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Equipment Discussions >> ATM, Optics and DIY Forum

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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Spray painting a 70" mirror??
      #5541876 - 11/27/12 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.


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Lynnblac
super member
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Reged: 04/06/09

Loc: Arizona
Re: Spray painting a 70" mirror?? new [Re: Dick Jacobson]
      #5541892 - 11/27/12 11:20 AM

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

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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: Spray painting a 70" mirror?? new [Re: Dick Jacobson]
      #5541905 - 11/27/12 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.

Mike


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rboeAdministrator

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Reged: 03/16/02

Loc: Phx, AZ
Re: Spray painting a 70" mirror?? new [Re: Mike I. Jones]
      #5541921 - 11/27/12 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.

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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Spray painting a 70" mirror?? new [Re: rboe]
      #5541969 - 11/27/12 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.peacocklabs.com/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


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Darren Drake
Carpal Tunnel
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Reged: 10/09/02

Loc: Chicagoland
Re: Spray painting a 70" mirror?? new [Re: DAVIDG]
      #5542224 - 11/27/12 02:20 PM

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

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tcmzodiac
professor emeritus


Reged: 11/11/11

Re: Spray painting a 70" mirror?? new [Re: Darren Drake]
      #5542255 - 11/27/12 02:41 PM

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

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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Spray painting a 70" mirror?? new [Re: Darren Drake]
      #5542452 - 11/27/12 04:40 PM

Quote:

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




That would explain the IR overcoat of magnesium fluoride then.

http://en.wikipedia.org/wiki/Hubble_Space_Telescope#Optical_Telescope_Assembl...

Best,
Mark


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polaraligned
professor emeritus


Reged: 12/26/08

Re: Spray painting a 70" mirror?? new [Re: mark cowan]
      #5542536 - 11/27/12 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.


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EJN
Carpal Tunnel
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Reged: 11/01/05

Loc: 53 miles west of Venus
Re: Spray painting a 70" mirror?? new [Re: Darren Drake]
      #5542541 - 11/27/12 05:47 PM

Quote:

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




The Hubble mirror was sprayed with Krylon Ultra-Chrome.


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Bob Myler
Carpal Tunnel
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Reged: 05/18/06

Loc: St Louis, MO
Re: Spray painting a 70" mirror?? new [Re: EJN]
      #5542964 - 11/27/12 10:22 PM

Quote:

Quote:

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




The Hubble mirror was sprayed with Krylon Ultra-Chrome.






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Lynnblac
super member
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Reged: 04/06/09

Loc: Arizona
Re: Spray painting a 70" mirror?? new [Re: polaraligned]
      #5543678 - 11/28/12 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.

Edited by Lynnblac (11/28/12 11:28 AM)


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EJN
Carpal Tunnel
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Reged: 11/01/05

Loc: 53 miles west of Venus
Re: Spray painting a 70" mirror?? new [Re: Lynnblac]
      #5543881 - 11/28/12 01:17 PM

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

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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Spray painting a 70" mirror?? new [Re: EJN]
      #5543905 - 11/28/12 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...



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.


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Project Galileo
scholastic sledgehammer
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Reged: 11/14/07

Loc: Jefferson County, Colorado
Re: Spray painting a 70" mirror?? new [Re: orlyandico]
      #5543929 - 11/28/12 01:49 PM

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

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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: Spray painting a 70" mirror?? new [Re: Project Galileo]
      #5543961 - 11/28/12 02:14 PM

Might contact the University of Arizona Optical Sciences Center at http://www.arizona.edu/colleges/college-optical-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


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rboeAdministrator

*****

Reged: 03/16/02

Loc: Phx, AZ
Re: Spray painting a 70" mirror?? new [Re: Mike I. Jones]
      #5544036 - 11/28/12 02:59 PM

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

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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Spray painting a 70" mirror?? new [Re: Mike I. Jones]
      #5544059 - 11/28/12 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


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Spray painting a 70" mirror?? new [Re: DAVIDG]
      #5544085 - 11/28/12 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.


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TxStars
Pooh-Bah
*****

Reged: 10/01/05

Loc: Lost In Space
Re: Spray painting a 70" mirror?? new [Re: Lynnblac]
      #5544161 - 11/28/12 04:15 PM

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


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Mary B
Vendor - Echo Astronomy and Electronics
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Reged: 05/21/10

Loc: Minnesota
Re: Spray painting a 70" mirror?? new [Re: TxStars]
      #5544207 - 11/28/12 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 well and my 6 inch RC

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cheapersleeper
Post Laureate
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Reged: 01/22/10

Loc: Sachse TX
Re: Spray painting a 70" mirror?? new [Re: Mary B]
      #5544332 - 11/28/12 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


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ed_turco
Pooh-Bah
*****

Reged: 08/29/09

Loc: Lincoln, RI
Re: Spray painting a 70" mirror?? new [Re: cheapersleeper]
      #5545545 - 11/29/12 01:27 PM

It is possible that this may be a joke.

