I have the AP/Baader binoviewer (bv). Brought it from Company 7 last fall. Binoviewers are fun and are great for observing planets, the moon, and brighter OCs. There is a neat 3-D effect on OCs and brighter nebulas such as M41. I have used it in all 3 of my current scopes: the AP 130, 9.25″ SCT, and
TV-85. So far, I have 2 sets of eyepieces: 19 Pans and 30 mm Ultrascopics (same as the Celestron Ultimas). For higher powers, I use the barlow, but figuring out what power you at depending on where you place the barlow (directly in front of the bv, ahead of the diagonal, or further down inserting the extender tube) requires a lookup table.
The S&T review was fairly comprehensive, but bvs are a complicated topic. In the article, Alan Dyer revealed that there is a new improved Baader bv in the works – improved coatings and the most importantly, individual eyepiece focusers which will be a big plus. The price is going to up to “around $1400″, so it will make the Baader bv even more expensive. However the Baader price does include the low profile high quality prism diagonal and the 1.25x corrector, which does allow me to reach focus in the AP130 and 9.25″ SCT without using the 2.4x barlow. IIRC, the review also didn’t mention one of the differences between the two bvs in that the Baader bv has the widest throughput of any bv at 28 mm, while the TV unit is limited to 26 mm. Not a huge difference, but the TV unit will start to vignette with low power 1.25” eyepieces. The lower cost bvs, which are often adapted microscope objectives, usually have 22 mm clear aperture.
One of the best writeups on bvs can be found at Todd Gross’s website http://www.weatherman.com/ under “All About BinoViewers”. Todd is a fan of bvs and has tried most of them at one time or another.
There is also a new review of bvs at http://www.cloudynights.com/accessories2/binoviewers.htm but this one does not include the Baader or Siebert units. What is surprising, given the price of the Baader and TV bvs, is that they haven’t all along had a low profile helical focuser for the right eye, similar to a binocular. For people with different focus points for each eye that is a problem. You can change focus by pulling one eyepiece out and re-tightening the screw, but that can be imprecise. Last year, someone offered an aftermarket focuser to fit into the Baader bv eyepiece, but it was a limited one of production run and was very expensive at $400 (IIRC) – and sold them all. I suspect this may have motivated Baader to improve their unit with new focusers.
Not everyone can easily use a bv. Some people don’t get the 3-D effect or have difficulty in getting the focus right for both eyes. It can also take time to get the interoccular spacing right; at 200x, the tolerance for the right spacing is a lot touchier than for 10x binocs. The mechanics of using a bv can also be cumbersome, even in a refractor or SCT. You have to get your head at the right
position to look through both eyepieces, whereas with one eye you can lean over and twist your head to look thru the eyepiece. The bv eats up focus travel which is why they are problematical for newts or mak-casses which haven’t been built for bv use.
The binoview effects on planets is fascinating. Some of the hard core planetary observers may scoff, but I could easily see more details of low contrast features on Jupiter using two eyes than just one. This is despite breaking one of the rules of planetary observing by putting a LOT of extra glass into the
light path. I’m looking forward to using the bv on Mars next year during opposition; Mars is nothing but low contrast.
A big drawback to the high end bvs is $. Not only do you have to buy the bv & the barlow; you then need to get 2 of each eyepiece and matched sets at that. I brought the 30 mm Ultrascopics from C7 as a matched set; they had them tested for a match in focal length before selling as a pair. The new 13 and 16 1.25″ Naglers are aimed for the bv market; but a matched set? Ouch, $ wise!
Before anyone buys a bv, I would suggest they try one out at a club observing session. I will often bring mine along to weekend sessions when I have the G-11 set up. If I have it in the scope, you are welcome to take a look.