In a different thread, I said that I would get around to testing a binoviewer configuration for the BF1200 when used on the LS80THa and I have completed that testing.
Before I begin, this is not my normal BV configuration. I use the BF1800 with a Baader 1.7x GPC so this gives about a full degree true field and I like the very bright image I get when I use lower powers, but my eyepieces are Nikon zooms with very small apparent fields. I just promised I would test this for someone and I decided to post it as a new post.
When using most simple 2x Barlows that are usually included with inexpensive binoviewers, the focuser tube will have to be moved in well in excess of 40mm, and the BF 1200 does not allow this much forward movement without loosing a pretty large amount of the fully illuminated circle.
Most standard Barlow lenses when used with a typical binoviewer are going to give well in excess of 2.4x.
Here is a solution that while not inexpensive, has the advantage of providing a full disk sun when using zooms, and makes the binoviewer portable to other telescopes.
The solution is to use the Televue 2.5x Televue 1.25" Powermate. Now the intresting thing about the 2.5x powermate not the 2x powermate) is that the longer the spacing to the focal plane, the lower the power will be, and when used with a William Optics type binoviewer, the actual magnfication factor will be more like 2.2x.
The tolerance is very tight, and the BF1200 will only fully illuminate a circle that is about 7mm in diameter when used with a regular eyepiece. Now the important thing to remember is that at f/7, every 7mm of forward movement of the BF will cause this fully illuminated circle to lose 1mm in size.
The configuration I used was a Burgess binoviewer with a 96mm light path, an RAF 28mm to T2 adapter, the Televue T ring adapter for the 2.5x Powermate, and the 2.25x powermate.
I first brought the telescope to focus with a standard eyepiece. This eyepiece has a field stop that was right at the top of the eyepiece holder.
Next, I removed the eyepiece and put it into the binoviewer, then put the binoviewer in to the telescope, racked it in until the telescope reached focus, and then measured the forward movement.
The movement I measured was about 12mm. This means that the 7mm fully illuminated circle would be reduces from about 7mm by about 1.7mm so that a fully illumined circle that was 5.3mm in diameter would remain. Now for the LS80, the sun is 5.15mm at the focal plane, so for this scope, the sun and a small ring around it would be fully illuminated.
With my Nikon zooms at the longest focal lenght (21mm) the full sun was still able to fit into the field of view and someone using a pair of 24mm zooms would have an even better fit.
(Note.. Many zooms require 5mm to 6mm more inward travel than standard eyepieces, with these, the fully illuminated circle would perhaps drop less than 5mm).
For a purely visual user than, using the Powermate would allow one to get by with the BF1200 and still give a full disk using zoom eyepieces (for years, my preferred eyepiece for solar work and ever changing seeing).
One last point is that the Powermate makes the BV pretty portable to other telescopes. Even most dobs will reach focus with a Powermate, though it might require a low profile 2" to 1.25" adapter for some scopes. While the Powermate is not cheap, it does offer potential beyond just use in an H-a scope.
Hope this helps someone sometime...
Edited by Eddgie, 15 August 2019 - 04:49 PM.