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Webster Telescopes Forum • View topic - The "truth" about Mirror Coatings
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 Post subject: The "truth" about Mirror Coatings
PostPosted: Wed Jun 15, 2011 3:11 am 
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Joined: Thu Jul 02, 2009 11:48 am
Posts: 80
Location: Detroit Michigan
EVERY brand of mirror coating, and thus EVERY optician has experienced a mirror coating failure. It has happened to Intermountian, Kennedy, Zambuto, OMI, Spectrum, Nova, Flabeg, Swayze, Lockwood, Rice, Pegasus - literally everyone.

Sometimes people online like to act like there is a big conspiracy going on behind everyone's backs. But the truth is, every so often a coating fails early.

Looking at our records, we return about 1 in 20 mirrors due to some coating problem. We test the coatings with a 500w halogen lightbox. We look for any unevenness or large holes. There are almost always some pinholes in the coatings of large mirrors (that is because the coatings are so thin - 1/4 of a wavelength of light thin).

How long can you expect a mirror coating to last? Some last 1 year, others last 15. Nobody has ever figured out why. I like to think that if I get 3-7 years out of a coating that I did alright. Mirror coatings are simply part of the cost of ownership of a large telescope. One of the reasons we tell people not to worry too much about standard, semi-enhanced or enhanced coatings is that you can always try a different coating the next time you recoat.

If you want to ruin your coating in a hurry, let dew form on the primary each morning in the garage. The dew mixes with dust and pollution from the air and forms an "acid rain" that will quickly eat your coatings. Always use a dew chaser or 15w light bulb under your primary to keep the morning dew from forming.

If dew forms on your primary in the morning at a star party, simply rinse off the dew with distilled water. Don't rub or wash the mirror, just rinse off the dew.

---------------------

In this picture you can see a mirror we are testing. Note that the coatings are uniform. Note the existence of pinholes. This mirror is 2.25" thick, so the pinholes are greatly magnified while looking through the back of the mirror. Note also that you can see the uncoated spots around the edge of the mirror where the clips held the mirror face down in the coating chamber. This is normal.


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 Post subject: Re: The "truth" about Mirror Coatings
PostPosted: Wed Jun 15, 2011 4:57 pm 
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Another common phone call about coatings is the places on the mirror where there is no coating. This is often mistaken for "chips" because they exist along the edge of a mirror.

The mirror is coated upsidedown in the chamber and the only way a 150 lb. mirror is going to stay upsidedown is to be held by three clips. Wherever the clips touch the mirror edge, no aluminum gets deposited.

Take a look at this example below and see how the non coated areas could be mistaken for chips along the edge (this is what is typical for coatings done by Nova):


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 Post subject: Re: The "truth" about Mirror Coatings
PostPosted: Wed Jun 15, 2011 5:00 pm 
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In this picture, you can see how a secondary mirror is held by the clips during coating. Because the secondary mirror edge is masked off by the secondary holder, many customers never notice the lack of coating until they disassemble the holder for cleaning (This was a coating by OMI):


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 Post subject: Re: The "truth" about Mirror Coatings
PostPosted: Fri Jun 17, 2011 12:06 am 
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We will post some actual mirror coating problem pictures and some "It's time to recoat" pictures soon. If any of you have some, feel free to post.


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 Post subject: Re: The "truth" about Mirror Coatings
PostPosted: Fri Jun 17, 2011 6:04 pm 
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Posts: 80
Location: Detroit Michigan
OK, I see we did not do a very good job describing how dew forms in the garage. We have already had phone calls and emails saying "What do you mean dew in the garage? How could dew form in the garage?"

Let's use the word "condensation" rather than dew and see if we can make this make sense.


If we take two pizza pans out of the oven, one a metal sheet, and the other a Pyrex sheet, we know that the pizza will still be warm 30 minutes latter on the Pyrex sheet, because glass stores the heat much better than metal.

Now the same theory works in reverse. If you have a cold mirror in the garage, it takes a very long time for it to "warm up" along with morning's rising ambient temperature.

If you take a 45° F beer out of the garage refrigerator, it immediately gets covered in condensation. Guess what happens when you take a mirror that cooled down to 45° during the night and expose it to the morning air? Yes, the same condensation.

Why doesn't the secondary mirror dew up in the morning? Because the secondary mirror is a relatively small amount of glass, so it quickly keeps up with the rising morning temperature.

The primary is a heavy (sometimes 200 pound) chunk of glass, it takes a long time to cool and a long time to warm up. This is often called the "Flywheel Effect".

Let's recap: The Primary mirror becomes very cold in the garage during the night. As the morning temperatures rise, the Primary lags far behind; still cold from the night. Condensation forms on the cold mirror in the morning hours. The condensation mixes with dust and pollution on the surface of the mirror and becomes acidic. The acid eats through the incredibly thin aluminum coatings.

Many customers have never seen this condensation, because it has evaporated by the late afternoon. If you wanted to see it, check the primary around 10am.


How to prevent this from occurring:

Place a 15w lightbulb, dew chaser, or even reptile heater on the floor of the rocker box - under the primary mirror. This will keep the Primary just barely warm enough to not form condensation.

If you put the dew chaser on a timer (let's say from 4am to 11am), it will heat the mirror during the dewy morning hours, but let the mirror cool down for quick use in the evening (and save some electricity).


At a star party, aim the mirror cell East when you tie down for the night. The rising sun's rays will help warm the mirror from behind. If you have any portable 12v fans, aim their airflow at the face of the mirror.

If you still get some condensation, rinse with distilled water.

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