Binocular resolution

[Astronut]

I'm curious about binoculars.

http://medusa.as.arizona.edu/lbto/optics.htm

Now I know the LBT in Arizona is not a visual instrument, as in each eye placed at separate eyepieces, but given that it uses 2 x 8.4m mirrors which have the equivalent light gathering ability of a single 11.8m mirror and the spacing of both mirrors is such that it has the diffraction-limited image resolution of a single 22.8m mirror, does the same principle apply to an ordinary pair of binoculars?

I've been given to understand (after alot of reading) that, for example, a 100mm binocular has the same light gathering ability as a refractor of approx. 140mm so does the resolving power principle also apply or do the objectives have to be much closer together (as in the LBT) for this to have any effect? In other words, are binocular objectives too far apart in respect to their diameters for increased resolution. I understand that the relatively low magnitude of binos would probably not allow you to see the extra resolution (if any) so it might be a moot point.

This resolving power principle is applied to multi radio telescopes (eg. the VLA) and their dishes seem to be placed much further apart than their respective diameters. Does this theory hold true or am I talking out my rear end?

I'm just curious as I'm considering getting a pair of big binos to take with me during the week and I'm suffering a little from info overload.

[Nostromo]

It's the surface area of the lens which is a primary factor in determining light gathering ability. The larger the area the more light. Also it determines resolution delivered by each objective but light and resolution are separate things really.

For example, a 100mm binocular would have a lot less light gathering power than a 140mm telescope - and less resolution.

Take a 66mm Astro Tech/William optics scope as "100%" and you can compare the relative light gathering ability and resolution to the previous scope as follows:

Objective dia:

66mm 3419 mm2 100%

80mm 5024 mm2 147% area of previous

96mm 7235 mm2 144% area of previous

102mm 8167 mm2 113% area of previous

120mm 11304 mm2 138% area of previous

140mm 15386 mm2 136% area of previous

Compare a 100mm to a 140mm scope and you get:

100mm 7850mm2 100%

140mm 15386mm2 196%

That's almost twice the light gathering ability (and resolution) of the 100mm.

Now imagine you used binoviewers to compare a "binoscope" (one 100mm objective for each eye) to a 140mm telescope with a "bino-viewer" attached. The best binos on the market will still lose about 5% of light due to prims absorbtion ect. Then they split the light into two almost equal parts one for each eye so you're getting 40% of the light in each eye respectively.

This isn't all bad though because although you've lost a lot of light in the process by splitting the light from the 140mm with binoviewers - you haven't lost resolution because the aperture hasn't changed.

Your eyes evelved with some very good software (or wetware rather between them which works best with 2 images to compare. That's why binoviewers give such a stonkingly good performance on bright objects like planets but such stonkingly bad performance on DSOs unless you've got a massive scope.

So there's a point where aperture and portability come into play I guess. There are various "binoscope" kits on the market and they all have various issues. I've used a 128mm binoscope extensively and it has given awesome views of Jupiter and globulars sheer luxury but it's been beaten by a 7" Mak on planets and I couldn't justify spending the money on such a monster.

Having said that I believe there is a good argument for a small 100mm binoscope - for travel purposes. Recently I saw a pretty cool one at Kelling Heath I think it was made by a gent called Dave but may be mistaken.

anyway just some thoughts, Nick

[Astronut]

My question wasn't about binoviewers and binoscopes but about binoculars but thanks anyway.

I've done some more reading and I have now been lead to believe that 100mm binoculars have the equivalent light gathering ability of a 118mm refractor (not 140mm). No info on possibly increased resolution though. :?

[Nostromo]

Hi Jon sorry mate I meant to say the same "formula" applies to binoculars and telescopes if you see what I mean. It's really just because you can't compare one to two eyes - unless you're using binoviewers. Anyway I have probably confused things so I'll shut up now :oops:

Best of luck with your choice tho

Nick