The Practical Telescope #2

The Practical Telescope #2
by Dennis Allen

I went shopping the other day. While waiting to check out, I noticed people looking over the 60mm refractors on display. I've seen this situation repeated several times. Somebody will buy one of these "department store" telescopes, take it home, try it once, maybe twice, and get quite frustrated. The result: Another person turned off to astronomy and another telescope gathering dust in some closet.

These frustrations are caused by a number of factors: Cheap finder scope, sloppy mount, and a "kiddy" eyepiece. At the top of the list, however, is the mount. On most small refractor mounts, you have to unlock a swing arm, locate your object, and re-lock the arm. These swing arms are not very rigid, so the act of letting go of the swing arm causes movement. So much so, the object you worked so hard to find slipped out of the field of view.

I believe there's a solution: Dobsonize your refractor. Replace your cheap mount with a home-built dobsonian mount. The following is a material list, available at most hardware stores.

1 4.5" ¼" 1/20 thread round head bolt (bearing box adjuster) 2 2.5" ¼" 1/20 thread countersink bolts (bearing bolts) 2 1.5" PVC end caps, also known as spigot plugs (bearings) 3 ¼ wing nuts (for bearing bolts and bearing box adjuster) 4 felt furniture feet (to cradle bearings) 1 1.5" ¼" 1/20 hex bolt (azimuth bearing) 1 ¼" wood insert (holds the azimuth bearing) 14 1.5" countersink screws (thin black ones) 6" square piece of Formica (bottom of rocker box) Small pieces of felt (sticky on one side)

If you original refractor tripod doesn't have a ¼" 1/20 thread attachment, you might want to purchase a camera tripod. The more stable, the better.

As for the wood, use scrap ¾" plywood. I prefer the cabinet A-B Grade ¾"material leftover from my last telescope project. What you need is three pieces of 4"square wood, and two or three pieces of 12"-by-6" wood. These measurements assume you're making a mount for a 60mm refractor, 2.5" wide. If you have a 90mm telescope, you'll want to add 1.25" to all measurements.

A dobsonian mount has three components: Bearing box, rocker box, and rocker plate. The first thing I would build is the bearing box, which surrounds the telescope. All other measurements depend on the bearing box's size.

Bearing Box

Cut your three 4" square pieces of wood. Cut one of these pieces to a 2.5" width. This piece will be your bottom; the other two pieces will be the sides. For the sides, drill a ¼"hole dead center. These holes will be for the bearings. Also drill a ¼" hole ½"from the top. These holes will hold the 4.5" ¼" round head bolt.

It's important that both sets of holes line up with each other. Otherwise, your bearings won't work properly or you might not be able to insert the 4.5" bolt all the way across.

On the bottom, use a 1/8" drill bit to drill out room for you're screws. Use a ¼" drill bit to countersink the screw heads. On what will be the insides use a 3/8" drill bit to countersink the bearing holes.

Once you've screwed in the bottom piece, use your telescope to make a rough check. Your telescope should fit snug between the side pieces. If not, you might have to do a shave a little wood off your bottom piece.

For final assembly, drill out ¼" holes in the center of your 1.5"PVC end caps. Secure the PVC end caps with your bearing bolts and wing nuts. You might want to cover the heads of the bearing bolts with felt. Place your telescope in the cradle and insert the 4.5" bolt in the top holes. Tightening down on this bolt ought to secure the telescope.

Rocker Box

Take one of the 12"-by-6" pieces of wood. Drill a 3" hole, dead center. Now cut this piece in half. You should have two 6" square pieces of wood with half circles at the top. These will be your side pieces.

At this point, it's important to check these half circles. The distance from the circle to the bottom needs to be the same. Otherwise, your bearing won't rotate properly. If they don't match, drill and cut out another 12"-by-6" piece of wood. By having four sides to choose from, it's easier to find two that match.

For the rocker box bottom, cut a 6"-by-6" piece of wood. For added safety, you might want to leave the width a little fat. To this bottom piece, drill out your 1/8" screw holes and ¼" countersink heads.

After you've screwed the bottom to the sides, use your bearing box to check out the rocker box. You bearings should fit comfortably within the half-circles. If not, one or both sides may be tilted. To square the sides, cut a scrap piece of 4.5"(or the actual distance between by sides, along the bottom) -by-4" wood. Drill, countersink, and screw this piece in.

At this point I prefer to glue Formica to the bottom of the rocker box. It's up to you. There isn't much weight, so the friction of wood-on-wood isn't a big deal.

For final assembly, drill a ¼" hole for the azimuth bearing. Also install your four furniture feet in your half circles. Space them 90 degrees from each other. Far enough apart so they'll grip the PVC ends, yet not allow the ends to rub wood.

Rocker Plate

Take a 5"-by-5" square piece of wood. Drill out an 11/32" hole in the center. Screw in your ¼" wood insert, until flush to the tripod side. Now you can shave off the corners, changing a square piece of wood into something of an octagon. Add three pieces of felt, at 60 degree angles from each other.

Final Assembly

For that finished look, I prefer to use wood putty to cover the screw holes and other defects. Give everything a final sanding, then stain or paint your wood.

Give your telescope a test run. I'm sure you'll need to make a few adjustments. That's to be expected. If the PVC bearings slip, for example, use sandpaper to rough them up.

Conclusion

Since the initial work, I made a couple of improvements. For example, I added rubber washers to the azimuth bearing. They do help reduce wobble. I also added Teflon feet to the rocker plate. So now I can set up either on a camera tripod or on a solid table.

Oh, I did buy a hybrid diagonal for the refractor. I like it! Now I can use "real" 1.25" eyepieces, not just "kiddy" .96" eyepieces.

I went down to K-Mart. For $15, I bought a BB-Gun laser-sight. Very Nice! Works just like a Telerad. Of course I had to go to a local gun shop to get a mount, but I found out Orion catalog also has these mounts.

With a little work, anybody can turn a "worthless" department store telescope into something actually usable.

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This web page was last updated 04/03/08