The bottom of the Newtonian reflector
A problem with Newtonian reflectors is that in very cold weather, the back of the primary mirror can cooled by being exposed almost directly to the cold, ambient air. This can result in the front of the primary mirror dewing up even though it is at the bottom of a long telescope tube.
One way to ameliorate this problem is to use foam packing material, cut to size and attached to the sheet at the back of the primary mirror. Two layers of this material can be used (depending on thickness) and they can be attached using double sided sticky tape.
The foam insulation in place
This insulating foam protects the back of the primary mirror from the cold, ambient air. However, if any stray light should happen to fall on the bottom of the scope, the white, semi-translucent foam will direct some of it outwards towards the edge of the telescope tube. This presents the danger that some unwanted light might make its way from the back of the scope onto the edge of the primary mirror. Sources of such light could be street lights, light from windows, street light reflected off building walls etc.
What is required is to cut a disk of matt black, 1mm styrene Plasicard and attach at least one sheet to the foam insulation and with a slightly greater diameter than the foam insulation. This serves to prevent extraneous light from falling onto the foam insulation, as well as adding another thin layer between the ambient air and the back of the primary mirror.
The black styrene Plasticard on top of the insulation foam
A finishing touch could be to attach a black shower cap to the bottom of the scope. In addition to extra darkening, this would trap another layer of air at the bottom of the scope to act as extra insulation. 1mm black Plasticard, A4 styrene sheets can be obtained from Amazon.
