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cnchillstar
super member
Reged: 12/29/08
Posts: 102
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Hi, I have two 12" reflector in hand. and find a local coating supplier. but their answer realy confuse me.
1.enhanced aluminium coating can get a 98% reflect ratio, but he said that will change the mirror figure a little.espicially for precise mirror with a 1/10 wave or higher.
2.ordinary coating will get a more than 80% ratio.
but I know enhanced coating is very ordinary in US, how about the precision change for the mirror?
wish coating master to teach me which kind of coating should I use for 12" mirror.
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3463
Loc: Doylestown, PA
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I have the enhanced coating on my 18" mirror and when the seeing is good (for the USA NE) 700x plus gives very sharp views. There is much discussion and debate about coatings. Check the threads in the reflector forum going on about different coating types and their effect. You may find some info there.
OMI has a good article on coating here that may be helpful.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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Standard aluminum coatings are more like 89-90% reflective in the visible.
The reason why the figure may change slightly with enhanced coatings is that it is a multi-layer process to apply them. Even thou the the coatings are applied in a vacuum, there are slight thickness variablity with multiple coatings that will add up.
You can actually take a spherical mirror and by using a special mask in the coating chamber vary the deposition of the metal, cause the coating to be deposited so the mirror will be parabolized.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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HiggsBoson
Pooh-Bah
Reged: 02/21/07
Posts: 1118
Loc: Kal-li-fornia
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Cnchillstar
As stated above standard aluminum coatings produce reflectances in the 88 - 89% range over the visible spectrum. Enhanced coating go from 96 to 98% depending upon the vendor. I have heard about enhanced coatings causing change of figure and being less robust but I am satisfied that this information is not applicable to modern coatings available today. Enhanced coatings are a high tech preimum product of interest only to the most demanding views. It is unlikely that they would survive as a viable product if they ruined every mirror on which they were applied.
I am not an expert but I have made a small mirror and am quite happy with the 98% coating I have. Someday I will finish my larger mirror and I plan to use the same vendor and the same coating. I choose the star test as the ultimate arbiter of quality in a finished mirror. My small mirror measured substantially better than 1/10 wave on a Foucault test and I used a star test for final verification. Since its coating I have repeated this test many times and am satisfied that my mirror was not degraded by its coating.
If you are concerned about this I suggest the book ‘Star Testing Astronomical Telescopes’ by Harold Richard Suiter. Learn to perform the test and let it be you guide. One does not need fancy equipment or to know how to make mirrors to run this simple test. Also the star test includes the entire telescope therefore it includes alignment, thermal and other factors. If you can get a good test under these condition then the system is verified good.
You may wonder ‘if the star test is so good, why do you use the Foucault test during the making of the mirror?’ The reason is the Star test requires a star and a complete telescope. When making a mirror one may use the Foucalult test after only a few minutes of figuring. It is not practial to have to wait for darkness each time one wants to test. Also when making a mirror one would like to test just the mirror without the other sources of aberrations in the system. The Foucalt test is a good shop test that assuming a skilled operator is good to better than 1/10 wave. The Star test is actually more sensitive than the Foucalt test but does not readily produce quantities results. Skill is required to use the star test effectively.
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Michael
ATM: 6" F/9 Newtonian Travel Scope
ATM: 12.5" F/4.5 Real Soon Now...
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cnchillstar
super member
Reged: 12/29/08
Posts: 102
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thanks a lot! after two weeks, I will get two 12" reflectors. I think I will have a try for enhanced coating, that lab tell me 110 dollars for two mirrors. maybe I can add a 10" inside their oven....:)
yes, I will read OMI article and other matieral to learn deeply.
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