Building a Telescope

The hardest part of building a telescope is beginning.

Fortunately, I had a good reason and a good example late last winter to take on this project. There was a competition fostered by one of the forums on the CloudyNights website to build a telescope that cost less than a hundred dollars. Matt, my San Antonio astronomy friend, posted the details on our local Yahoo group. He mentioned that he had found a place to buy a 4 inch mirror and a diagonal for $40 including shipping.

On a whim, I ordered the mirror and started thinking about how to build a scope. I decided that my goal was not only to design an inexpensive scope, but one that would be easy to build too. Maybe a family or a school science club would take up plan and build one of their own.

After the mirror had been sitting in my hobby room for a month or more, I began thinking of actual plans for building a scope. It seemed like I needed to divide the project into several areas: Optics, Furniture, and Mechanics.

For the Furniture portion, I thought of different ways that I could build a simple Dobsonian scope with as little use of wood working equipment as possible. I knew that a base could be constructed much like the base of my XT10. I needed to think about a way to build an optical tube. After scouring the local hardware store, I came up short on tubes that would fit the mirror, without being way too big. I decided that the easiest way to create the tube was to make a long box with open ends using standard lumber.

Next, I started working out the optics. On a clear night with a bright moon that shone nearly overhead, I put the mirror on my driveway, just in front of my garage door. My garage faces south, so I was able to find the moon’s reflection from the concave mirror on a piece of paper that I held along the door frame. I moved the paper up and down until the image was in focus on the paper. When it was, I made a small mark on the door frame and then measured the distance from the surface of the mirror to the mark. It was 36 inches, and I knew that the optical length from the mirror to the underside of the eyepiece needed to be very close to that distance.

The mechanical distance needed to be the distance from the eyepiece to the center of the secondary mirror plus the distance from the center of the secondary to the surface of the mirror itself. Knowing that distance, I knew that the optics needed three pieces of machinery. I had to find a way to hold the secondary mirror, a way to move the eyepiece in and out to focus, and a way to columnate the secondary mirror.

For focusing, I decided that I would use the oldest and cheapest focus trick: I would have a friction fit between the optical tube and the eyepiece so that I could move the eyepiece in and out for fine focus. How would I achieve such a fine tolerance? By applying just the right number of layers of duct tape to the inside of a hole cut with a 1-1/4 inch hole saw.

For the secondary mirror, I borrowed an idea from the web: using hack saw blades to spring fit a spider into the optical tube. However, with a four sided tube as I envisioned, I would need to have a spider with four legs, each leg resting in a corner of the tube. To make the spider, I slotted a piece of PVC to receive two crossing hack saw blades. I measured the distance from corner to corner and cut each hack saw blade an inch short of that measurement. I fit a piece of dowel into the PVC pipe with a 45 degree angle cut at its end. I used some clear silicon bathtub calk to attach the secondary mirror to the angle cut on the dowel. That gave me a platform that I could move around in the tube to have the secondary under the eyepiece, and in which I could rotate the secondary to correctly reflect the primary into the eyepiece.

For the eyepiece holder, I elected to make the eyepiece hole in one corner of the optical tube box. I used a hole saw to make the hole, making sure that it was pointed straight at the far corner when I was cutting the hole. After measuring twice, I cut the hole about 2-1/4 inches from the open end of the optical tube.

I jumped past the primary mirror cell construction in my description, though it was actually the first thing I made. I cut two pieces of 2x6. One was square and one was just a little longer, so that it could be attached to the sides of the “tube” made with 1x8 standard lumber.

For the cell, I suspended the square by counter sinking 1-1/2 inch bolts in the floating square, and then placing a spring between the floating square and the cell frame. I used wing nuts to adjust the floating mirror holding square from the bottom of the scope. For the springs, I obtained a screen door spring and pulled the spring until it came a little “unsprung”. I cut the spring at three coils for each of the adjusting bolts.

The tube itself was four 1x8 pieces of lumber 3 feet long.

The base was created from several pieces of 1x12. The two rotating base boards are 11” square. I originally used an old record as my turning device, and later found that it worked even better to use three furnature glides on the bottom board turning against the record. There are two vertical pieces to the base, braced by a piece of 1x6 on the forward end.

The tube turns in altitude on two 4 inch PVC pipe caps.

A piece of pipe can be used to aim the scope, or perhaps a red dot finder can be added to the set-up.

That is a building history for the wood box scope. Since I came up with the idea in Helotes, Texas, I began calling the design the Helotescope.

My friend Matt did mostly the same design. He added a used focuser instead of my friction device. That works very well too! The picture at the beginning of this blog shows our two scopes at a meeting of the San Antonio Astronomical Association this last winter.

I have taken this scope to several public star parties. It never fails to attract some young men that look carefully at the scope. I can see the wheels turning in their minds. However, I have not yet seen the third Helotescope.