This is the third rebuild of my 14 inch binocular telescope. A project inspired by my wife, a math/science major in university. She would drag me out into the night to look at some far off event in the dark sky. So eventually we had to get an 8 inch Schmidt-Cassegrain telescope. Nice scope, except one soon realizes that in telescopes, bigger is definitely better. Which means I want to get a big scope. Buying one would be simple, but where is the challenge in that? I bet I could make one. That would be a fun project. I reasoned that if we can see great things with only one eye, two eyes at the same time should be even better. Build a binocular telescope. For a good discussion for the advantages of binocular vision and visual issues in aligning a binocular telescope read Arie Otte at arieotte-binoscopes.nl .
Two telescopes perfectly matched, perfectly aligned with each other, side by side, acting as one great telescope. Both eyes seeing into the night, combining their images in the mind to form one great picture. That’s what I’m envisioning anyway. However there’s one problem, I know nothing about building telescopes. The solution would be to get a book, or three very useful books from Willmann-Bell publishers:
“Engineering, Design and Construction of Portable Newtonian Telescopes” by Albert Highe
“Telescope Optics” by Rutten and Van Venrooij
“The Dobsonian Telescope” by Kriege and Berry
I managed to find two 14 inch mirrors. They were not a perfect matched set, their focal lengths were within 1.5% of each other. The mirrors should be of the same focal lengths so that the resulting magnifications are the same. It is obviously easier for the the mind to put two images together into a whole picture if the images match to begin with. At this point in the project, the bigger issue was could I design and build a scope so that it actually worked. I”ll deal with mirror matching later. I decided to design the scope using the optical properties of these two mirrors. If the mirrors didn’t work well together, I could have them reworked to make their focal lengths match.
Working 5-6 hours a day, reading the books, engineering the design, building most the parts by hand, it took a solid year to finish the scope. I was pleasantly surprised at how well it worked. You could actually see stuff with it. The view from each mirror was clear and sharp. The difference in magnification between the telescopes didn’t appear to be a major problem. The eyes were able to accommodate the differences to give one clear picture.
In any design project, often what looks good on the drawing board doesn’t always work so well in the real world. The scope worked, but there were many areas that could be improved. This first year was really a big learning experience and I expected that there would be design deficiencies. So I set to work re-engineering it with improvements. After another year the second telescope was finished.
The Second Design:
The second design was a definite improvement. but still there were issues. The altitude bearings were on the opposite ends of the telescope. This required a large box structure to hold the scope. This box then had to carry the heavy load toward the center so everything could pivot about the azimuth bearing. As the box rotated about the azimuth, any flexing of the box would be telegraphed up to the scope via the vertical bearings. This would result in the telescope itself flexing, putting the mirrors out of alignment. Then I’d have to stop viewing and realign the mirrors. This issue had to be addressed.
One solution would be if everything hung off of a central column, that would eliminate the base box issue entirely. The biggest problem I saw with a central pedestal is that the scope would now have two heavy mirror structures hanging off each side of the column. The telescope would want to act as a seesaw, rocking back and forth, causing a possible vibration problem. The 8 inch Schmidt-Cassegrain telescope is carried on a single pivot point which results in its biggest design weakness. Any touching of the scope sets it to vibrating until the force loads dissipate. A pedestal design would need to be solidly built with ridged connections from the ground board up to the telescope attachment point. No looseness, no flexing, no vibration. Back to the CAD program (Sketchup) to redesign it for the third time.
The Third Design:
The mirrors were moved 4.25 inches further apart than on the first two designs. This was to make room for the central column which is 4″w x 5.5″d.
As can be seen on the above photo, all the electronics, the computer module, the power pack, the vertical axis motor have to fit on the 4 inch wide pedestal. Space is at a premium, everything is very tight. I utilized the ServoCAT/Argo Navis computer control and drive system from StellarCAT. This system had the flexibility to be used in a very tight space. The drive motors are able to easily handle the 100+ pound weight of the complete optical unit.
The pedestal itself is attached to the base plate which is made of MDF board. MDF was used because it is very rigid, it will break before it bends. The issue is to avoid any flexing in the pedestal base plate unit. Unfortunately, MDF has very little structural strength. Therefore a 1/8 inch thick steel plate was bolted to the bottom of the column, and then the plate is fastened to the MDF base board using t-nuts around the perimeter. MDF offers another advantage in that it will not crush. as would be the case if regular plywood was used.
