Telescopes come in two main varieties: the reflector and the refractor. The reflector telescope has an opening in the front which is aimed toward the object being viewed, and the light travels to the rear of the telescope where it is then reflected back toward the front where it is then gathered by a secondary mirror which directs the light into the eyepiece. A refractor has a lens in the front and also a lens at the rear. The front of both telescopes should be covered when not in use.
Measure the outside diameter of the front of the telescope for which the cover will be made. Use this measurement when selecting the tin can to be used, making sure the can's inside diameter is about the same width as the outside diameter of the telescope. The can must be able to slide easily over the front end of the telescope.
Apply 4 to 6 pieces of double-sided tape to the inside of the can so the tape extends lengthwise on the interior, from the bottom to the top lip of the can. Space the tape lengths so they go all the way around the interior, with equal space between them. Place an additional 3 to 4 pieces of the double-sided tape onto the bottom of the can around the edges, with one piece in the centre.
Cut a round piece of felt from one felt square, large enough that it covers the entire bottom of the can. Insert the round piece of felt into the can and paste onto the tape so it is secured to the bottom. This piece of felt will prevent scuffs to the finish of the lip on the front of the telescope.
Cover the entire length of the inside of the can with additional felt, sticking it to the tape strips. Add felt as necessary until none of the metal sides of the can may be seen through the felt sections in order to prevent rubs and scratches from occurring to the exterior tube finish of the telescope when installed.
Gently slide the felt-lined can over the end of the telescope to cover when not in use. The felt-lined can will do a very good job of keeping dirt and moisture from accumulating on the lens of a refractor telescope or on the rear mirror of a reflector telescope.
If a temporary cover is needed in a rush, such as if a dust storm is approaching while caught outside with the telescope, stretch a piece of cloth across the front of the telescope and tie it on securely. Though this won't prevent moisture from reaching the lens or the mirror, it will prevent either from being sand blasted by the blowing dirt. Blowing debris can pit and scratch the sensitive lenses and mirrors of a telescope. If viewing lenses will be removed from a telescope when being stored, the hole of the lens tube should also be covered. Cut a 2-inch piece of tape and place a cotton ball in the centre of the tape. Insert the cotton ball into the eyepiece and press the tape onto the edges of the eyepiece tube to hold it in place. This will prevent both dust and moisture from entering the inside of the telescope through the eyepiece tube while the eyepiece is removed. Store the telescope in the case or box in which it arrived for the best possible protection. Not only will this protect the interior of the telescope from dirt and moisture build-up, but will also protect it from falls. This will keep the exterior looking like new for many years.