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Short notes on Asymmetric Binocular Telescopes
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Short notes on Asymmetric Binocular Telescopes
By Dr. Alex Tat-Sang Choy
I've always dreamed of having a pair of big binocular telescopes since teenage. In 2002-03, I built a pair of asymmetric binocular telescopes with what I believed to be the simplest design, using very simple tools. This design gives essentially the same optical quality for each telescope tube and is very suitable for amateur ATM. Recently I notice a new cloudy nights article by Bill Zmek showing a binocular telescope he intended to build with a design he discovered independently. Since I already built one and used it for a long time, I think I should share some thought so anyone who tries to build binoculars with this design will have the benefit of my experience.
If we are given a pair of 40mm refractors with diagonals, it would take 5 minute to build a simple binoculars. Simply find a piece of wood, drill a few holes and secure the refractors in parallel and we're set. The separation of the two eyepiece's center would have to be the interocular distance of our eyes, which is usually between 5-8 cm.
However, if we're given a pair of larger refractors, say 120mm in diameters, such simple design would not work. The asymmetric binoculars is a design to solve this problem. (Bill Zmek has drawn nice figures and explained the principle clearly, so I would not repeat here.)
When building binoculars with this design, the following details may be helpful:
Let me call the uncut OTA as tube A, and the cut OTA as tube B. Since tube B is cut and rejoined at 90 degrees, in order to retain the original optical quality, it is crucial to have a mechanism to align the optical axis of tube B, or else the optical quality of the final product will not justify the hard work of making the binoculars. I learned this the hard way.
There must be mechanism for alignment of optical axises of A and B. Unlike regular binoculars, which goes to as much as 40x magnifications, these binocular telescopes are expected to work at 100x or 200x. At these powers, a minute off in optical axis will result in 1-2 degrees off in the visual image. Although the human eyes/brain can adjust for minor mismatch in images between the two eyes, it may only take a few minutes of observation to make one tired and stressed. The result is reducing productivity and enjoyment.
The design shown in Figure 3 does not have alignment mechanism between tube A and B, giving me a lot of trouble in actual observation. I have since reworked the mounting plate between A and B, and added an x-y fine adjustment for tube-B, which makes my observation night a lot more enjoyable.
- Rigidity of the assembly is important. The distribution of weight of this pair binoculars is strange, parts holding the OTAs in place must be strong, and vibration dampening may be necessary.
- Center of gravity is important. The center of gravity for this binocular telescope is somewhere strangely placed, both in the up/down and front/back direction. The binocular telescope itself is very heavy and have large moment of inertia, slight mismatch between the mount axis and the center of gravity of the assembly can make the scope hard to use.
- This is a heavy scope and requires stronger mount. Fortunately I had a LXD55 handy. My LXD55's 'polar' axis can be adjusted to point upwards, and the controller can be made to believe it is an LX90. My binoculars on the azimuth LXD55 is usable, although not great. But an 120mm GOTO binoculars can draw some attention. In any case, strong mount is important.
- There are three 90 degrees binoculars designs I know of, they have their disadvantages:
- Matsumoto design. It uses two mirrors per tube, and give erect images, which is great for wide angle panning in star fields. I thought it is a brilliant design, but it is probably not easy to build and not cheap to buy.
- Asymmetric design. Mirror image, which could be hard to get used to for scanning and panning in the star field. I used only a few days to build the prototype, but a lot more effort to improve upon it. I had only simple electric drills and saws, if I could build one, it's not difficult.
- Three mirror per tube design. It's interocular distance is adjustable, but buying six mirrors and aligning them could be not favourable for ATMs. The multiple reflection also place strong requirement on the quality of mirrors.
- Interocular distance adjustment is overrated. I had great trouble finding a simple, strong, and low cost solution for interocular distance adjustment. In the end I decided to build first, think later. My binoculars have a fixed interocular distance of 5.5cm. I have since shown them to some members of the Columbus Astronomy Society and even once to the public at the Perkin's Observatory, to my surprise, only about 1 in 10 people complaint about not able to merge the two images. Large percentage of the public grasped at the sight of a 23x moon. (And since they never used my telescope for long observations, I do not know if they would still feel comfortable in the long run.) Therefore, from an ATM's point of view, NOT building the interocular distance adjustment could be a decision that saves more than 40% of the effort, a nice trade off in my opinion.
- The viewing. While I was still in
the US, where dark skies were accessible, this pair of binoculars gave
some of the most memorable views. In a good night using a pair of
mid-priced 30mm 80 degree eyepieces, I could view the entire M31
filling up most of the 80 degree apparent field of view, while at the
same time clearly see the dust lane and the companions. M42 was 3D
like. The whole Veil Nebula could be seen together without any filter.
And the Milky Way was simply stunning. Unfortunately, the 120mm
achromat was not good for planet observation. Despite that, the most
touching view I had was during a total lunar eclipse, at 23x, when some
faint cloud passed in front of the pale orange moon, the color and
perception of 3D was breath taking.
Dr. Alex Tat-Sang Choy