Telescope Mirrors: Maintaining and Cleaning
Telescope Mirrors do get dirty over time and need to be cleaned, but it may not be as often as you think. It is generally advised that unless the dirt is drastically reducing the quality of the image you are trying to see, then don't bother cleaning the mirror. This is because the cleaning process is a bit risky and can permanently damage the mirrors with scratches if not done correctly. The first step is to maintain your mirrors so that you do not have the necessity of cleaning them often.
The main defense against dirt is to put on the lens cap whenever you are not using the telescope. If you don’t have one, improvise with a plastic bag or a shower cap. The eyepiece also should be kept covered with cloth or a plastic covering. Never touch the face of the lens with your fingers, as chemicals on your skin can cause havoc to the mirror. Always handle the mirror by gripping the sides.
When you find the dirt on your telescope mirror is seriously affecting your view, then it is time for a thorough cleaning. First, you need to prepare a tub with about a gallon of distilled water with a few drops of dish soap so that it’s a bit bubbly on the top. Then, you need to carefully remove the mirror from the tube assembly without scratching it. Place the mirror in the tub and slosh the liquid over it with your hand, without touching the mirror.
If there is stubborn dirt, do not rub the mirror with your hand. Wet cotton wool and very gently run it over the mirror which is still soaking in the soapy liquid. After each wipe, discard the cotton and take a new one. Now, take the mirror out of the tub and pour fresh distilled water over it until all the soap gets washed away. Place it on a towel and leave it to dry. Do not use any drying device or try to wipe it. It will not take very long for the mirror to dry, and then you need to fit it back in the tube.