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Uncoated mirror and the Moon

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18 replies to this topic

#1 leonard

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Posted 01 July 2014 - 04:30 PM

Hello ,

I have read and been told at different times over the years that an uncoated mirror will give the very best lunar views . The explaination is that the coating will disrupt the wave front coming off the mirror to some small degree because of its micro roughness .
Does anyone do this , that is use a mirror uncoated for lunar viewing and does it improve the detail in the image ?????
Leonard

#2 David Knisely

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Posted 02 July 2014 - 02:31 AM

Nope, uncoated mirrors might work on a bright moon in large apertures, but I prefer having all the light I can when viewing it, especially when pushing the magnifications. You can always make the moon fainter by using a simple filter, but with an uncoated mirror, you can't make it brighter. Clear skies to you.

#3 kfrederick

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Posted 02 July 2014 - 09:14 AM

On a 20+ inch telescope uncoated works great The contrast and color and it is not so blinding bright .It is different I like it . I may keep one of my 35 uncoated for it and planets

#4 youngamateur42

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Posted 02 July 2014 - 06:50 PM

I read about these. There's this 28" scope without an aluminized primary from Webster:

http://webstertelescopes.com/lunar.htm

#5 Pinbout

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Posted 02 July 2014 - 09:40 PM

I use my homemade wedge without the nd3 filter and no polarizer on the moon. the colors really pop.

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#6 Astrojensen

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Posted 04 July 2014 - 09:16 AM

I've heard of this. I tested it with a 6" f/10 newtonian with aluminized secondary and unaluminized primary. I found the view dim and unusable, even at low magnifications.

A herschel wedge gives the same result. Dim and uninteresting images. Filters are even worse.

What works a LOT better is to avoid dark adaption of both eyes, so that observing the relatively bright Moon with one eye doesn't create a huge contrast with the other eye.

But what works SUPERBLY well is to use a binoviewer AND prevent dark adaption, while keeping the magnification low to moderate, thus getting a bright image. This brings out all kinds of subtle colors and nuances I otherwise don't see. Using a binoviewer has been the most significant improvement in my lunar observing ever. It is very relaxing and brings out the details like nothing else.


Clear skies!
Thomas, Denmark
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#7 JimK

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Posted 04 July 2014 - 10:30 AM

... But what works SUPERBLY well is to use a binoviewer AND prevent dark adaption, while keeping the magnification low to moderate, thus getting a bright image. This brings out all kinds of subtle colors and nuances I otherwise don't see. Using a binoviewer has been the most significant improvement in my lunar observing ever. It is very relaxing and brings out the details like nothing else.

Clear skies!
Thomas, Denmark

Yes! This is exactly how I like to view the Moon on a relaxing evening, using binoviewers with some white lights on in the area (which also allow for easy chart review). I fully agree with Thomas on this technique.

#8 Aquarist

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Posted 06 July 2014 - 03:21 PM

Binoviewing is superb.

#9 Sarkikos

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Posted 07 July 2014 - 11:44 AM

But what works SUPERBLY well is to use a binoviewer AND prevent dark adaption, while keeping the magnification low to moderate, thus getting a bright image. This brings out all kinds of subtle colors and nuances I otherwise don't see. Using a binoviewer has been the most significant improvement in my lunar observing ever. It is very relaxing and brings out the details like nothing else.


Clear skies!
Thomas, Denmark


+1 :waytogo: Besides, binoviewing and keeping the magnification moderate makes eye floaters much less obvious.

Mike

#10 GlennLeDrew

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Posted 08 July 2014 - 05:01 AM

With weaker transmission efficiency comes the possibility of proportionally larger extraneous scatter, requiring better baffling so as to mitigate contrast-robbing veiling glare. This could be worse than coating scatter.

And the dimmer image will impact higher power view--for all apertures. Remember, object surface brightess scales as the area of the exit pupil, irrespective of aperture. A given exit pupil will deliver the same lunar brightness whether the aperture is 2" or 200". This is why a big scope can profit from an uncoated mirror; it can deliver suitably high magnification with a still-large exit pupil. But it becomes something of a limited, specialist instrument, which is a shame as it precludes fainter object observation--what the big aperture really helps with.

#11 jnewton

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Posted 09 July 2014 - 07:17 AM

So why doesn't someone buy one of these 28" Websters and review it for us? :D
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#12 Mason Dixon

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Posted 24 July 2017 - 09:29 AM

Was reading a large uncoated mirror creates a unique contrast when observing the moon, this the case or can the same be achieved with a moon filter?



#13 Tropobob

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Posted 25 July 2017 - 03:44 AM

I've heard of this. I tested it with a 6" f/10 newtonian with aluminized secondary and unaluminized primary. I found the view dim and unusable, even at low magnifications.


Clear skies!
Thomas, Denmark


I also tried this once, with a 4 inch F8 mirror. It was very, very disappointing.

Absolutely useless really.
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#14 Headshot

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Posted 26 July 2017 - 05:21 PM

I recall reading about using uncoated mirrors for lunar and solar work in some publication a long time ago. One of the main issues was scattering caused by the back of the mirror. It had to be ground flat, but at an angle, not polished, and treated (with lampblack?) to prevent light scatter from reaching the secondary.

 

I don't recall the book's name, but we are talking about 40-50 years ago.

 

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#15 stargazer193857

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Posted 10 August 2017 - 02:11 PM

I did not think of the back side. Yep. Horrible idea.

#16 stargazer193857

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Posted 10 August 2017 - 02:12 PM

I deal with lunar glare with bright adaptation, or extra magnification. I don't use lunar filters.

#17 stargazer193857

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Posted 10 August 2017 - 02:16 PM

Surely the atmosphere is worse than the aluminum. I heard a good coated can get it smooth.

#18 BJS

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Posted 13 August 2017 - 07:20 PM

I have talked to Eric at Webester about the "uncoated moon" telescope.  It is not an idea that will work on smaller scopes.  You need the large mirror; 28" in this case.  He says it is something you have to see for yourself, it is hard to explain.  I have no reason to doubt him, but that is a very pricy one object telescope.  I wonder how many of these scopes exist?  Has anyone looked through one? 

Brian



#19 Tony_Gondola

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Posted 14 January 2020 - 12:40 PM

I know this is an old thread but I had to comment. I tried this with a 10 inch mirror that I was re-figuring so it was un-coated. 4" or 6" is too small an aperture to really work but the image was certainly bright enough at 10". As others have said, the difference in the view between coated and un-coated is hard to describe. To my eye it looks a lot smoother and less harsh with a really lovely tonality to everything, like looking at wet clay or fresh concrete. I can't say definitively that it was sharper, just more pleasing.


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