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Uncoated Newtonian lunar scope

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#1 ccaissie

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Posted 20 January 2013 - 07:11 AM

I dug out my old Mirror #1, a 5" f/13 sphere from Edmund. Probably part of a Schlieren set. Was given to me by a physicist friend of mine when I was 10, and started this whole thing going. It got scratched and chipped at some point, and I had it recoated back in 1990. Stunning performer.

I dissolved the coating recently, but some Beral remained, so I pressed a 4" lap and slowly polished it away. I rechecked the figure and it now tested as a smooth asphere at 1/56 wave, which is about the same as the original sphere, which was equivalent to 1/20 wave.

The numbers mean little, but my plan is to mount it back in the original square wooden tube, and use it uncoated as a lunar scope. My star testing experience with uncoated mirrors showed me very beautiful lunar views.

Anyone familiar with building such a scope? Hints?

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#2 Mirzam

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Posted 20 January 2013 - 07:27 AM

I have been very impressed with the use of uncoated mirrors for lunar observing. The impression (using a 14") is that both atmospheric seeing effects and intra-ocular "floater" effects are substantially reduced. I do wonder though whether your 5" mirror will be a little bit too dim, even on the moon.

JimC

#3 Mirzam

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Posted 20 January 2013 - 07:38 AM

Another thing that comes to mind is the possibility of secondary reflections off the back of the mirror. It might be a good idea to blacken the back, maybe even to frost it a bit first. Or build some sort of light trap behind the mirror.

I thought about looking into partially reflective coatings (say 20%) to allow use of smaller mirrors for lunar observing with most of the benefits of an uncoated mirror.

JimC

#4 Darren Drake

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Posted 20 January 2013 - 09:02 AM

Webster offers a 28 inch version but I don't think they're exactly flying out the door.
http://webstertelescopes.com/lunar.htm

#5 Pinbout

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Posted 20 January 2013 - 09:33 AM

I just tested star tested a 8in f4.5 mirror I've been working on and the moon looked very cool. Even Jupiter was alright, I was surprised at the detail for something more than 1~ over. [I didn't over correct it, I thought I bought a blank and it came hyperbolic, I've been slowly taking it down. :lol:]

#6 ccaissie

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Posted 20 January 2013 - 01:57 PM

We've got some TiO2 flats/splitters that are about 20%, but no imaging mirrors.
Like the Webster, if I don't like it, I'll coat it.

#7 sopticals

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Posted 20 January 2013 - 04:12 PM

Another thing that comes to mind is the possibility of secondary reflections off the back of the mirror.

JimC


I have been using uncoated 22" and 25" uncoated mirrors for Lunar and planetary viewing and there is no problem with reflections off the back of the glass. So no need for frosting or painting glass back. I do suspect though that the 5" aperture only returning 4% of light incident to its curved surface will give sufficient illumonation at the eyepiece to be of any use. Even my 25" uncoated mirror returns to the eyepiece only about the light of a 4" coated mirror, so image brightness and contrast begin to suffer beyond 150x.

Stephen.(44deg.S.)

#8 Pinbout

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Posted 20 January 2013 - 05:11 PM

since its a small aperature long focal length, how about a solar newt?

#9 Mirzam

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Posted 20 January 2013 - 06:16 PM

I admit that I did not notice any back reflections from my 14" when trained at the Moon. I was not looking for them either!

With 4% reflection from the first surface 96% is transmitted with some loss due to absorption. Then there is 4% reflection from the rear surface, but this surface is not oriented to form an image and would tend to reflect light in many different directions. Some of this light would be transmitted back through the front surface and who knows where it would go? I think it may be worthwhile from a contrast standpoint to do something about it.

JimC

#10 careysub

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Posted 20 January 2013 - 06:42 PM

Another thing that comes to mind is the possibility of secondary reflections off the back of the mirror. It might be a good idea to blacken the back, maybe even to frost it a bit first. Or build some sort of light trap behind the mirror.


Hmmm. If you build a light trap behind the mirror, rather than applying treatments directly to the glass surface you would still get the reflection off the back (such as it is) unchanged. This would only affect transmitted scattering off telescope hardware - though it would block light from behind the tube if the bottom is open (not normally transmitted through a mirror surface) which might be worthwhile also.

#11 ccaissie

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Posted 20 January 2013 - 08:15 PM

In this case, the rear surface is the molded pyrex surface, so no issues there. I'll report in on this unit asap.

I'll have to be dark adapted to see anything.

#12 highertheflyer

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Posted 20 January 2013 - 09:36 PM

I built a version of Grotsky's solar scope, a 4.5 inch F16 and have used it for viewing the moon with no apparent reflected glare from the blackened back of the primary's uncoated mirror.
The secondary is also uncoated, yet you could use an aluminized secondary in this case to brighten the image, but gosh, never ever attempt to use it then as a solar scope!
Hope this helps, Jim

#13 careysub

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Posted 20 January 2013 - 10:17 PM

I built a version of Grotsky's solar scope, a 4.5 inch F16 and have used it for viewing the moon with no apparent reflected glare from the blackened back of the primary's uncoated mirror.


