TUBE
General The tube should meet several objectives
- Provide a rigid support for the position of mirror - secondary - focuser.
- Protect the optical elements against damage.
- Prevent unwanted light from reaching the eyepiece.
- Minimize air currents in the light path.
- Provide support for a finder.
- Provide a means to attach the optical elements to a mount.
There are two main types of tubes, the fully closed tube (called just 'tube') or an open framework called a truss tube.
See also: "Tube Dimensions"
Materials The most common materials for closed tubes are aluminum, fiber, paper and wood. Paper tubes should be impregnated for greater strength and water resistance.
The Inside The tube inside should be made nonreflecting. There are several techniques for this. Installing baffles is generally considered the best method, but also the most laborious. This is seldom seen in commercial scopes. Lining the tube inside with black velvet or flocked paper is the second best solution. Just painting the inside black is the easiest method, but also the least effective. This is because every paint tends to be reflective if the incoming angle of the light is very shallow. To improve the black paint, some people use additives in the paint to get a rough surface (cracked walnut shells are popular).
The minimize air currents inside an aluminum tube due to thermal effects, the inside can be lined with a thin sheet of cork.
The truss tube of course does not have an inside. To prevent stray light, a shroud may be wrapped around the trusses.
Back End The back end of a tube is often left open to improve temperature adjustment of the primary. A fan may be used to speed this process. If the back end is open, care should be taken that no light can enter via this gap. It should not be possible to look through the focuser (without eyepiece) into the secondary and see out of the end of the tube. Or maybe easier; it should not be possible to see the secondary when looking into the back end of the scope (past the primary).
Diameter To suppress air currents due to thermal effects (cooling of the tube) it is best to leave at least 20 to 30 mm (1 inch) between the tube and the primary mirror. The tube diameter should thus be at least 50 mm (2 inch) more than the aperture. If you want to install baffles, then allow a margin for this as well.