I recently received my Seibert Black Night binoviewers from Siebert Optics (http://www.siebertoptics.com/). I also purchased the Siebert 1.2x OCA that allows my Lunt 50 to achieve focus without over-magnifying the image. I have found that eyepiece pairs in the 15mm - 20mm range work best but higher magnification (around 12mm) is also usable.
Here are my Seibert Black Night binoviewers mounted on my double-stacked Lunt LS50. One of the great features of the Black Nights is their light weight, which is a good match for a small scope like the Lunt 50. I can achieve good balance on my small equatorial tracking mount by sliding the scope forward in the dovetail mount. The double-stack module which screws onto the front of the scope also helps to balance the added weight of the Black Night binoviewers.
After a bit of troubleshooting with Harry Siebert, I discovered the best placement of the two lens elements that comprise the Siebert 1.2x OCA so that I can achieve focus with various eyepiece pairs.
The 1.2x OCA has two lenses numbered #3 and #4. The #3 lens cell is threaded into the binoviewer's 1.25-inch nosepiece. The #4 lens cell is threaded into the rear of the Lunt diagonal's nosepiece (after removing the O-ring on the #4 lens cell), shown here:
I have been able to achieve focus with 15mm and 20mm eyepiece pairs. I also tested this configuration with the one Lunt zoom eyepiece that I have, and the Lunt zoom can achieve focus across its entire zoom range.
The pair of Baader zooms I have did not quite come to focus - I needed a bit more inward focus travel than the Lunt Helical focuser allows, even with both diopters fully lowered on the binoviewer. Here is a photo of one of my Baader zooms next to the Lunt zoom; the Baader zoom has a slightly longer optical path length:
I also tried threading the #4 lens cell into the front of the diagonal's nosepiece as far as I could. I found that I needed a bit more outward focus travel than the Lunt Helical focuser allows. I was able to just achieve focus with the Baader zooms by extending the height of the diopters, but I would much prefer to use the Helical focuser because it's more convenient than having to fiddle with both diopters just to get focus.
It appears that if I could move cell #4 further inside the diagonal nose, I could achieve focus with the Baader zooms. Harry has told me he can make the cell #4 placement adjustable, so I am just waiting to hear back from him about this.
The Black Night binoviewer is very easy to use, and the self-centering diopters are very well executed. Even the Baader zooms with their undercuts on the barrels stay fixed in the eyepiece holders and don't wiggle around. However, I am going to replace the Baader zoom barrels with the custom smooth barrels currently being fabricated by Gary Susik (vkhastro1), so that they will work better with the eyepiece holders on my Binotron which I use with my C-14 SCT.
It is fantastic to be able to use both eyes for solar viewing. The double-stack module on my Lunt 50 does not significantly reduce the overall brightness of the solar disk, so that's not a concern for me. I do use a Dark Skies Apparel Observing Hood (the white one with 2-layers of black lining) which I find is indispensable for solar viewing.