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OK, this episode of Astronomy Hints is putting the cart well before the horse. In fact I think that cart is in another state. We haven’t even discussed the various types of telescopes & their advantages and here I am cleaning parts of one. But we had a chance to do some work on a telescope mirror, so we broke out the camera phone and took some shots of us cleaning our mirror.

OK, so first and foremost: do not clean telescope mirrors lightly. MORE TELESCOPE MIRRORS HAVE BEEN DAMAGED BY EXCESS CLEANING THAN ANY OTHER CAUSE.  You should only do a mirror cleaning about once every 2-3 years and even then only when it needs it. Your mirror will actually work quite well even with a few small spots on it – especially if there are in the center of the mirror where the ‘shadow’ of the secondary mirror falls.

But if you left the telescope in a damp area and it gets water stains, or if leaving it out without proper dust protection has resulted in some unacceptble staining or a layer of ‘grime’ has formed then you might need to clean it. If a few dust particles have fallen on on the mirror, just try to blow them off with a ‘blower bulb’ or compressed air (don’t blow on the mirror – moisture is bad for it) rather than doing a full cleaning.  However if your mirror looks like this:

Dirty Mirror

You can’t see the lawyer of grime that was also on this.

It might be time to clean it.

To clean your mirror you will need:

1) Some dish soap

2) Rubbing alchohol

3) A towel

4) Paper towels

5) Cotton balls (lots)

6) A sink or bowl big enough to hold your mirror

7) A phillips-head screwdriver

A ‘blower bulb’ may also be helpful.

First thing you need to do is remove the mirror cell from the telescope. This varies from model to model but there are usually 3-4 screws on the side of the tube that hold the mirror cell in place. Take the cell off the telescope (be careful, removing that much weight may cause your tube to tip in odd ways). You should have something that looks like this:

Mirror cell designs will vary

Now first of all, DO NOT TOUCH THE MIRROR SURFACE! I cannot stress how critical this is. Grab the cell by the metal cell parts.

You do not want to wash the cell, so you will need to remove the mirror from the cell. This usually involves a phillips head screwdriver.

There are many variations on how mirrors are held in cells, but this is typical.

Once you have all the screws and holders removed, remove the mirror by holding it on the edges. This is easy with a small 6″ mirror like we have here but with large mirrors you have to be very careful.

Place the mirror on a towel, mirror side up. At this point you want to fill a bowl (if it a small mirror) or a sink (larger mirrors) with warm water. Make certain the bowl or sink is clean. If it is not, do not clean it with an abrasive cleaner. Rinse it heavily to make sure there is nothing that will scratch the mirror’s surface.

Put a little bit of dish detergent and a capful of rubbing alchohol in the water. Then carefully place the mirror in the bowl or sink. Let it sit for a few minutes, or hours if it is very dirty.

Soaking in warm water with detergent and alchohol

Now you are ready to actually clean the mirror.Keep the mirror in the bowl or sink. Get the cotton balls and clean the mirror by making single, straight strokes from top to bottom. Use a new cotton ball for each cleaning stroke – don’t re-use them. Continue cleaning in this fashion until the entire surface is clean.

Straight cleaning strokes – from top to bottom. Use a new cotton ball for each cleaning stroke.

Once you have cleaned the mirror, hold it under a stream of lukewarm water. If you have cleaned the mirror properly the majority of the water should slide right off:

Again, avoid touching the surface. Keep the angle such that the water slides off.

Now most of the water will slide off, but some drops will remain. You want to dab off the larger droplets with a paper towel. Be extremely careful and touch the water drops with the towel, not the surface of the mirror:

EXTREME CAUTION should be used to avoid touching the towel to the actual mirror. Just soak up the droplets with the edge if you can.

You don’t want the water to dry on its own as it will leave spots. Once the big drops have been toweled up, put the mirror back on the towel (again, mirror side up). Put a tissue over the mirror while it completely dries.

You’ve done it! Your mirror is clean. Then replace it in its cell, then put it back into the telescope. You will doubtless need to collimate the telescope after doing all that.





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