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Stopping down large Binoculars

Started by Brian Goodman, 08/31/2004 04:43PM
Posted 08/31/2004 04:43PM Opening Post
It's pretty well known that stopping down large Achro telescopes kind of "cleans" up the image, but at the cost of light gathering. I never have heard to much about this concerning Binocular performance though. Yesterday I read Alan Adlers article in S&T, Sept 2002. "Some thoughts on choosing and using Binoculars for Astronomy". His explanation of the Astro Index comparisons of magnification vs objective diameter got me to thinking if I could clean up the image in my 100mm Oberwerks without loosing to much light gathering and resolution. I put two Orion 120mm objective covers with the center cap's off which stopped my 4" objectives down to 2". I guess this was turning my Obies into 22x50's (!). Under a full moon last night, I was shocked and rather delighted at what I saw. Any problem that my Obies had with flairing on stars was eliminated into nice pinpoint stars. I spent a couple hours comparing open clusters with the caps on and off. The light gathering difference was actually very slight and the resolution and sharpness seemed to jump up a notch. I saw no color on the full moon, and the contrast and resolution seemed very much improved. I assume I'm lowering my exit pupil and raising the F/ ratio by stopping these down?. Anyway, I thought it made a much nicer, more enjoyable image. Any thoughts on this would be great...Bri
Posted 08/31/2004 05:32PM #1
>>"Some thoughts on choosing and using I could clean up the image in my 100mm Oberwerks without loosing to much light gathering and resolution. I put two Orion 120mm objective covers with the center cap's off which stopped my 4" objectives down to 2". >>

With 50mm opening you have approximately 25% of the original light grasp and theoretically only 50% of the original resolution. However, at 20x, you would not see any loss of resolution - the power is too low for your eye to resolve the difference.

As far as light grasp, the only reason that there might not be a dramatic difference is if the original 100mm objectives were internally vignetted by field stops or undersize prisms. In that case, you might have had only 70mm -80mm binos to begin with. Personally, I have examined a number of giant binos at amateur conventions, and found them all to be internally vignetted.

Roland Christen