Fiberglass Telescope Tube Assembly

Telescopes need a tube. A tube is the easiest method used to hold the mirror cell and the spider and focuser all in position.

It is very important that the telescope stay in alignment. We mean by alignment that the mirror reflects the stellar light up the tube along the center of the tube, then the secondary mirror or diagonal reflects this light at 90 degrees out the side of the tube, where it now enters the focuser which holds eyepieces for magnifing the stellar image.

All of the pieces on the telescope must stay in position during viewing to obtain the best quality image.

There are several things to consider when building you telescope tube.

First, the tube needs to be ridged, water proof so it does not warp, and easy to handle. You don't need a heavy tube.

Second the tube length needs to fit into your car. So don't make a telescope that is too long and won't be easily transportable.

Third the tube protects all of the optical elements, the mirror and the secondary mirror. The tube needs end caps to keep dust and things from falling into the tube.

Here are some materials used for tubes.

1. Aluminum tube with thin wall. This is not a good idea, it is heavy, and the aluminum can hold heat. When it cools the heat is released to the inside of the tube which causes heat waves. Heat waves ruin the image quality.

2. Solid fiberglass tube is a bit expensive but very good for really permanent telescopes. The fiberglass tube can stay outside with little damage. Fiberglass weight about the same as aluminum tubing.

3. All wood tube. If you feel like cutting lots of long wood lengths and mitering them together to make a round or multisided wood tube it will be heavy.

4. Cardboard tube with no reinforcement. You can use just a cardboard tube but water moisture will eventually cause it to delaminate. Cardboard tubes are cheap and easy to fabricate.

5. The ideal tube can be a cardboard tube that you fiberglass with one coat on the outside of the tube. This helps keep water moisture from soaking the cardboard from the outside. Purchase a cardboard sonotube, burke tube, crete-tube, from your local hardware store. They come in sizes about every 1/2" in diameter.

What size tube do you need for a give telescope you ask? The answer lies in the focal length of the telescope. If you have a telescope mirror that has a focal length of about 48" or less, you have a rich-field telescope with wide field of view. You will need a tube that is about 2" larger in diameter than the mirror. On the other end if you have a telescope with a focal length longer than say 60" you can use a tube that is only about 1" in diameter larger than the mirror.

For the current telescopes of 6" we will use an 8" inside diameter tube.

Figure 1: Cardboard tube of 8" ID

Fiberglassing the telescope tube

The steps are straight forward.

First, clean the cardboard tube so it has no bumps or gouges. Do not remove the wax coating from the outside of the tube.

Second you need a warm day.. maybe 60 to 80 degrees. If it is too cold the epoxy resin will not set up and if it is too hot the epoxy may set up before you get the whole tube painted.

Start by placing a wood rod like a broom handle through the tube. Place the ends of the rod up in the air about 40 inchs so you can roll the tube around the rod during the epoxy painting.

Cut your fiberglass matt so it will wrap around the tube and over lap about 1/2". Let the ends of the fiber glass stick over the end of the tube about 1/2". The fiberglass matt will shrink as you it gets resin in the little mesh squares.

Get started by mixing enough epoxy resin to do the hole tube outside area? This may require some calculations and estimates of resin hardener needed to caust the epoxy to set up hard. You will need to ware rubber gloves that will not melt when handling the epoxy resin.

Mix your resin and hardener in a paper bucket. Get a cheapo paint brush about 2" wide.

Lay the fiberglass over the tube and start painting on the resin. Paint from one end of the tube to the other end in a swath about 4" wide.

Now roll the tube a few inches and paint more. Paint all the way around the tube.

Figure 2: Fiber glass laid over the tube.

Lay the fiberglass over the tube and start painting on the resin. Paint from one end of the tube to the other end in a swath about 4" wide.

Now roll the tube a few inches and paint more. Paint all the way around the tube.

Once your have the fiberglass all wet and sticking to the tube you need to trim the overlap so there is no overlap.

Have a sharp puddy knife or razor box knife ready. Cut along the over lap fiberglass so that there is one smooth seam edge.

Paint the ends of the cardboard tube with some resin so it will seal off the cardboard from potential water and moisture.

After the resin has setup, one day sitting around, you can give it another coat or two. This will thicken up the resin so you can sand it smooth.

Once you have all the coatings of resin as thick as you like you can cut the fiberglass overlap off the ends of the tube. Sand the tube as smooth as you like.

Now paint the outside so it is any nice pattern you like.

Figure 3: Fiberglassed Cardboard Tube

Strip and Paint the Inside of the Tube

The inside of the cardboard tube usually has a wax coating. This is to keep the cement form from delaminating when wet cement is poured inside. You need to strip out the wax layer.

This is done easily by starting at one end and peeling the first layer of cardboard and wax. The layer is a spiral of wax paper going down the tube. If the tube is very long you will find it difficult to pull the layer without tearing it.

But you need to get the whole waxy layer out of the inside of the tube. Paint will not stick to the wax layer very well. If you paint over the wax layer on the inside of the tube it will just be a terrible reflecting surfcase that causes ghost images.

So, strip out the inside wax layer and then purchase ultra-flat-black spray paint.

Spray the hole tube insides from end to end.

Then look down the tube and besure it does not show any relflection surfaces on the tube inner wall.

You will notice when you pull up the waxy layer that little cardboard fillaments stick up all around the tube. This is oky, just paint all the inside from both ends of the tube to cover all the cardboard.

You can also line the inside of the tube with black felt material. It is hard to glue this material to the inside of the tube.

Some have lined the inside of the tube with cork sheets. Be sure to paint them well before you glue them inside.

You are now ready to mount the mirror cell assembly. Do not cut the focuser hole or mount the secondary mirror diagonal holder tell they are ready for final assembly.

Design support and fiberglassing of a cardboard tube is a due it yourself and learn method.

I don't know if you will find any thing on Google.com about fiberglassing a telescope tube.

Some things just require you to go do it and trial and error.

Good Luck.