Sonotubes are designed to make cylindrical concrete pillars, but they make fine telescope tubes.
For more than 50 years, contractors have been using Sonotubes to easily create concrete forms to support buildings. Sonotubes are heavy-duty cardboard tubes sturdy enough to hold concrete and light enough to be easily positioned. Those same advantages of lightness and sturdiness are exactly what's needed for a telescope, so both commercial and amateur telescope makers often build their telescopes with Sonotubes. The most common Sonotube telescope is the reflective Newtonian design.
Instructions
Build a Newtonian Reflector in a Sonotube
1. Newton's telescope collects light with its primary mirror.
Select a focal length for the primary mirror. Somewhere between 700 mm and 1,000 mm is reasonable.
The primary mirror is the large, light-collecting element of the telescope, so the telescope can be no better than the primary mirror. Select a wavefront quality of lambda/8 or better (better being smaller, like lambda/10).
2. Place the mirror in its mount, called a mirror cell. Using a permanent black marker, draw short "target" lines at the exact center of the mirror. The mark should look like a half-inch-long plus sign centered on the mirror, except leaving three or four millimeters at the very center unmarked.
3. Put the mounted mirror temporarily in place at the end of the Sonotube.
4. Mark the location of the eyepiece focusing tube. Subtract the radius of the mounting tube from the focal length of the mirror. Measure that distance up the telescope tube from the center of the mirror and mark the spot. Don't worry about it being accurate to the millimeter, just measure from a spot on the outside of the tube that is approximately at the height of the mirror center.
5. Remove the large mirror and cut the telescope tube. A sharp utility knife or a keyhole saw can do the job, but a small rotary cutter such as a Dremel tool will generally make smoother edges. This could require two cuts: first, a round hole to fit the eyepiece focusing tube. Second, if the telescope tube extends more than a couple inches beyond the eyepiece hole, cut the tube shorter. If using a spider, drill mounting holes where necessary. Also drill holes for mounting the finder scope and the focusing tube.
6. Trim and sand around the cuts as needed. Soaking "super thin" cyanoacrylate glue into the cardboard at the edges will help keep them neat. Paint the inside of the Sonotube black. Let it dry completely.
7. Replace the primary mirror and install the focusing tube and the mirror stalk or spider assembly.
8. The eyepiece modifies the light collected by the primary mirror so that your eye can easily focus on the image.
Select an eyepiece focal length. Somewhere between 15 and 50 mm would be an appropriate choice.
9. If desired, align the telescope. See the next section for alignment instructions. Alignment is highly recommended.
10. Insert the eyepiece lens in the focusing tube. Use the adjustment mechanism on the tube to focus the telescope.
Aligning the Telescope
11. Shine the laser pointer directly through the center of the focusing tube. Laser collimators are available to make this job easier.
12. Adjust the position of the small flat mirror to center the laser spot.
13. Adjust the angle of the flat mirror so that the laser spot falls in the center of the target earlier marked on the primary mirror.
14. Adjust the angle of the primary mirror so that the laser spot is reflected right back to the laser.
Tags: primary mirror, focusing tube, focal length, laser spot, mirror that, telescope tube