The pedestal itself uses hickory as the structual material used to attach the metal plates on the bottom and top. Again, if a softer wood was used, it would be crushed by the weight of the telescope shifting from side to side. Any crushing would loosen the entire structure, destroying its stability. The cutaway view illustrates shows the design, with 1/4 inch baltic birch plywood used for the sides. Baltic birch plywood is excellent in its strength and stability.
The pedestal is cut away at the bottom, to make room for the azimuth encoder.
The mirror assembly is mounted to the pedestal by a detachable axle unit. Detachable to make the unit more compact for moving and storage. The axle is 3/8 inch stainless steel, enclosed in a mounting block which holds the axle firm, preventing any rotation. The mounting block has a tongue and groove design which is matched by the top of the pedestal. Thus when the mirror assembly is mounted onto the pedestal, it can only be clamped down when it is properly aligned.
Most of the odd hardware for this project was sourced from McMaster-Car, McMaster.com. This entire design would be very different without their wide selection of hardware.
The mirrors are mounted in a 10 inch deep housing, which is hung from the axle unit by four bolts. As each mirror box is only supported on one side and cantilevers out 20 inches, the box needed some depth to resist sagging under the load of both the upper and lower optical units. Plus the box unit needs to be able to handle some rough treatment in the back of a van as it’s being transported.
Inner Pupillary Distance Adjustment:
The distance between our eye pupils is different for each individual, therefore there has to be some way to vary the distance between the telescope eyepieces to accommodate each user. This design accomplishes this by rotating each upper optical unit. The units only have to rotate 1/2 inch to move the eyepieces together or apart by one inch. This rotation is achieved by a simple plate the moves in or out by turning a screw drive.
The 90 degree diagonals were not designed for use in a binocular telescope. Therefore when they are used at an angle to each other, their corners prevent the eyepieces from moving close enough to accommodate the required distances between our eyes. Solution, grind off the corners of the diagonals.
Secondary Mirror Support:
The upper optical unit is supported by 1 inch 2024-T3 aluminum tubing. The fasteners are cut from 1/8 inch 6061-T6 aluminum angle.
The support for the secondary mirror was constructed with spring loaded thumb screws to adjust the mirror.
Mirror Adjustment-the Lower Optical Unit:
I use a laser collimator to first align the mirrors in each telescope individually. Then the two telescopes need to be aligned with each other. To accomplish this final adjustment, the primary mirror positions are designed to move horizontally. Each mirror cell is mounted on a low-profile sleeve bearing-rail system obtained from McMaster-Carr (McMaster.com). The right mirror can move in the X axis, and the left mirror moves in the Y axis.
These pictures show the placement of the low-profile sleeve bearing-rail system. Here I have placed on rail on top of one mirror support arm. There is one rail placed under each mirror rocker support arm. The primary mirror cell rides on a triangular base which has a vertical adjustment motor at each corner.
Aligning the Two Telescopes:
To align the two telescopes to each other, the mirror cell horizontal positions are controlled by a 1/4 inch threaded rod anchored to the supporting frame. A t-nut is placed on the rod and is rotated by a hand knob belt system. This carries the t-nut horizontally along the rod and the mirror cell with it.
In this binocular telescope design, I found that during the initial alignment of the two scopes together it is best to use a big easy target. On the initial attempt to coordinate the two telescopes, one has no idea where the two scopes are pointed in relation to each other. Using a big target, such as the moon, will greatly help in getting a rough alignment of the two telescopes together. You can easily find the moon in either scope, and then you can determine the relationship between the two scopes. (e.g., If the right scope is aimed too far left, then you need to adjust it to the right, and if the left scope is looking too high in relation to the right scope, then it has to be adjusted down). On the final alignment, I found that Polaris was a great star to use. I didn’t have to chase it across the sky, which allowed me to take my time fiddling with the mirror positions.
The trick to aligning the two scopes together is to move the primary mirror in the opposite direction than one would assume looking through the eyepieces. As the mirror is moved horizontally by turning the adjustment knob, the adjustment field in one scope can be seen moving in relation to the target field in the other scope. In the adjusting scope, move the adjustment field further away from the target as seen in the other scope. Having moved the adjusting field away from the target field, then realign the mirrors in the adjusting telescope. This will swing the field of view back toward the target field of the stationary scope.
Adjust in the X axis first, then the Y axis. Our eyes will try to make a clear picture out of two visual inputs. When looking at two views which are offset horizontally from each other, our eyes will snap the two views together into one. So if we adjust the telescope views horizontally first, our eyes will combine the images before they are actually aligned in the telescope. Leave the images vertically offset, move them so they are aligned horizontally. Then move them into alignment vertically. Once the two telescopes are aligned with each other I found I didn’t have to make any further adjustments, therefore I lock the manual adjustment knob in its position. This can be done using a set screw. Fine adjustments for final mirror alignment will be done with the motors at the mirror support points.
