First light: Williams Optics binoviewers

[SkyHiker]

Clearly, if the BV tube becomes shorter and/or thicker (its focal ratio as I defined it gets smaller), the vignetting will be less and eventually will go away. If the BV focal ratio is exactly that of the OTA the central light bundle will be unobstructed but the outer ones are still partially obstructed. Hope this explains the thought that I wanted to get across.
bv.jpg

I will admit that I am a bit confused but I think it is mixing terms that do not apply.
The BVs have no focal ratio but they have a focal length, so the f/r will not match between BVs & telescope.

What matters is that the aspect ratio of the box defined by the BV tube (length/diameter), and the aspect ratio of the objective (focalLength/diameter) are the same. The figure below shows a case where they match exactly; the central beam does not get clipped (the off-central ones will still get clipped though, more the further off center you go). It does not matter to me if we call it aspect ratio or focal ratio, I guess aspect ratio is more general.

Using the OCA just negates the f/l of the BVs so focus can be achieved.

It also effectively increases the focal length of the objective so in the picture, imagine the objective is further to the right corresponding with the multiplication factor caused by Barlow; in that case the central light cone is narrower and there is more clearance at the BV light entrance side, and less or no vignetting of the off-center light beams.

The picture is pretty much what is happening if you exceed the clear aperture of 22 mm

Not quite, the eyepiece represented as a box had a height (corresponding with the field stop size) that was less than that of the BV tube, in the previous picture. In the picture below the orange box represents an eyepiece that has a larger field stop than the BV tube diameter, which leads to the vignetting by the BV tube exit as you pointed out, and a green one that has a field stop smaller than the BV tube diameter. So there are 2 places where vignetting can take place, at the BV tube entrance and exit. Hope this explains it. Sorry if my earlier sketch and explanation were not clear enough, it was a quick draw with some hand waving about what the boxes mean.

[Bigzmey]

Let's consider practical outcome. The purpose of using 32mm is to maximize the FOV while using BVs. It will serve the purpose. Due to vignetting the edges of the FOV may get darker, but you still will be getting the max FOV available for BVs and TFV (even if augmented by vignetting) will still be wider than with 20mm 66d deg EPs.

I agree that you are trying to maximize TFOV but you will not do that with a set of 32 mm eyepieces due to vignetting.
Better to pick a set of eyepieces that give maximum FOV but only exceeding the clear aperture by 1-2 mm, no major vingetting that way and you are getting the maximum resolution from the BVs.
Using the prism in a SC negates the use of a OCA so will not be the same as using a newtonian or refractor.

This is a chart made by EdZ from CN who has worked out for prism BVs.

1420743-Eyepiece FS Diameters for Binoviewers.xls

I used to follow discussions on various aspects of vignetting: secondary mirror vigentting Newts/DOBs, baffle tubes vignetting Maks/SCTs, diagonals and BVs vignetting, etc. I don't any more. The science is complex and even professional opticians can't provide exact answer. Here is info from Celestron.

https://www.celestron.com/blogs/knowled ... ota-itself

I would like to quote

"Vignetting is normal in any real optical system." and

"When combining various components for observing with your scope, determining the degree of vignetting under the actual focused spacing without ray tracing is difficult."

So, I realized that the simplest way is to get EP(s) and try in your scope. I like what I see with 32mm in my BV, so no guy from CN will convince me otherwise. Your mileage may vary.

[SkyHiker]

I used to follow discussions on various aspects of vignetting: secondary mirror vigentting Newts/DOBs, baffle tubes vignetting Maks/SCTs, diagonals and BVs vignetting, etc. I don't any more.

I would be less concerned if the vignetting is just around the edges, as is the often the case, also for instance with what Gabi pointed out when the field stop of the eyepiece is larger than the BV tube diameter. That is vignetting at the exit of the BV tube.

The situation at the entrance of the BV tube can be throughout the field. For instance, if the F ratio of the objective is F/4 and the aspect ratio of the BV tube is F/5, you will only receive (4/5)^2=64% of the maximum for the central light beam, even less for the non-central ones. You probably would not notice that because it is far away from the field stop and gradual over the entire field. You should be able to tell when switching them with BVs that have a larger BV tube aperture.

The numbers in the exampl would be valid for my 12" GSO astrograph and the WO BVs if I were able to use it without Barlow (which I can't). My Z12 is F/5; at the center I would receive 100% of the light, which is good enough for me. But you could get better performance by BVs with larger BV tube diameter.

The vignetting at the BV tube exit due to eyepieces with too large a field stop like those 32 mm ones that I ordered, will be clearly visible at the edges but no have effect at the center, because the blockage is closer to the field stop.

So, I realized that the simplest way is to get EP(s) and try in your scope. I like what I see with 32mm in my BV, so no guy from CN will convince me otherwise. Your mileage may vary.

For sure, I think I will like it too. I know I talk too much about the math, it's just that I try to understand how things work. That is as much fun to me as actually using it.

[Bigzmey]

For sure, nerding about science behind the hobby is always fun.

BV with larger aperture certainly have less vignetting, the trade off is that they are larger and heavier. I like the small factor and light weight of the BV I have. I can fit them into EP case with plenty of room left for the EP pairs, they are also easier to balance on the mount. Finally, for my application (Lunar and planets) 20mm aperture is good enough since the most of observing is done with EPs with smaller field stop than that.