With a 30mm F1.8 (0.656 inch), I should get a TFOV of 43 degrees and 2.3x magnification. This would be ideal, except they are no longer making 30mm F1.8 lenses, so the chance of getting two identical ones seems slim.

If the idea seems good I will probably test it on a 50mm F1.8 (Which I already have), which should be 1.09 inch F1.8 scope, and with 13mm Ethos should give a TFOV of 26 degrees at a magnification of 3.83x.

I notice that the field stop of the 13mm Ethos is 22.3mm and the die size of a film lens is 35mm (or with a DX lens around 23.3mm), so I am not sure if this means I would need to baffle it or that I would lose edge illumination or get vignetting.

Has anyone ever tried anything like this before? The one advantage to this design is that these could be binos without prisms, so they would effectively make a Galilean binocular.

I think I can get the T-mount for it from a manufacturer of CCD cameras (it is not the regualr T-Mount), and I think the Ethos 13mm should hopefully get close enough to the lens in the T-mount to focus, but does anyone have any idea if this would work?
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Old 25-11-2011, 12:33 PM
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racecar (David)
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Nobody seemed to know whether this might work so I thought I'd try it. It didn't work out too well. I used and 13mm Ethos and a 50mm F1.8 lens. The blue colour fringing was terrible. Worse than a cheap set of binos. The only thing I can think is that the Ethos is designed to go down to about F4 or F3 with a Paracorr. A 21mm Ethos with a 50mm lens stopped down to F2.8 might perform better. Actually, I haven't tried stopping down the lens to see if that helps. It might help the aberrations, and diagnose the problem, but it would ruin the exit pupil which would kind of defeat the purpose.

I have been told that camera lenses and telescopes were designed with different purposes in mind and that this idea is unlikely to work anyway. Perhaps I may need to try this with a small telescope made up of small surplus lenses rather than a camera lens. Still haven't given up on the idea as yet.
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Old 25-11-2011, 04:14 PM
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The problem is in exit pupil - this is the diameter into all light collected by objective lens is put through.
the formula is:

De = Do/mag

where:
De - exit pupil diameter
Do - diameter of the objective
mag - magnification

It doesn't make any sense to have this diameter much larger than 8mm (maximum diameter of the human eye iris) because the light outside this will not enter the eye anyway.
For magnidfication of 2.3x the diameter of the objective is 18.4mm... anything wider is a waste of aperture.

more here:
http://www.nikon.com/products/sporto...c/basic_05.htm

Also, you have to consider the fact that photographic objective lens i designed to have flat focal plane.. while eyepiece is designed for curved focal plane, produced by telescope lens.. and another issue - eyepieces are designed for much slower objectives.
I tried something similar a long time ago but gave up.
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Old 26-11-2011, 07:03 AM
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I have some binoculars made from 50mm F1.8 camera objectives. I use them for looking at the milky way with 24mm panoptics ( 2X and 34 degree field).

I'd say don't spend too much time or money on them - the 12mm or so used aperture at 2X is pretty uninspiring.

Its amazing how much more detail you can see at 2X but due to the small aperture you will lose interest pretty quickly I would predict.
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