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Wednesday, October 15, 2008

How to Clean Your Telescope's Mirror

Over time, your reflector's mirror is going to become dirty. It's not a matter of if, but when. The trick is knowing when you actually need to actually clean the mirror. The surprising thing is that a thin layer of dust will impact your viewing so little that it's not worth some of the risks involved in cleaning. Cleaning your telescope's primary mirror should only be done when the mirror is so dirty that it could possibly affect your views.

Even most smudges will not affect your views (I know from experience!) This being said, I would recommend that you not clean your telescope's mirror unless it is really dirty or grimy (as mine was) or it really, really just irks you to have a dusty mirror.

Cleaning your mirror is not as scary as it sounds, however, and can be done by any amateur astronomer with a bit of patience and some distilled water! Just follow this guide, and I'll show you how to do it safely. (Speaking of safety, don't do this outside on a sunny day, or in a sunbeam indoors, you might catch a magnified sun reflection in your eye!)

Things you will need:

  • A screwdriver to remove the mirror cell
  • 3-4 gallons of distilled water (not spring water)
  • Dye-free (preferably scent-free) dish soap
  • A tub to put the soapy water and mirror in
  • A towel to set the mirror on while air-drying
  • Cotton balls (Optional)

Cleaning the mirror

  1. First, add one gallon of distilled water to the tub, and put maybe 5-6 drops of the dish soap in there. Slosh the water around with your hand to bubble it up a bit. We just want a bit of bubbles on the top of the water. Don't bubble it up too much, because it will make the rinsing step more difficult.

  2. Remove the mirror cell from the optical tube assembly. If you need help doing this, check this tutorial for instructions. Next, carefully remove the primary mirror from the mirror cell. The mirror is held down by 3 clamps on the edge of the mirror. Use a screwdriver to remove the screws and remove the clamps. Then very carefully lift the mirror out of the mirror cell. Don't touch the face of the mirror, just grab it by the uncoated sides.

    Look how dirty my mirror was!

  3. Once you've removed the mirror from the mirror cell, carefully lower it into the water bath. If the face of the mirror is not submerged, add distilled water and small amounts of soap as needed. The water should be 1-2 inches above the face of the mirror. Then slosh water over the mirror for 10-20 minutes, being very careful not to actually touch the face of the mirror.

  4. Simply sloshing the water back and forth for 10-20 minutes should remove almost all of the contaminants from the mirror. If your mirror has some real stubborn dirt on it, you can use cotton balls to clean the face of the mirror. This is where it gets real risky, though, so be extremely careful during this step. In fact, skip this step if your mirror looks good after a simple bath.

    If you decide to do this step, wet the cotton ball first. Then lightly drag it across the mirror surface. Don't put any pressure on it, just kind of let the weight of the cotton ball do the work. Use one cotton ball for one pass, wiping from the center outward, then throw it away. Reusing cotton balls can drag dirt across the mirror, potentially scratching it.

  5. Now we can rinse the soapy water off the mirror. Lift the mirror out of the water and run clean distilled water over it. I used an entire gallon of distilled water for the rinse stage. It helps to have two people for this step: one to hold the mirror and one to pour the water.

  6. Once you've got all the soap off the mirror, set it on a towel to air dry. Do not use anything to dry the mirror, as it will leave streaks and potentially scratches. Just tilt the mirror, making sure it is securely supported, and let it air dry. Mirrors air dry surprisingly quick, so resist the temptation to dry it.

Now you have a clean primary mirror, ready to be reassembled into the mirror cell in the reverse order that it was removed. When you tighten the clamps, don't tighten them down too much, as this can distort the mirror and affect your views. Just tighten them until they make contact, then a little tiny bit tighter.

If you follow this advice and work patiently, your mirror should come out sparkly clean and without any damage!

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Clear skies!

Disclaimer: I cannot take any responsibility for any damages, to you or to your property, occuring before, during, or after performing this telescope mod or resulting from any advice taken from this website. Do so at your own risk.


Anonymous said...

I have an auction find that is in need of help. It looks as if someone put tap water on the mirror cause it is covered with water spots. I tuned it up this afternoon and this evening I put it on Jupiter with disappointing results. So, it does indeed require cleaning. I'll follow your advice tomorrow, Thank you for posting this procedure.

Sean Welton said...

Actually, water spots won't affect your view in any appreciable way. The poor results are most likely due to bad collimation. Sky and Telescope has a really good article on collimation at http://www.skyandtelescope.com/howto/diy/3306876.html

Clear skies!

dennie assasin said...

Thanks For Another Magnificent Article. Where Else Could Anybody Get That Kind Of Info In Such A Perfect Way Of Writing? read here I Have A Presentation Next Week, And I'm On The Look For Such Information.

Anonymous said...

Thanks so much for this article! My old dob collected a lot of dust in storage and this guide is just what I needed :D

Unknown said...

I have a used telescope that was left in the garage for years without the telescope cover. Primary mirror has a dusty white film and after trying to clean it. It had minor results. Is there any recommendations on how to restore the mirror?

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