A simple solution to adjusting the knobs while at the same time looking through the eye piece is to use a PVC adjustment rod. The end is notched to fit over the knob. Works great.
Mirror Motors and Control Box:
#1 and #6 motors are on the X and Y axis of the left and right mirrors. After the telescopes are aligned with each other, they will need very fine adjustments to finalize the fields of view. That is accomplished with the motors raising and lowering the individual legs of the mirror supports. With the motors on the axis, their movement will have the same effect as the horizontal movement of the mirrors themselves, but with much finer adjustments.
The motors to drive the legs of the primary mirror support are operated by a control box. With the control box, each individual motor at each pivot point on the primary mirror cell can be controlled separately. I used 2RPM motors, with a variable speed controller. This allows for very fine control of the motor speed, and thus the mirror movement. It handles the load with no problem at all. ServoCity.com was the source for the motor control assembly.
Aligning the Eyepieces:
The two eyepieces must be parallel to each other or their fields of view will not line up, one will be at an angle to the other. The high tech solution I use is to take two wood dowels, the same size as the eyepiece seat and put the dowels into the diagonal base. Simply make the dowels parallel and that’s it. You can tell they are parallel by running your fingers between them, any small variation in distance will be felt.
The 8 inch Schmidt-Cassegrain telescope is a good telescope, but it isn’t very steady, and it has a very small focus range. I had no idea how my binocular telescope would compare. Well it works great, better that the 8 inch I bought. Even with the pedestal design the binocular telescope is very steady, very little vibration, and it will hold its focus point. When the telescope is rotated vertically or horizontally I have not noticed any frame distortion. The mirrors stay aligned. An initial star test of the mirrors was very good. After three years work, I am very pleased with the final result.
I wrote this review with two objectives. First, hopefully other binocular telescope builders might pick up a few ideas they can use. Secondly, to show off my toy!
Thanks for visiting.
Murray Robert Ayerhart
Safety Tip: Stay Out of the SUN!
Or: How to Set Yourself on Fire with a Telescope.
Sometime in the distance past you have probably played with a small magnifying glass, focusing the sun’s rays on some paper, setting it on fire. Well, I can testify from personal experience it works even better with a great big lens, like maybe a 14 inch magnifier. By my rough calculations, a 14 inch lens will concentrate about 200 times as much heat on its subject as that 1 inch magnifying glass. Now substitute for the paper a not thinking too clearly telescope maker who decides it would be nice to work on his telescope outside on a beautiful sunny day. I’m working quietly on the telescope with the sun nicely warming my back. With a Newtonian telescope you are looking at a reflection of the object in the mirror lens.
You are driving at night. Suddenly you are being blinded by a car’s headlights in your rearview mirror. Question: Where is that other car in relation to you?
A. In front of you.
B. Behind you.
C. Sorry, I wasn’t paying attention. What’s the question?
If you answered B (behind you) you’re correct. Unfortunately I answered C.
I’m definitely not paying attention to the primary mirror, the big one. I’m minding my own business working on my telescope, the sun shining brightly behind me. I’m not paying attention to the fact that the sun’s nice warm rays are also hitting the mirror. Nor am I paying attention to where that 14 inch lens is focusing those nice warm rays, focusing them into a pinpoint of incredibly intense light and heat. Evidently the mirror was aiming that incredible heat directly at me. Being the observant type, I noticed that I was on fire when the flames and smoke were in my face. Leaning back to see what was going on, I watching the death ray from the lens track up my jacket. As I moved about, the heat from the mirror kept burning a path across my chest until it started another fire closer to my face. I wasn’t keeping exact time, but I can confidently say it takes less than a second to set yourself on fire with a 14 inch lens. Doing some quick thinking, I decided it might be smart just to jump out of the line of fire. After the flames were out and the smoke cleared, I realized I was really lucky.
You have probably look up at the sun on a clear day. You know how intense even a very quick glance can be. Depending on a your pupil size, on a bright sunny day, a 14 inch lens will gather up to 30,000 times as much light as your eye. So if the light beam had tracked across my face, it would only take a fraction of a second over the eyes to do extreme permanent damage. I moved the telescope into the shade where there was less chance of being interrupted by the fire alarm going off in my head. Lesson learned, don’t work on a telescope in the sun!