Ed Turco


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Spray painting a 70" mirror?? new [Re: ed_turco]
      #5545639 - 11/29/12 02:27 PM

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

Cheers,


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Lynnblac
super member
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Reged: 04/06/09

Loc: Arizona
Re: Spray painting a 70" mirror?? new [Re: ed_turco]
      #5545651 - 11/29/12 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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seryddwr
Innocent Bystander
*****

Reged: 02/19/10

Loc: La-la land.
Re: Spray painting a 70" mirror?? new [Re: Lynnblac]
      #5545662 - 11/29/12 02:50 PM

I met this man about 10 years ago. It's not a joke. I suspect that they will chemically silver the mirror, using a spray gun to distribute the chemicals. Nothing else makes sense.

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Scout92
member


Reged: 12/20/11

Loc: Hoquiam, WA.
Re: Spray painting a 70" mirror?? new [Re: seryddwr]
      #5570271 - 12/14/12 02:42 AM

Cool article

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Bill Whitehead
sage
*****

Reged: 07/11/09

Loc: Exit 135 - Garden St Parkway
Re: Spray painting a 70" mirror?? new [Re: Scout92]
      #5570492 - 12/14/12 09:06 AM

Howdy People,

It sounds like the 70" guy wants to observe with this monster. It also sounds like this thing is a really fast scope, like an f 1.5 or something close to it. Since it was supposed to be a spy satelitte it was created for a ccd camera in the place of the secondary.

So here is my question for you optics experts. Can this mirror focus an image that can be registered by a human eye? And, if so, what kind of eyepiece can be used?

peace, Bill


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cheapersleeper
Post Laureate
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Reged: 01/22/10

Loc: Sachse TX
Re: Spray painting a 70" mirror?? new [Re: Bill Whitehead]
      #5570497 - 12/14/12 09:11 AM

The structure he is building certainly does not look like it is for a 70" f1.5.

B


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Bill Whitehead
sage
*****

Reged: 07/11/09

Loc: Exit 135 - Garden St Parkway
Re: Spray painting a 70" mirror?? new [Re: cheapersleeper]
      #5570603 - 12/14/12 10:28 AM

B,

I didn't really get a good look at the structure in the video I saw. However, there are limits to what you can put into space, especially in the 1970's. I'm assuming they weren't planning on using a Saturn rocket for this, so the tube would have to be a reasonable size. Not much bigger than a Gemini capsule, for example?

Bill


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danjones
super member


Reged: 09/26/12

Re: Spray painting a 70" mirror?? new [Re: TxStars]
      #5571823 - 12/15/12 12:16 AM

loptics.com check out the big mirrors that guy does. he even lists companies that he works with. he does all his mirrors on his own grinding machines. they are huge. good luck with spraying, and with an automotive gun? really? if it runs off a hose and pump, i'd worry about pump oils entering the mix.

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cheapersleeper
Post Laureate
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Reged: 01/22/10

Loc: Sachse TX
Re: Spray painting a 70" mirror?? new [Re: danjones]
      #5571941 - 12/15/12 02:19 AM

I am amazed at the oddball comments from our Cloudy Nights crowd. The article refers to Steve Dobbs who owns Nova Optical Systems as being involved in this project. So an ATM who already has owned very large scopes and the owner of a company that does coatings are being subjected to cries of anguish from our peanut galler? Seriously, people, read the article and look up the names. I think these guys know exactly what they are doing.

Regards,
Brad


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Spray painting a 70" mirror?? new [Re: cheapersleeper]
      #5593059 - 12/28/12 06:05 PM

Looks to me like they'll be using a variation of this method of silvering.

http://angelgilding.com/PumpSpraySilverVideo.html

Dave


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mrlovt
member
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Reged: 09/08/12

Loc: Smyrna, TN
Re: Spray painting a 70" mirror?? new [Re: cheapersleeper]
      #5600218 - 01/01/13 07:47 PM

Quote:

I think these guys know exactly what they are doing.

Regards,
Brad




Cheers from this corner of the peanut gallery! These guys have a MASSIVE mirror on their hands; I can't wait to see what they do with it. Never second-guess a guy who loads a 40" telescope into a pickup truck and takes it around to show off the heavens!


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Jim Romanski
Carpal Tunnel
*****

Reged: 01/02/05

Loc: Guilford, Connecticut
Re: Spray painting a 70" mirror?? new [Re: EJN]
      #5601453 - 01/02/13 03:42 PM

Hopefully they'll remember that the flat black paint is for the tube and not the mirror.

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