CN FORUMS ARCHIVE Telescope Reviews: Re: Is there a scope that's best for binoviewing?

A few things you should know about binoviewers and scopes.

For almost every SCT that is listed as nominal f/10, as soon as you put a diagonal on the back end of it, it is operating about f/11. The Denk optical nose reducer, which is listed as nominal 0.66x, may be 0.66x if you stick it directly into the back end of an SCT, but in the diagonal of my SCT, it produces a 0.80x factor. So, my nominal f/10 5" SCT which starts out at F=1250, lengthens to F=1370 with a diagonal then gets factored at 0.80x with the Denk optical nose, and therefore has a BV with reducer focal length of about F=1100mm ( and not the F=825 that some people think). You can change the aperture, but you can't change the f#s and the factors. So using a Denk optical nose reducer, assuming all the SCTs are nominal f/10, here's approximately what you get in an SCT:

125mm SCT F=1100
150mm SCT F=1320
200mm SCT F=1760
250mm SCT F=2200

So, let's just say we want the widest field of view we can get at the lowest power that won't look like we are looking thru a straw. If you put a 40mm eyepiece in a binoviewer that has a 26mm clear aperture, you get an effective Afov of 38°. So I won't use a 40mm eyepiece. I'll set a low limit of Afov=45°. So, I'll use a 32mm eyepiece. Therefore, the lowest powers with a minimum orthoscopic Afov of 45° that you could get with these SCTs would be:
5" = 35x, 6" = 42x, 8" = 57x, and 10" = 72x.

What about the field of view. Well the Denk has 26mm of clear aperture and I've found I get field of view using 27mm+ filed stops that max out at just under 27mm field stop. A 32mm 50° plossl would have a 27.9mm field stop (actually, that's no field stop, that's just the inside of the barrel). So you won't get all the field of view out of a 32mm plossl. The 32mm plossl will be restricted slightly and the view will look like the eyepiece has an Afov of 47°. Not to bad. A 30mm Ultima will give you almost exactly the same Tfov, but since it has a field stop of 26.4mm, it will view like you are getting the full 50° Afov.

So what's the maximum field of view you can get in an SCT using the 1.25" Denk with optical nose reducer? Well with that 32mm eyepiece at lowest possible powers and widest possible Tfov you would get:

125mm SCT 35x @ 1.38°
150mm SCT 42x @ 1.15°
200mm SCT 57x @ 0.85°
250mm SCT 72x @ 0.67°

Without any reducers three different binoviewers that I measured, including the Denk, all operate between 1.23x and 1.25x in an SCT. So without a reducer all the lowest possible magnifications would be approx 1.25/0.80 = ~1.5x higher and all the true fields of view would be 2/3rds of that above. That’s what you would get with the Denk with it’s hollow nose, directly in the diagonal.

However if you use one of thee entry level binoviewers with 20mm clear aperture, your field of view will be restricted by the clear aperture. I've found two different 20mm CA binoviewers gave near exactly the same results. It would appear you can get the full field of view from eyepieces in the range of 22-23mm field stops. All maximum fields of view are about 15% smaller than the Denk. So, while you could get the same low powered views with a 32mm eyepiece, the maximum Tfov you get shrinks, and the view with that 32mm eyepiece would look as if the Afov is 40°, orthoscopic.

That's why the distributors recommend these 20mm CA binoviewers are best with eyepieces about 23mm out less. In fact if you stick with plossls for you low powers, you can go to 26mm. Even a 30mm 50° plossl when you put it in a 20mm CA binoviewer is going to view as if the Afov is only 43°.

In both 20mm CA binoviewers:
A 26mm TV plossl with a 21.7mm field stop did show the entire field of view.
A 25mm SMA eyepiece with a 22.3mm field stop did not show the full FOV.
A 28.7mm RKE with a 22.9mm field stop showed a 5% reduced FOV.
Three eyepieces from 24mm to 30mm with 26-27.3mm field stops all show about a 15% reduced FOV.

Not only did my 24mm 60° eyepiece and my 30mm 50° Ultima have a 15% reduced measurable TFOV, but approximately the outer 5-6% of the remaining visible field was a black ring with no distinct field stop. Clearly this shows not only the reduction in TFOV but also the vignette of the light in the outer edges of the field.

If you go the 20mm clear aperture route, the recommendation is stick to eyepieces with field stops 23mm or less and you will get the entire Afov out of the eyepiece.

Maximum field of view in a 20mm clear aperture BV is restricted to aproximately a 22-23mm field stop eyepiece. That is approx a 25mm 50° plossl, a 24mm 52° plossl or a 22mm 60° eyepiece or a 19-20mm 68°Afov widefield. Stellervue's 23mm eyepiece measures up at 52° Afov and gives very close to the maximum field of view that you can get with a 20mm CA binoviewer. It would be a waste of money to purchase wider field combinations than these for use in a 20mm CA binoviewer.

If your goal is to get the widest field you can get in an any scope, then get a binoviewer with 26mm clear aperture (or more, but not a 20mm CA).

How about using an f/5 refractor? Well, I'm not sure there is a 0.0x OCS available to use in a stock refractor, but I am sure there is a 1.25x corrector, although I'm not sure it fits the Denk. Anyway, using a stock f/5 refractor with a 1.25x OCS here's what you would get with that same 32mm eyepiece in a 26mm CA binoviewer. (There are other options that must be considered if using a 2" diagonal, not included here).

So what's the maximum field of view you can get in an f/5 Refractor using the 1.25" Denk with 1.25x OCS? Using the same 32mm eyepiece at lowest powers and widest possible Tfov you would get:

100mm F=625 23x @ 2.5°
120mm F=750 23x @ 2.1°
150mm F=937 29x @ 1.66°

Very simple adjustments can be made to these values if you are not using an f/5 refractor. For instance, if you use an f/8 refractor all magnifications will be 8/5=1.6x higher and all Tfov will be 62% of those above. If you don't have a 1.25x OCS and you need to use higher just factor it. If you need to use a 2.0x OCS then all magnifications will be 2.0/1.25 = 1.6x those listed above and all Tfov will be 62% of those above.

If either one of these factors needs to be applied to the refractor/BV combo then comparing to the SCT of similar size, the SCT would have a very slightly wider field of view. Otherwise the f/5 refractor would give the widest Tfov.

The plethora of 20mm CA binoviewers do not come with a reducer. If you end up using a binoviewer in an SCT without a reducer, even the Denkmeier, then the binoviewer is acting at 1.25x. Your SCT would be operating at f/13 to f/14. In that case even a 2.0x stock issue OCS on an f/6 refractor would give a wider field of view.

Many Newts are f/5. The Denk low OCS factor for a Newt is 1.4x. Therefore the Newts are close to the refractors. Assuming f/5 Newt at 1.4x OCS:

150mm F=1050 33x @ 1.5°
200mm F=1400 44x @ 1.1°
250mm F=1750 55x @ 0.9°
300mm F=2100 66x @ 0.7°

If you use one of the entry level binoviewers with 20mm clear aperture, again your field of view will be restricted by the clear aperture of the BV. All 20mm CA maximum fields of view are about 15% smaller than the 26mm Denk.

So there you have it. There's a little more meat for you to sink your teeth into when you go to make that decision, which scope suits you best for your binoviewing experience. These are just the lowest magnifications you could get without going below Afov 45°, and these are the widest possible fileds of view that you will ever get no matter what.

edz

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