Initial Impressions & Thoughts
This post was initially a response on a singastro forum, but was so long, I thought I may as well blog it.
From Teleskop Service in Germany, I bought:
- The Baader Maxbright binoviewer kit with 1.25x glass path corrector and 1.25" nosepiece.
- Zeiss Jena 90 degree prism T-2
This is the first time I've used Teleskop Service. Good prices, easy to use site, same day shipping and everything was packaged well. Very happy with their service.
From Starbase in Tokyo.
- 2x 18mm Takahashi orthos (24.5mm)
- 1x 12.5mm Tani ortho (24.5mm). I already have another 12.5mm
Great service as per usual.
On opening the binoviewer and parts
The binoviewer is nicely made, with coatings well applied. The tension is just right, and there is a fair amount of focus travel for each eyepiece. The helical focussers for the eyepieces are smooth. On the prism, however, I was surprised to see a tiny chip on one surface. I doubt this will have much, if any effect, but still, I'd expect it to fail QC, especially with the Zeiss prism being the more expensive option. There is a plastic ring on the male threaded side of the prism. This ring is there to provide some gradual pressure when you tighten the thread, and worked well with some T-thread accessories I tried. However, to fully screw the prism to the binoviewer, I had to remove this ring. Without the ring the binoviewer jams and "gives" suddenly. This is a bit disappointing.
Also, there is the usual Baader problem of 'sticky' threads. Every Baader filter I've ever had has had the threads very slightly off. Not sure why this is, but again the prism and binoviewer have this problem.
The 31.7->T adapter is well made and has a 34mm thread internally to attach the glass path corrector. This works well.
First light
Of course it was cloudy when I got all the parts delivered and I couldn't try them out on the sky. I just looked at some far-off buildings and lights. So, these are fairly useless initial impressions.
I tried the binoviewer and prism on my FSQ-85ED, which has a lot of back-focus (>200mm) to support long imaging trains. The adapters for visual use take up some of this, quite a bit as it turns out.
There are basically two configurations:
- Binoviewer - corrector
- Binoviewer - prism - corrector
With 1, I could get focus with the 18mm orthos. But with a Televue maxbright 1.25" diagonal inline I couldn't get focus.
I then tried configuration 2, and couldn't get focus. I changed the Tak 2"->1.25" adapter set (which takes up about 30-40mm or so) to the low profile adapter, and was able to get focus. There was quite a lot more back focus available when using 31.7mm eyepieces, so I could probably just use the normal adapter. This setup is close to the stated requirements.
- The quality of the machining shows - I had no problem fusing the image and getting good focus for each eye
- Very nice, high contrast, sharp images
- Some lateral colour
- Some softness at the edge of the field
- Internal reflections
By far the worst and most intrusive was 5: internal reflections. This was very, very bad. Far worse than any I've seen in cheap binoculars. In daytime viewing, this will probably lead to reduced contrast. It's probably not much of an issue when viewing star fields, but these reflections will probably cause problems viewing the Moon. These were so obvious and so bad, I'm curious if there's something weird about my setup. I can't think what it would be - they were visible with or without the prism.
Point 4 is worrying, but I need to try this setup under the stars to see what's going on. There's certainly no field curvature on the Q, but it's possible the light cone is still a bit too steep for the orthos, or even the prisms.
Some things to heed:
- The Tak 31.7->24.5 adapters are nice, but take up even more back focus. DOH. So, don't do what I did. Use 31.7mm eyepieces.
- The Baader T-2 prism allows you to connect the binoviewer with the minimum increase in light path. I'd recommend this, or another T-thread diagonal.
- You will almost certainly need one of the glass path converters. Unless you're using something like a Borg, Tak Sky-90/Q or other scope with a lot of back focus, you will need a corrector with magnification higher than 1.25x.
- The entire setup is quite heavy and greatly increases the moment on the rear of your scope (assuming a refractor or compound).
- Due to the weight, you're not going to be able to use heavy eyepieces. If you want to do this, it would make sense to do so on a larger scope and get the more expensive Mark-V binoviewer.
- I've not tried the setup with my Mak or Mewlon yet, but I'm worried about the ergonomics. It looks like the scope might get in the way. There is a 45 degree Amici prism that may be more suitable.
- Eye placement seems to be more critical than with a single eyepiece.
- SCT owners are going to focus way, way off from where the system is optimised. Expect even worse off-axis performance than usual, and probably some SA on axis.
- The system diagram I got was for the older binoviewer system which was not T-2. There are some configurations not possible with the T-2 system, and others that are no longer necessary. This might lead to confusion.
