Tubes for Reflecting Telescopes（管对反射望远镜）.pdf

Tubes for Reflecting Telescopes By: Jeff Beish, A.L.P.O. Mars Section (31 May 2017) INTRODUCTION It is no secret that many amateur telescopes take excessive time to equalize with the surrounding air, or cool down, so to speak. It is important to remove heat from components within the telescope to allow air in the optical path to stabilize. Even external parts, such as the mount, pier and surface area, will cause unwanted heat to enter into the telescope tube and spoil the stable air in the optical path. So, it is desirable to equalize the inside of the tube with the surrounding air as soon as possible. In our atmosphere cooler air tends to drop towards the ground because it is denser, therefore heavier than warm air. When cool air descends, often down through a telescope tube, it mixes with rising warm air to cause turbulence in the optical path and form what some telescope designers refer to as thermal cells. Because the density of air varies with temperature and the refractive index of air varies with its density, these thermal cells act like lenses floating around inside the tube. The result is similar to the effects of bad astronomical seeing. For serious planetary observers a portable telescope is usually not important. However, deep sky telescope user’s desire portability and weight becomes an important design constraint. Another major concern is keeping the optics from losing collimation when moving the telescope around. The cost of materials is important when considering telescope design and while materials such as paper or fiberglass may seem to be cheaper than steel or aluminum this author found the opposite to be true. This article will point out methods to reduce weight and annoying tube currents for both types of telescopes, and to reduce the overall cost of the telescope construction. However, a important rule to remember that we all subscri