CN FORUMS ARCHIVE Telescope Reviews: Re: Binoviewers and clear aperture.

If selecting one of the 20mm CA binoviewers on the market, not only is there a light loss at the outer edges of the field stops, but also the Maximum True Field of View that you can get will be restricted to eyepieces with field stops about 22-23mm max.

For instance you can get a lower power by using a 30mm Ultima vs the 23mm SV (fs=20.9mm) in the SVB3. BUT, the 30mm Ultima has a 26.1mm field stop. You would think you would get 26.1/20.9 = 1.25, a 25% wider field of view than the SV23mm eyepiece. That is exactly what you do get when both eyepieces are used in the SCT without the binoviewer. But that's not what you get with the 20mm CA binoviewer in place. Likewise, with the 26mm Meade 5k eyepiece. The Meade has a 27.3mm field stop, but you do not get 30% more fov than the SV23mm eyepiece. You do get a slightly wider field of view, about 10%, but you will not get the full benefit of the wider field stop in the Meade 5k or the 30 Ultima when used in a 20mm CA binoviewer.

I've been drift timing eyepieces in three different binoviewers, the Burgess, the Stellervue BV3, both with only 20mm of Clear Aperture, and the Denk Big Easy which has 26mm CA. These three binoviewers have tested to have almost exactly the same magnification factor in my SCT when used with the hollow nose. You can see the drift times of various eyepieces in the 15mm to 21mm range match to within a few seconds in all three binoviewers. For example, the 20mm TV plossl drifts for 137 seconds in the Burgess, 139 seconds in the SVB3 and 138 seconds in the Denk. (Drift times are corrected for cosine declination).

Then in the range of field stops about 22-23mm, for both the 20mm CA binoviewers, length of drift time or FOV stops increasing, regardless of the eyepiece. That's how I was able to determine both the 20mm CA binoviewers restrict field of view and at what point the veiw becomes restricted. The Denk has a 26mm CA and it does not stop the FOV from increasing as expected. It is especially easy to see the differences when using eyepieces with field stops in the range of 26mm to 27mm, such as the Meade 5k 26mm, which has a 27.3mm field stop. Only the Denkmeier (26mmCA) allowed the full field of view to be seen. The Meade 26mm 5K eyepiece drift time in the Denk varied from the baseline, the drift time without binoviewer, by almost exactly the amount of the magnification factor determined for the Denk, 1.25x.

In both 20mm CA binoviewers:
A 26mm TV plossl with a 21.7mm field stops 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 10% of the remaining visible field was a black ring with no distinct field stop. Clearly this shows no only the reduction in TFOV but also the vignette of the light in the outer edges of the field.

So the recommendation is, stick to eyepieces with field stops 23mm or less and these bargain BVs will do just fine.

Maximum field of view in a 20mm clear aperture BV is restricted to aproximately a 22-23mm field stop eyepiece. That is approx 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.

edz

Edited by EdZ (02/01/07 06:58 AM)

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