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.
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 tube inside should be made non-reflecting. 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 put additives in the paint to get a rough surface.
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.
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 focusser (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).
The maximum length of the tube (as measured from the mirror front) can be calculated to be:
This is the maximum length of a tube for which no vignetting will occur due to the tube. To this you have to add the desired length from the mirror front to the back of the tube. If the calculated maximum length is lower as the minimum length (see below) than increase the tube diameter (and thereby the distance 'm').
The minimum length of a tube is up to you. It should be at least so long that no light can come past the edge of the tube, into the focuser down to the focal plane. If you plan to use a barlow, it should not reach the barlow.
Another advantage of a long tube is that it takes longer to dew up the mirrors.
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.