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Clear Aperture & vignetting

Started by mxb, 03/14/2004 01:56PM
Posted 03/14/2004 01:56PM Opening Post
Hi,
I'm researching BV's, and I notice that they are available with clear apertures from around 20mm to 26mm, depending on brand/model.

Has anyone had any problem with vignetting caused by using say, a 24mm Panoptic (or maybe even a 20mm WS III) in a smaller aperture BV?

Thanks,
Mark
Posted 03/14/2004 02:58PM #1
Hi Mark,

I have a Black Night Binoviewer and I have a pair of 24mm wide angle Plossls that I constantly use without any noticeable vignetting problems. These are the EPs that I normally start with. I normally center the object of interest in them and then switch to a higher magnification. They have a really nice wide view and they make it very easy to find objects.

I have also used a combination of 32mm Plossls with a .5 focal reducer with my Celestron 9.25. At that time I did not notice any vignetting problems but I have to admit that I really was not looking for them.


The Black Night has a 22mm clear aperture. The manufacturer says that there is roughly a 10% reduction in apparent field on eyepieces 30mm and higher.

Hope this helps you
Don Durbin
Posted 03/15/2004 05:01AM #2
Mark,

This might ruffle some feathers, but I'm going to post it anyway. This was a post on the Yahoo binoviewer group where we were discussing how to measure the clear aperture of a binoviewer.

http://groups.yahoo.com/group/binoviewer/message/4626

My numbers do not agree with what others have measured or advertised, thus the rub. I'm not sure that bothers me too much. I know what I measured and it is what it is. Why it is, I can't explain that.

"I just went out and used the sun and that took care of the expanding circles issue. The diameter of the illuminated circle appeared to remain constant as I drew the piece of paper away from the ep holders. That takes care of that. Then I taped paper across both ep holders on both my Denk standard and the Televues and marked the width of the illuminated circle with a sharp pencil. I would guesstimate my accuracy at + or - .25mm. Here's what I measured:

Denk standards left and right were 24.5mm
Televue binovues left and right were 26.5mm

I took the measurements 3 times using 2 different methods and it all came out the same.

As far as the different height of the ep holders, I think that gets normalized in that the ep is higher therefore the ep field stop is higher (on the TV's). I think what we are interested in is the diameter of the clear aperture at the ep field stop. I think measuring it anywhere else is relatively uninteresting.

One interesting thing that I noticed and maybe this is an indicator of a flaw in my method. This applies to both binoviewers. I held the binoviewer input end up to the sun and then tilted it until I got a fully illuminated circle. I had removed all ancillary equipment from the nose. As I was tilting the binoviewer around, you could see the illuinated field clipping. I assume this is due to the beamsplitter and such inside the binoviewer. But here's what was interesting. When I got the tilt just right, I could get a complete circle fully illuminated, but it was not in the center of the ep holders, it was offset to the inside a bit. This was true for both binoviewers. With the illuminated circle centered in the ep holder, parts of it were clipped. Not sure what that means."

As far as seeing vignetting, I have not seen any in either binoviewer. It must be there, but I haven't noticed it. I have to use an OCS with a mag of 1.38x, so maybe that eliminates the need for a 27mm clear aperture to illuminate a 27mm field stop ep (24 Pans).

Clear skies,

Tom
Posted 03/17/2004 04:18PM #3
Vignetting in Binoviewers is caused by the size of the entrance aperture and the focal ratio of the scope used. The faster the scope, the more vignetting there will be. A typical binoviewer has about a 5 inch (127mm) optical path length (some are longer, some are shorter). When used with an F8 scope, as an example, the beam will expand from the eyepiece focus point to about 16mm diameter (127/8 = 16mm). This means that a Binoviewer with a 16mm opening will just barely illuminate fully the very center of the eyepiece field. For every 1mm more of prism opening you will have 1mm more fully illuminated field. A 20mm Bino will have a 4mm field, 25mm will have a 9mm fully illuminated field, etc.