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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12565
Loc: Cumberland, R I , USA42N71.4W
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was there heat involved?
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Alan French
Night Owl
Reged: 01/28/05
Posts: 1484
Loc: Upstate NY
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Aluminum is evaporated by heating, usually on a Tungsten coil. There is a very nice chapter on this in John Strong's "Procedures in Experiemental Physics."
A thin layer of the aluminum coating does oxidize, and this top layer is harder than the original coating and serves to protect it somewhat.
Clear skies, Alan
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1531
Loc: Prescott, AZ
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Quote:
Actually the thick film roughness of the aluminum spatter coating may be greater than the smoothness of the prism surface. isn't that why mirrors scatter light?
edz
I'm not a coating guy, but I understand electron beam deposition and dielectric coatings are significantly better than standard aluminum. 1/20th wave Newtonian quartz diagonal mirrors with interferrograms are easily available at reasonable cost. Mounting and adjusting such mirrors is very familiar to the amateur telescope maker.
As to the surface accuracy of prisms, the only information I have seen is on the Denkmeier webiste. The specs on their standard models are 1/4 wave, premium model is special ordered by Denkmeier to 1/8 wave. Since most binocular manufacturers primarily build for terrestrial uses, I seriously doubt they would go the expense of figuring faces to an accuracy higher than required for terrestrial use. My understanding of prisms is that the angles between the faces also must be manufactured to a a high degree. Perhaps this is easy from a manufacturing perspective, I don't know, but it is a potential error source one does not have with mirrors.
A reason that comes to mind for prisms to be favored by manufacturers is that prisms are forever whereas in 10 years a mirror may need a recoat, neccesitating the return of the unit to the factory.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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BoyntonStu
sage
Reged: 01/22/06
Posts: 263
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Quote:
was there heat involved?
Yes and do.
Sometimes the substrates were heated. Mostly not.
OTOH All other non-heated areas that 'saw' the tungsten coil were also coated. I never saw any black coatings.
What would react with AL to produce 'black'?
Al and Si can produce 'purple plague' and AFAIK that is the only other color I have seen associated with Al.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12565
Loc: Cumberland, R I , USA42N71.4W
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I don't know. that's a good question for Roland Christen over in the Astromart Forums. I don't think I misinterpreted his comments.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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jkevn
super member
Reged: 03/02/06
Posts: 175
Loc: Tucson, Arizona
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Coating telescope mirrors with Al and SiO, no, the mirrors are not heated.
See this link:
http://home.covad.net/~alcoat/alum_coa.htm
I've coated mirrors using a similar procedure.
jkevn
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refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1014
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If you can see coatings in mirrors- usually a good brushing after with an isopropyl alcohol rinse will do the trick. ;-)
Jess Tauber
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KennyJ
Reged: 04/27/03
Posts: 10142
Loc: Lancashire UK
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< If you can see coatings in mirrors- usually a good brushing after with an isopropyl alcohol rinse will do the trick. ;-) >
Extremely amusing post :-)
Kenny
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Two eyes and a preference to use both
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Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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DJB
Pooh-Bah
Reged: 02/23/05
Posts: 1267
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Hi all,
We used the very same technique with our vendor when coating very large mirrors, like better than 130".
No heat was involved. However, that may be a different process altogether.
We used a deposition process that was accomplished by a charged ionic system.
Regards,
Dave.
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