If you blacken the back directly then there should be no second reflection. That happens at the glass/air interface, so blackening the back is quite different from putting a black baffle behind it.

#14 Pinbout

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Posted 20 January 2013 - 10:57 PM

The secondary is also uncoated, yet you could use an aluminized secondary in this case to brighten the image,



it's easy enough to keep two secondaries and swapping them out for proper target.

#15 GlennLeDrew

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Posted 20 January 2013 - 11:55 PM

There is no problem with the small aperture in and of itself. Image surface brightness scales as the square of the exit pupil diameter. A 5" and a 50" working at the same exit pupil diameter will deliver identically bright images. It's just that the 10X bigger objective delivers an image 10X bigger.

For reference, a 4% reflectance is a diminution of 3.5 magnitudes. Two uncoated mirrors result in 0.16% reflectance, which is a diminution of 7 magnitudes.

#16 ccaissie

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Posted 21 January 2013 - 04:30 PM

There's an idea...make a pair of swappable secondaries. Coated for lunar, Uncoated for solar. Plus an auxiliary filter at the eyepiece, I'd imagine. .04 x .04 is still bright for the sun.

#17 John_C

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Posted 22 January 2013 - 08:23 PM

There's an idea...make a pair of swappable secondaries. Coated for lunar, Uncoated for solar. Plus an auxiliary filter at the eyepiece, I'd imagine. .04 x .04 is still bright for the sun.


Not only is it still bright but I would think you would want to check the reflectance of uncoated glass in IR and UV - if they are more than 0.04 you could be looking at eye damage!

#18 Pinbout

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Posted 22 January 2013 - 08:34 PM

from groski's white light newtonian page

For safe viewing, the image of the Sun needs to be reduced by 100,000to 1 in brightness across the full spectrum. This is an Optical Density of 5
( LOG(100000) = 5). Bare glass reflects about 5% of the light striking it. Thistranslates into an OD of about 2. The two reflections in a Newtonian would
results in an OD of 3.5. Still too bright for comfortable viewing so a third
reflections is needed. I used a homemade Herschel wedge to achieve this. I also
took advantage of the fact that when light is reflected off of bare glass at an
angle it becomes polarized. The closer the angle of reflections gets to the Brewsterangle of 57 degrees, the more complete the polarization. With both the diagonal
mirror and Herschel Wedge placed at 45 degrees the light becomes crossed polarized
and the image brightness can be farther reduced.


so can you use a ND filter and a polarizer to get from 3.5 to 5 OD instead of a wedge?

#19 michalh

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Posted 22 January 2013 - 09:15 PM

In the January 2013 Sky&Telescope, Gary Seronik had an article on the solar dobsonian, an old design that uses a half silvered flat mirror doing double duty as a filter AND secondary. A clever design and very safe. Might be an idea for your mirror.

The example Gary highlighted had a square tube, but a cylindrical version is shown here:

http://planitarium.net/sfsa/sunscope/

#20 Pinbout

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Posted 22 January 2013 - 09:28 PM

That's a John Dobson solar newt
I want to do that one also

#21 michalh

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Posted 22 January 2013 - 09:43 PM

Yeah, sweet in its elegant simplicity. Got a couple years before that solar eclipse...may be enough time...

#22 careysub

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Posted 22 January 2013 - 10:27 PM

from groski's white light newtonian page

For safe viewing, the image of the Sun needs to be reduced by 100,000to 1 in brightness across the full spectrum. This is an Optical Density of 5
( LOG(100000) = 5). Bare glass reflects about 5% of the light striking it. Thistranslates into an OD of about 2. The two reflections in a Newtonian would
results in an OD of 3.5. Still too bright for comfortable viewing so a third
reflections is needed. I used a homemade Herschel wedge to achieve this. I also
took advantage of the fact that when light is reflected off of bare glass at an
angle it becomes polarized. The closer the angle of reflections gets to the Brewsterangle of 57 degrees, the more complete the polarization. With both the diagonal
mirror and Herschel Wedge placed at 45 degrees the light becomes crossed polarized
and the image brightness can be farther reduced.


so can you use a ND filter and a polarizer to get from 3.5 to 5 OD instead of a wedge?


100,000* reduction is the number you are looking for, but a transmittance of 0.05 is an optical density of 1.3, and two reflections is 2.6, leaving you 2.4 or so short. He probably did need the polarization trick to bring the brightness down with just three reflections as a light reduction mechanism.

Shade 7 (0.27% transmittance) or greater welding filters would provide enough reduction.

*I find a range of 30,000 (minimum for safety) to 300,000 (depending on how bright you find optimum for comfortable observation) cited. 100,000 is right in the middle.






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