astro binoculars, focal length?

[starguest]

Magnification isn't something that has "units" but is it fair to say that seeing an attribute of 10x, 20x, 40x means the same across the board, so to speak?

This was started when looking at binocular specs, and those for the giant astro types with aperture of 70/80/100mm and mags 15x/20x/30x/40x, you don't see a figure for Focal Length. You might see a FOV angle in degrees and you could compare that to other specs for scopes and photographic lenses and get an equivalent FL. And of course some of them state they can use normal 1.25in eyepieces. So, knowing what mag an EP gives in a scope of known FL should allow the working out of the binocular's FL.

Let's use a small refractor of 100mm, the makes all vary but assume the FL is 600mm. The given formula says a 30mm EP yields 20x mag and a 15mm EP yields 40x. That means the 40x-100 bino has a FL about 600mm. Reasonable assumption?

If I had this set up beside a 40x-100mm binocular pointing at Jupiter and had a pair of 15mm EPs, should I see Jupiter the same size in each instrument when swapping EPs? (for FOV I'm aware that a normal 15mm EP would give a narrower fov than a WA EP, but the same mag of the central object)

Further, aware of quality variations amongst the various makes, another formula states useful magnification is about 2x aperture in mm. Might a giant bino be able to take EPs approaching the 5, 4 or 3mm if that translates to their useful mag size, and has anyone done this?

[Peter Drew]

Hello Starguest. If the objectives of the refactor and the binocular are of the same focal length then the magnification in each will be the same with a given eyepiece/s used on either. I've made several astro binocular telescopes from 80mm-300mm, the problems with using high magnification are the difficulty of maintaining collimation and the eventual loss of the nice "3D" effect which is most impressive at low to medium powers.

[starguest]

Peter hey,

Right then. I have imagined using a pair of small refractors but I get to the point of the eyepiece placing. Then how the spacing is going to be achieved to match eye IOD, Would that need proper bino prisms, I can't quite see it being done with another pair of diagonals? Unless the 2nd pair meant you had to look into the EPs in a forward direction - along the tube length - as opposed to looking down like with a normal diagonal.