Binoviewer vignetting

[bosun21]

How badly would a pair of 82 degree Nirvanas vignette in a Baader Maxbright II binoviewer?

How is this calculated? Many thanks  

   Ian 

[Louis D]

What focal length?  It's dependent on the field stop of the eyepiece, not the AFOV.  Pretty much any eyepiece with a FS of about 26mm or less would not vignette since that is BM2's clear aperture.  That covers all but a few widest TFOV eyepieces like a 32mm Plossl and 24mm Panoptic/ES-68/APM UFF which have ~27mm field stops.

The widest 82 degree Nirvana in a 1.25" barrel is the 16mm which has a 21mm to 22mm diameter field stop, so it shouldn't vignette.  However, since it is a negative/positive design, we also need to look at the field lens diameter which is 25mm because we don't want to block the edges of it.  Again, it is less than the BM2's clear aperture of ~26mm.

[bosun21]

2 hours ago, Louis D said:

What focal length?  It's dependent on the field stop of the eyepiece, not the AFOV.  Pretty much any eyepiece with a FS of about 26mm or less would not vignette since that is BM2's clear aperture.  That covers all but a few widest TFOV eyepieces like a 32mm Plossl and 24mm Panoptic/ES-68/APM UFF which have ~27mm field stops.

The widest 82 degree Nirvana in a 1.25" barrel is the 16mm which has a 21mm to 22mm diameter field stop, so it shouldn't vignette.  However, since it is a negative/positive design, we also need to look at the field lens diameter which is 25mm because we don't want to block the edges of it.  Again, it is less than the BM2's clear aperture of ~26mm.

Thanks Louis, it’s actually the 16mm Nirvanas that I was looking at. Either that or I could buy one more Morpheus to make a bino pair.

  ATB  

    Ian 

[Louis D]

3 hours ago, bosun21 said:

Thanks Louis, it’s actually the 16mm Nirvanas that I was looking at. Either that or I could buy one more Morpheus to make a bino pair.

  ATB  

    Ian 

Watch out for the space left between them for your nose.  I tried it with two Astro-Tech AF70 17mm eyepieces, but my large nose prevented me from comfortably using them.  I would have had to approach them with chin tucked downward and looking through the very top of my eyeglasses to use them.  I experimented by pairing my 17mm with my 13mm in my BV, pulling the shorter one outward in its holder to mimic having a a pair of 17mm eyepieces.  If their tops had been sufficiently tapered, it might have worked.

Ultimately, I went with narrower, simpler 65-68 degree designs that perform well once Barlowed by 3x to reach focus.  Last night, I used a pair of 20mm Svbony UWA 68 degree eyepieces to view Jupiter for 1.5 hours.  They were comfortable, sharp, contrasty, and displayed no ghosting even with super bright Jupiter.  The long eye relief even allowed me to hold filters between my eyeglasses and the eyepiece (one side only seemed to work best).

Edited September 29, 2022 by Louis D

[bosun21]

1 hour ago, Louis D said:

I used a pair of 20mm Svbony UWA 68 degree eyepieces to view Jupiter for 1.5 hours.

A lot of people say that the SVbony UWA suffer from kidney beaning. So I presume that you didn’t have this? I have tried two Morpheus in the binoviewer and they were just fine. I do like the weight saving aspect of the SVbony UWA’s though 🤫 Decisions, decisions.

[Louis D]

Only the two negative/positive design versions suffer from kidney beaning.  Those would be the 6mm and 9mm versions.  The 15mm and 20mm versions are straight positive designs like a Konig variant, so they don't have that issue.  Kidney beaning tends to happen mostly in negative/positive designs or when pairing a positive only eyepiece with some Barlows in my experience.  Of course, without the negative section, they don't perform so well in faster systems.  Even with the Barlow element on the BV, I didn't notice any kidney beaning with the 20mm Svbonys.  Since I was able to pick up the pair for under $50, I was willing to roll the dice with them.

I'm glad the Morpheus pair worked fine for you.  The added weight and lever arm can be an issue as you say.

[spartinix]

Vignetting at the exit of a binoviewer, as opposed to at the entrance (aperture reduction), depends on the exit clear aperture, the field stop of the eyepiece AND how close its field stop is to the exit clear aperture of the BV (some baffle).
When an eyepiece's field stop is low in its barrel and thus fairly close to the exit baffle (exit clear aperture of the BV), a sharp falloff can occur (sharp vignetting).

From a certain distance, it is said that, in general, an eyepiece field stop can be 10% larger than the clear aperture of the BV without being noticeable or particularly annoying.