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NOTES ON 8" BINOCULAR TELESCOPE
1. What is the ideal focal length for an 8" binocular telescope?
I think the ideal focal length is one where you can point the telescope at the zenith and observe comfortably with your feet firmly on the ground. I am 5' 10" (178 cm) and at F8 I have to just stand on my tip toes. F7.5 would have been perfect. Binocular eyepieces tend to get very close to the ground at the horizen, so the more focal length the better.
I originally chose F8 so the field of good definition would be enormous, and having no knowledge of the mecahnical tolerances required, I played it safe. If I had made F4 first off the project would have initially failed... On this topic, I would make one mirror movable temporarily in the X-Y plane before you glue them down so you can adjust them so you have good collimation at high power in each side when the images are merged. This would be a very sensitive adjustment below F5.
2. What is the tolerance on the focal length of the primary mirrors? I there a % tolerance ?
The figure I see repeatedly in the books is that the brain can accomodate difference in image size of 1%. If we make .5% our top tolerance for safety, we must decide on our desired high magnification. I usually use mine at 200X tops, only because that is the limit of eyepiece comfort. This is achieved with an 8mm focal length. Our 0.5% tolerance represents a magnification difference of 1X which of course means an allowable focal length difference of 8mm. My mirrors are within 1mm FL but that is probably overkill. I found when I was employed in optical mass manufacture of telescope mirrors that two mirrors coming in succession off the same tool would frequently be within a few mm FL. If your mirrors are ground alternate wets on the same tool, this should be fine.
3. When using the eyepieces can you use two eyepieces that the manufacturer states as 16mm for example, or do they have to be matched to close tolerances ?
I was suspicious, and working for a retailer at the time enabled me to borrow eyepieces first to test their suitability. I tested pairs by viewing a distant paper scale through a barlowed C8 to check for magnification difference. I found no detectable differances. If you want to see the effect, try putting in a 15mm on one side and 16mm on the other! I frequently try pairs different pairs of eyepieces at star parties, and have never had any problem with differing magnification.
4. As I recall you had a control box with push buttons to make the alignment change and cause the two images to coincide. Can you please provide me with some details as to how this control box does the alignment? Is one of the primary mirrors shifted?
I set up my motorised right primary, not so much for myself, but to help other people view through the binoculars. For some inexplicable reason their are subtle differences between collimation settings betwen people. I think it has a lot to do with how accurately thay set their inter-pupil distance, because the problem seems much less marked at higher powers, where the eye pupil is overfilling the telescope exit pupil. People rarely have to adjust collimation at X100 but offten at X50. You also get small errors in eyepiece centering with the higher powers. I found it very difficult to collimate the binos for people when I was doing the adjustment at the back via verbal instruction from the observer. People not familiar with binocular telescopes just don't understand what they are seeing. I found with the two axis push button control that people could play with the buttons and collimate without instruction. When they are in control it seems to come naturally, even with people unfamiliar with using telescopes.
In my set up I have my right mirror on a plywood plate about 10" square. I have Edmund 2 rpm 3 volt DC gear motors mounted on diagonal corners with their shafts pointing to the bottom of the tube. In the other corner I have a cap head bolt going through the mirror mount base plate into a t-nut hammered into the front of the mirror mount plate.It goes through a spring sandwiched between the two boards. This way if I need to make an adjustment to the overall foacl plane position on the right side, I can turn this bolt from outside the telescope. Then turn the two motorised XY collimation bolts to level the mirror up again. I have only done this once. The motor shafts are attached to small collars which attach the motor shafts to a lengths of 5/32" threaded rod which like the corner bolt pass through sandwiched springs and then into t - nuts hammered into the main mounting plate. One corner is thus unsupported, but I have only noticed a very slight loss of collimation at high power when I go to the horizen.
To clarify then, we have the vertical motor in the top left corner of the mirror plate, the corner pinion bolt in bottom left corner and the horizontal adjustment motor on the bottom right corner. I found the 2rpm motor driving the 3/16" rods just right for this scope.
As far as wiring the handbox, I mounted a battery pack in the handbox and they have been going for 5 years. I wired a simple circuit with two discreet connections to the battery for each motor, giving reverse directions. I'm sure it could have been done more elegantly. An expert friend tells me I could short the thing if I hold down two buttons at once, but I've had no problems yet. For my next binocs I will probably dispense with the motors and have two rods running down the tube to drive the mirror adjustments, particularly if my 20" binocs ever get built.
5. We intend to make the binoculars as a Dob design. Is there any differeces when making it fit two telescopes ?
There is no problem using the dobsonian mount. Most Newt dobs are built like this. I think the side boards need to be thicker due to the extra load of the tube assembly. I cut big holes in my side boards to save weight and now they are flaring out under the weight of the tubes. I like box section sides ie pine frames with thin plywood skins: they are very rigid and light. My 24" is built this way.
Just a couple of minor points on mirror collimation.
1. When you are adjusting the the collimation it is good to throw the focus on the motorised side out till you see a small `donut'. Then move the `donut' around till it is centered on the in focus star in the other eye. I found that if you have them both focused, your eyes will chase ahead of the motors and try and strain the images to merge. It can then get very confusing and stressful trying to collimate this way. When you are more experienced you can collimate by moving the image around while your eyes have pulled the images together, finding the point at which your eyes feel most relaxed and comfortable. I find after using them for years that my eyes have much better accomodation for fusing images than most.
We are used to converging our eyes to see close in stereo but most people have very little vertical accomodation. Your scope must be rigid in the vertical direction at least. Even if the images are 25% of the field apart in the horizontal you can often pull them in. The result is a strange bowl effect to the field and overlapping field edges. If people get way out with the collimation I often just center the star in one eye, moving collimation on the other mirror to also center it in its own field then open both eyes for fine adjustment.
2. Our accomodation for field rotation is ZERO ! The adjustment of tertiary mirrors rotating around their own optical axis affects this adjustment. I am talking here about the tiniest nudge ! If the fields are rotated slightly you cant see it easily but the view looks strange at lower power and you can get a headache that lasts for days...
The way I adjust this, is to go to the highest power, picking two stars that are separated by most of the field. I then decollimate in a vertical direction enough to see a parallogram of 4 stars when I have both eyes open. If the two sides of the parallelogram aren't parallel then you have field rotation. A slight rotation of the tertiary mirror ( around the axis coming out the side of the tube ) will move two stars (as seen in that eye )in the field but also rotate the optical axis. Move the stars again by primary collimation to form the parallelogram until you see parallel sides. As you get better you can move the star pair till thay are almost merged for fine tuning.
Doing these adjustments will ensure you get no eye strain at lower powers.
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