Telescope Reviews: Re: Undersized prisms

As promised, I did up an illustration using my (crude) Binoplan software, with fiddling done in Photoshop.

A commercial 20X100 available 20 or more years ago was measured out. It had rather fast f/3.5 objectives which made it impossible for the prism front aperture to fully take in even the on-axis cone of light, much less any off-axis light. The result is an effective aperture of about 87mm.

The prisms were standard 1-inch jobs, with the front and rear apertures being the same. One might naively think that using larger prisms would help, and that using smaller prisms would be sheer folly. To show that there would be no real difference, either way, I checked to see how another common prism size would perform, namely those with smaller 20mm clear apertures.

In both analyses, I chose typical prism-to-field stop distances, they being equal to the prism's clear aperture. You may be surprised to learn that as laid out, the smaller prisms actually resulted in a slight *gain* in effective aperture! But they do cause somewhat worse vignetting, although not enough for even the most discriminating observer to discern.

In both bino configurations, getting the field stop as close as possible to the rear prism aperture will result in a gain in effective aperture. This is why many binos do indeed have their focal surfaces (hence field stops) in about as far as physically possible. But unfortunately, with objectives faster than about f/4 there will almost always be a reduction of aperture.

Here's the link to the image...

http://www.cloudynights.com/photopost/showphoto.php?photo=15120&password=&sort=7&thecat=500

EDIT:

I should add that the software does take into account the effects of refraction within the prisms. For an index of refraction of 1.5, the focus is moved back by an amount equal to 1/3 the prism's physical length. So for standard 1" Porros, the glass length is 110mm, and therefore the focal position is moved back by about 37mm.

The two layouts shown in the illustration are scaled a tiny bit differently, in that the objective-to-focus distances are drawn as exactly the same length. In reality, the lower layout using the smaller 20mm prisms should be a little bit shorter (and smaller overall) because the focus with them is moved back 29mm, 8mm less than with the larger prism set. But this is small enough to ignore here.

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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV

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Edited by GlennLeDrew (06/22/08 02:32 AM)

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