Telescope Making

My first telescope making project is a 12.5 inch f/6 of my own design. Of course, there are not any really original design ideas, since I have borrowed from every source I could find. I didn't want to build the same "Obsession" style clone that so many people build so I decided to build something that was adaptable to different philosophies. I didn't obsess about getting a low center of gravity. I actually think it is rather funny when I look at some of the ultra lightweight scopes. They are indeed very light, but somewhere in one of the pictures you will see 30 pounds of lead strapped on below the mirror! Why bother! My scope is set up to work on a fork mount in the future, a rather heavy 8x50 finder was used to make room in the future for the weight of a camera. The mirror cell was set to use a sling or two point edge support or to have the mirror glued to the nine points of the cell. It has a Truss tube system using 6 poles above the center support tube and six below to attach to the mirror cell.

First Light March 21, 2004

These pictures were taken on March 21, 2004. I tried to use the scope two days earlier but the scope was set up a little to long (on purpose). After adjusting the length of the poles I was all ready to go on Saturday night but the weather did not cooperate. Drat! I took the pictures on Sunday and then waited all day for it to get dark. Right after dark I finally got FIRST LIGHT and really enjoyed it. I saw Venus, Saturn, the Orion Nebula and a few doubles, and then the clouds rolled in. Very short, but spectacular none the less.

Here is an overall view. When you use six truss poles instead of eight the whole thing looks very lopsided. I thought the same thing when I looked at this. I got out the tape measure and checked. The top end cage and the center section are indeed parallel. Trust me!

Looking at the rear end of the scope shows the aluminum mirror cell, the lower truss poles, the wooden mirror cover and the cooling fan.

Looking at this closeup of the cell reveals that the mirror is sitting on 1 inch round teflon pads and using a two point edge support. I am currently using two 1/4-20 nylon screws as point edge supports. I can already see that this wont work. When I twist the collimation knob the mirror support pulls away from the mirror! There is way too much friction on the edge support. Soon I plan to modify the cell to get the whiffletree pivot points as close to the mirror as possible and then RTV the mirror to the triangles. Stay tuned!

Looking right down the barrel!

This is the classic "Boy and his Scope" picture. Happy, Happy!

Mirror Cell

Construction started with the mirror cell. It is constructed of aluminum and built using only woodworking tools. There were lots of challenges but a background in machining (even though it is 30 years ago) and a little recent experience on another project Zen Amplifier made it all work out OK. The tools consist of a heavy duty router, band saw, drills and even a table saw. The most important tool was the bench belt sander. Very important for shaping and finishing.

Here is a picture of the nearly completed cell. It covers a lot of bases as this cell was made adaptable to three different mirror mounting methods. The primary method is to use RTV pads to glue the mirror to the cell. It is a nine point system and there are three 1 inch diameter aluminum pads cut into the top of each triangular piece. There is also mounting hardware for a two-point edge support (not shown) and also holes to mount bolts to support a sling.

This view shows the cell with a dummy mirror in place. Since this cell was designed as it was being built this dummy helped a lot.

The mirror supports are mounted on teflon lined Ball Joint Rod Ends from McMaster-Carr. This prevents any side-to-side movement and locks the whole mirror support system in place. The triangular piece is made from 1/2 inch thick aluminum plate from the surplus metal dealer.

This view of the ball joint is looking in from the side of the cell.

The mirror cell is attached to the base plate directly by the 3/8-24 collimation bolts. These were ordered from Bob's Knobs and feature a steel rod and plastic knob. To keep the cell from moving laterally the collimation bolts are mounted in a press fit Bronze bushing from McMaster-Carr. The base plate is 3/8 thick aluminum and the bushing is 3/4 inch long. A spring from the local hardware store completes the collimation and lateral support system.

Here is the plastic knob and bottom of the base plate. It still needs some feet to protect the knobs and the fan and a few more tapped holes.

This picture shows the mirror clips. They just clear the top of the 1.5 inch thick mirror and are protection against the mirror falling out if the RTV should fail. This is one place were the dummy mirror came in handy. Of course it gave me an easy way to locate the bolts, but also it demonstrated graphically that the first set of bolts I made were to short!

These templates are the secret to cutting those large plates with woodworking tools. First the template is cut to the exact shape of the finished part. Next, that pattern is traced onto the aluminum sheet. A band saw is then used to cut the part to shape just outside the lines. A portable jig saw (Sabre Saw) can be used but is a lot slower. Then clamp the template securely to the part, put on a full face shield and ear protection and run a router around the template. The bit can have a ball-bearing at either the top or bottom. I must admit that the 3 inch holes were cut with a carbide tipped hole saw that cost me $40. I'm sure I will find lots of other uses for it to help amortise the cost.

After building this wonderful cell and holding it in my hand I see it a little different. I worry about the moment arm created by the ball joint pivot being over an inch behind the mirror and causing bending in the mirror. I drew up a sketch to illustrate what I think could happen if the mirror was glued to this cell. For comparison I have shown what I think would happen with an edge support. I don't know what to think.

To get a better feeling for the forces involved with this arrangement I drew another sketch. This one shows the actual three point support as generated in PLOP and then a cross section through it with an attempt to figure out the forces. To get the worst case, I assumed that this triangle was at the bottom of the mirror and oriented so it's short length was vertical. 1/3 of the mirror weight, or 4 pounds is assumed to be acting vertically through the center of gravity of the mirror. This point is 1 7/8 inches from the pivot point and so it generated a moment of 7.5 in-lbs. The reaction forces are shown to the right of the diagram. With forces this big, I think I will use edge support rather than RTV.

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