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How to Prepare a Sonotube for a Telescope

Good telescope mirrors are among the most precise surfaces ever manufactured. Cardboard tubes are used to hold toilet paper. It doesn't sound like a match made in heaven. But in fact, sonotubes --- cardboard tubes used as cylindrical forms for pouring concrete --- provide economical mechanical strength and stability that's just right for amateur astronomy.

Things You'll Need

Sonotube
Drill
Utility knife
Rotary cutting tool
Super thin cyanoacrylate glue
Sandpaper
Paint

Instructions

- 1
  
  Sonotubes are stiff, heavy-duty cardboard tubes that can serve for years, if properly prepared.
  
  Trim the sonotube to the desired length. The cut can be made with a utility knife or with a rotary cutting tool --- such as the Dremel brand --- with a thin wheel attachment.
- 2
  Soak a few drops of the super-thin cyanoacrylate glue into the cardboard at the cut edge. The idea is to firm up the layers where the cut has been made.
- 3
  Sand the cut edge. This is where the soaking step pays off. If you use glue, the edge will stay nice and firm as you sand it smooth, rather than flaking off. The glue will also help seal against moisture when you use your telescope.
- 4
  Drill a hole large enough for the focusing tube. Apply glue and sand it smooth.
- 5
  Drill mounting holes for the screws that will hold the secondary mirror spider (if you're using one), the primary mirror cell, and the focusing tube. Soak some glue into the holes. Sand the inside and outside of the tube to remove any burrs.
- 6
  Dust out the tube. You may use a lightly dampened cloth if necessary to remove the cardboard debris.
- 7
  Paint the tube matte black on the inside. You should also paint the exterior of the tube to protect against the environment.

Astronomy