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Cleaning a telescope's mirror

  1. Jul 30, 2014 #1

    I have access to a pretty big telescope, at least when compared to others I've played with. I'm itching to set it up but the mirror is not in great shape. It has been in storage for a long time and there looks to be some corrosion.

    I was wondering if there is anything I need to be aware of before I try to clean it, or if anyone has any tips?

    It's 300mm in diameter and the telescope was built in Stockholm in 1997 according to a brass plaque on its mount.

    I have attached a picture of the corrosion. It's a little hard to tell what you're looking at since it's a mirror but you can see it contrasted on the white background (off white, but center image) of a support strut. There are a few such marks.

    It's not really mine (a friend of the family's, but he is aware of it's condition and has given permission for us to use it in exchange for storing it :) ), so I don't want to go doing anything that's going to cause damage.


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    Last edited: Jul 30, 2014
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  3. Jul 30, 2014 #2


    Staff: Mentor

    Last edited by a moderator: Sep 25, 2014
  4. Jul 30, 2014 #3


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    If the coating is degraded enough to be visually noticeable, the mirror needs recoated. I'm guessing it still has the original coating, making it about 17 years old. That's about as much life as you can expect from a coating. The 'good news' is it only costs a couple hundred to recoat a mirror of that size. Cleaning is normally done using distilled water and isopropyl alcohol. It's not a task to be taken lightly. The risk of damaging the mirror/coating far outweighs any improvement you are likely to realize. It takes a rather shocking amount of dirt to noticeably affect performance.
  5. Jul 30, 2014 #4
    Thank you for your advice - i'd hate to do it any unnecessary damage by hastily cleaning it.

    I think i'll put it all together and see how it performs before I go spending that kind of money. I'm still very much a beginner and I think it might be more telescope than I can handle at this point anyway.

    Between the three of us at home that are interested, getting it fixed is affordable though.
  6. Aug 5, 2014 #5
    How I used to do it:
    dismount the mirror and hold it under running water to dislodge dust. Let most of the water drop off, and then "chase" the rest of the drops with one of those camera lens blower things - the kind where you squeeze a rubber bulb to make it blow air.
    Never touch the mirror with a solid object (like a rag or something), unless you want zillions of tiny scratches all over it.
    I let a particular mirror sit for about 10-15 years and this method no longer worked - the gunk became kind of cemented and would not dislodge. Never did get it clean.
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