Cleaning your telescope's mirrors


Cleaning your telescopes main mirror is something that needs to be done periodically. As a general rule, you should clean it when it gets dirty enough to make a difference. When exactly this point is reached is a matter of opinion, but suffice it to say you probably shouldn't clean your primary mirror more than about twice a year. More than this and you could be wearing out the delicate coating on the surface of the mirror.

So... the mirror is dirty and you've decided to clean it.. where should you begin?

Well, first off.. if you have a BIG Dobsonian... make sure that there are no electronics around to get wet. Obsession recommends cleaning the mirror while still mounted in the cell. If you've got to remove the cell from the tube, this obviously is not an issue.

Required Supplies:

Cleaning the primary

Clean your hands thoroughly. You don't want to have any oils from your hand make contact with the mirror.

Begin by spraying distilled water onto the mirror. The mirror should be secured so that it can be elevated at about a 45° angle so that the water and dirt will roll off. Spray from top to bottom... generously soaking the mirror. Do this 3 or 4 times. This will get any heavy dirt off the surface of your mirror.

Fill a clean bucket with distilled water and 1 drop of dish detergent. DO NOT use more than 1 drop or you'll get suds which can cause a film on your mirror. 1 drop is all you need.

Lay the mirror flat on it's back and fill the mirror about 2/3 with water. Since most telescope mirrors are parabolas, they'll hold some water.

Put a large piece of sterile cotton into the water and let it soak. Then, put the cotton on the center of the mirror. Do not put any pressure on the cotton. Using only the weight of the cotton, move the cotton around in a widening circular pattern. Work from the center towards the outside. Discard the cotton. It's not worth reusing the cotton. You'll save only a couple of cents but risk having some trapped dirt particles scratch your mirror. Repeat the process a couple of times.

Return the mirror to the tilted position used for spraying above. Liberally pour clean distilled water onto the mirror to rinse off any soap residue. Again, be generous in your use of the distilled water. It's cheap.. and you don't want any residue to form on your mirror.

Tear a large chunk of cotton and pat dry the mirror. Be careful not to turn the cotton over. This might allow oils from your skin to come in contact with the mirror. As soon as smearing of the water starts... discard the cotton and get another piece. Continue until the mirror is dry and free of spots.

Your mirror is now squeaky clean :)


Cleaning the secondary

The secondary is fully 1/2 of your optical system. It's performance directly affects the telescopes overall performance. The primary, however, is a much less expensive piece of the equation. Clean the secondary in a manner similar to the primary, but.. .feel free to clean it a bit more often. If it's coating begins to wears out, you can get it recoated or replaced for a fraction of the cost of your primary.