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Spray painting a 70" mirror??

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#1 Dick Jacobson

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Posted 27 November 2012 - 11:10 AM

A Utah amateur is building a 70-inch telescope (see 70" scope). Believe it or not, he plans to use a spray gun to apply the reflective coating. Quote from the article: "Although the mirror has been ground and polished, it still needs its reflective coating. Clements, with Dodds' help, plans to do this with a special metallic liquid and an automotive spray gun."

Has anyone heard of coating a mirror with a spray gun? I don't see how you could get a smooth surface.

#2 Lynnblac

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Posted 27 November 2012 - 11:20 AM

This coating method is generally used temporarily to test optics. It is incredibly cheap and easily removed.

#3 Mike I. Jones

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Posted 27 November 2012 - 11:27 AM

What??? The surface ripple will be so bad that mirror couldn't even be used for millimeter-wave work. But, nothing teaches better than personal hands-on experience. He'll find out soon enough.

A truly "smooth" optical surface polish and coating should introduce no phase errors across the power spectral density curve of the wavefront roughness exceeding around 10-15 degrees. The equates to about 1/20 wave max roughness, and 1/40 wave would be even better. This holds true across the EM spectrum, from radar to MMW to infrared to visible and shorter.

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#4 rboe

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Posted 27 November 2012 - 11:36 AM

If he uses static charge he can mist the area above the mirror and let the electric field carry the coating to the mirror and avoid much of that ripple. Not ideal, but better than your typical spraying technique.

#5 DAVIDG

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Posted 27 November 2012 - 12:01 PM

What is most likely going to happen is that they are going to use the typical chemicals used to silver a mirror. Companies like Peacock Labs http://www.peacockla...om/products.htm sells the premixed solutions that they recommend be applied with a typical paint spray guns. At one time Peacock sold these solutions in spray can kits that contained the three solutions that one sprayed on, one after the other, to apply a silver coating just like one would do if they followed the typical Brashear method to silver coat a mirror in a solution bath. I believe Ed Jones used the spray can method to silver coat a flat for his "window" telescope.

Happy Holidays,
- Dave

#6 Darren Drake

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Posted 27 November 2012 - 02:20 PM

Wasn't the Hubble coated in a similar maner???

#7 tcmzodiac

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Posted 27 November 2012 - 02:41 PM

"Honey, I will not be home for dinner; I have to wet-sand the mirror"....

#8 mark cowan

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Posted 27 November 2012 - 04:40 PM

Wasn't the Hubble coated in a similar maner???


That would explain the IR overcoat of magnesium fluoride then. :funny:

http://en.wikipedia....cope_Assembl...

Best,
Mark

#9 polaraligned

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Posted 27 November 2012 - 05:43 PM

At 70" you don't need a coating for testing.
Maybe someone should contact either Steve or the scope builder for more info. Steve Dodds is a professional optician and would know better. The article could be wrong.

#10 EJN

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Posted 27 November 2012 - 05:47 PM

Wasn't the Hubble coated in a similar maner???


The Hubble mirror was sprayed with Krylon Ultra-Chrome.

#11 Bob Myler

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Posted 27 November 2012 - 10:22 PM

Wasn't the Hubble coated in a similar maner???


The Hubble mirror was sprayed with Krylon Ultra-Chrome.


:funny:

#12 Lynnblac

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Posted 28 November 2012 - 11:26 AM

This is the best solution for an ATM, just crating and shipping this mirror would cost a fortune, plus the risk of shipping damage. Where could one have a 70" coated? I doubt it could be coated for the standard cost of $10/inch, $700+. In the past all mirrors were silvered, before aluminizing was available.

"In 1935 the silver coating used since 1917 on the Hooker 100 inch mirror was replaced with a more modern and longer lasting aluminum metallic coating that reflected 50% more light than the older silver method of coating. The newer method of coating for the telescope mirrors was first tested on the older 60 inch mirror telescope."

You might remember that Hubble established the scale of the universe with the silver coated 100" Hooker telescope.

Yes, I think Steve Dobbs does know better and this is why he is helping silver coat this mirror.

#13 EJN

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Posted 28 November 2012 - 01:17 PM

How many coaters even have a vacuum chamber able to handle a 70" mirror?

#14 orlyandico

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Posted 28 November 2012 - 01:31 PM

if an aluminum coating reflects 50% more light.. then that 70" with a silver coating would reflect as much light as a 57" with an aluminum coating...

:tonofbricks:

and whether its 40" 60" or 70" I don't think you're getting any more resolving power, due to the atmosphere - so it's all about brightness, and the silver coating really kills it.

#15 Project Galileo

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Posted 28 November 2012 - 01:49 PM

Wow! That is a huge mirror. Neat stuff. He looks like he is having fun with it too. Rock on!

#16 Mike I. Jones

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Posted 28 November 2012 - 02:14 PM

Might contact the University of Arizona Optical Sciences Center at http://www.arizona.e...tical-sciences. I think they have coater tanks plenty large enough, and their price might actually be affordable. It would be good practice for a grad student.
Mike

#17 rboe

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Posted 28 November 2012 - 02:59 PM

Thanks Mike; was going to suggest that myself. Might be other universities that can do the same.

#18 DAVIDG

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Posted 28 November 2012 - 03:16 PM

A fresh silver coating reflects about 98% of visible spectrum vs about 90% for aluminum. One of the issues with the chemically deposited silver coatings were they were burnished to remove the white haze that would sometimes form when the silver coating was first deposited on the glass. The burnishing process produced sleeks which scattered light. So for a telescopes like the 100" Hooker, when the switch was made from silver to aluminum, the aluminum coating scattered much less light then the burnished silvered one. The lack of stray light allows about 1/2 magnitude fainter to be seen and imaged.

- Dave

#19 Mark Harry

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Posted 28 November 2012 - 03:26 PM

Heard of the Peacock coating process-
*******
Let 'em spray it, and look at the full moon!!! Maybe go "moonblind" for the next day!!!!!
We could call it being "moonshined"!
At that diameter, I believe a good appraisal could be gotten from the bare glass alone.
M.

#20 TxStars

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Posted 28 November 2012 - 04:15 PM

You don't have a company ship something like this.
You rent a truck and drive it yourself. :p

#21 Mary B

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Posted 28 November 2012 - 04:50 PM

He drives truck, I think he may have sources for shipping... awesome project and makes me jealous, all I have is a 10 inch dob :lol: well and my 6 inch RC

#22 cheapersleeper

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Posted 28 November 2012 - 06:14 PM

Somehow, I have a feeling that the gentleman and his friends know what they are doing. This is not his first telescope...

Brad

#23 ed_turco

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Posted 29 November 2012 - 01:27 PM

It is possible that this may be a joke.

Ed Turco

#24 Scanning4Comets

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Posted 29 November 2012 - 02:27 PM

A joke? That telescope frame looks mighty REAL to me....That thing is gonna be HUGE when it is finished! :bigshock:

Cheers,

#25 Lynnblac

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Posted 29 November 2012 - 02:42 PM

I do not think the group that has been working on this for more than 15 years considers their 70" scope a joke.






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