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Blackening mirror edges?

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#1 tag1260

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Posted 03 March 2013 - 09:53 PM

Can someone tell me the purpose of blackening the edges of your mirror? Will they ALL benefit from it?
Thanks
.

#2 Jason D

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Posted 03 March 2013 - 10:11 PM

Can someone tell me the purpose of blackening the edges of your mirror? Will they ALL benefit from it?
Thanks
.


In theory, to minimize light scatter which in turn will enhance contrast? In real life, the difference will be subtle to none. Do not have high expectations.

Jason

#3 GlennLeDrew

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Posted 03 March 2013 - 11:44 PM

One can calculate the potential contribution of scattered light from a bevel, by comparing the projected area of the bevel to the area of the mirror. For example, a 1/8" bevel on a 12" mirror has 4.3% the area of the mirror. I would most definitely blacken a bevel of this relative area. Actually, I'd be strongly tempted to so treat *all* bevels on both primary and secondary.

#4 Jason D

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Posted 03 March 2013 - 11:56 PM

But the bevel will reflect star light towards the inner OTA walls and if the inner wall is flocked then most of it will not make it to the eyepiece.
Granted the bevel surface is not as polished and some of the scattered light will make it to the eyepiece but I would expect that to be minute.
As an experiment, a front aperture can be placed at the OTA/UTA opening to block star light from reaching the bevel area to see how much of a difference it will make at the eyepiece.

Jason

#5 Jarad

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Posted 04 March 2013 - 07:38 AM

It depends a lot on the design of the scope. The bevel is less of an issue in a solid tube OTA. In a truss dob, though, it is possible for light from an off-axis object (like the moon) to hit the bevel at an angle where it will reach the focuser.

Also, there is no downside to blackening the bevel. It is easy to do with a chisel-tip marker, takes maybe 30 seconds once you have access to the mirror.

Jarad

#6 Mirzam

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Posted 04 March 2013 - 07:42 AM

One place where blackening is important is the partially aluminized exposed edge of the secondary visible from the eyepiece, unless of course one is using a secondary shroud.

I have seen major glare using a 100 degree afov eyepiece that was solved by blackening the exposed secondary edge. If you have a steady hand you may as well get the bevel at the same time--just don't get paint on the mirror surface!

JimC

#7 FirstSight

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Posted 04 March 2013 - 08:09 AM

One potential downside of blackening the secondary edge with a black marker is when you clean the mirror (and use any sort of cleaning solution that might potentially act as a solvent) you risk dissolving just enough of the marking to make a smeary mess.

#8 dcoyle

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Posted 04 March 2013 - 10:45 AM

+1 on felt tip smear.

Black pigmented 5 minute epoxy is more resistant to solvents.

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#9 Sarkikos

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Posted 04 March 2013 - 11:42 AM

I blacken the bevels of primary and secondary with flat-black paint, such as supplied by Duchek. I tend to avoid "permanent" black marker. In my very dewy region it is not so permanent.

Blackening the bevel is easy to do. It might make only a vanishingly small improvement in many scopes. But it is so easy to do. Just do it and don't try to figure out beforehand the probable magnitude of the improvement. By the time you figure it out, you could have already painted the bevels. Unless you're one of those types who enjoy making precise measurements and mathematically formulizing all these things. :shrug:

Use a bright light and wear a strong pair of those reading glasses you can buy for cheap at bargain stores.

:grin:
Mike

#10 Starman1

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Posted 04 March 2013 - 03:46 PM

It depends a lot on the design of the scope. The bevel is less of an issue in a solid tube OTA. In a truss dob, though, it is possible for light from an off-axis object (like the moon) to hit the bevel at an angle where it will reach the focuser.

Also, there is no downside to blackening the bevel. It is easy to do with a chisel-tip marker, takes maybe 30 seconds once you have access to the mirror.

Jarad

1) During cleaning, some of the blackening may run/dissolve and get onto the reflective surface
2) When the mirror is recoated, removing the blackening on the mirror may be nearly impossible to do completely and the residual may contaminate the coating process.
3) The reflection from the beveled edge could only get into the light path if the inside of the scope around the mirror was white and scattered light was reflecting from the inside of the scope, off the bevel, and into the light path. That's a highly unlikely scenario.
4) Whatever chemical or solvent is present in the darkening agent (paint, ink, etc), there is the possibility for it to get into the aluminum at the edge of the mirror and cause contamination of the aluminum.

I do NOT recommend darkening the bevel on a primary mirror for the above reasons.

#11 careysub

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Posted 04 March 2013 - 04:20 PM

...
I do NOT recommend darkening the bevel on a primary mirror for the above reasons.


Good reasons.

I propose then that those who want to do bevel darkening might try doing it by "bevel baffling":

Take a strip of black paper.
Using a straight edge bend on edge of the strip at a right angle to be the same width as the bevel.
Snip this bevel baffle every inch of so with scissors so that it can be wrapped around the mirror.
Wrap the paper around the mirror edge snugly using some double sided tape.

Will this interfere with mirror cooling?

No:
1) A single thickness of copy paper slows heat transport only as much as 0.07" of glass;
2) mirrors lose very little heat from their edges anyway - only that portion of glass that is closer to the edge than either the front or the back loses heat this way.

If you were still concerned with cooling you could use very thin metal instead (and this might be preferable for durability in humid environments anyway).

#12 GlennLeDrew

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Posted 04 March 2013 - 05:20 PM

A ground bevel scatters light almost isotropically. The bevel angle will not preferentially direct light toward the tube. Of course, the fact of the near uniform scatter makes that fraction directed into the FOV rather less than the overly pessimistic calculation I made earlier.

Nonetheless, if one is going all out with other baffling techniques, why not reduce scatter from all shiny, non image-forming surfaces?

I've used acrylic flat black paint to good effect. It's water resistant and does not seem to dissolve (readily, at least) in alcohol and such other solvents commonly used on glass coatings. I'm sure it can be removed without much shattering of nerves if re-coating is desired.

#13 precaud

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Posted 05 March 2013 - 01:04 PM

+1 to Glenn's solution. I have a 2,000 sq ft warehouse whose roof I coated with white acrylic 5 years ago. I've been very impressed by its adhesion and how well it stands up to the elements. On that basis, I applied flat black acrylic with a cotton swab to blacken one of my primaries.

#14 Binojunky

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Posted 05 March 2013 - 02:51 PM

To be honest the improvement is so minor you would see the same result in observing with a clean eypiece compared to a dirty one, or a clean primary versus a dirty one, in other words barely noticable, if at all to the eye, another case of making a mountain out of a mole hill, JMPOV,DA.

#15 mark cowan

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Posted 05 March 2013 - 04:43 PM

There's no good reason to blacken primary bevels, and plenty of reasons not to, including cleaning issues and recoating contamination. All paints can contaminate mirrors, if you even get organics (from fingerprints) distributed on the coating once with solvents, they don't come off again.

Under test on optical benches, the bevel of a mirror NEVER shows under light incident along the optical axis. Coating doesn't change the relationship between the polished surface and the bevel, as both end up with the same coating reflectivity. There is NO reason to expect ANY change in contrast transfer due to bevel blackening - the surface doesn't reflect coherent light and it scatters very much next to nothing towards the focus.

It's simply a bad idea all around. :shrug:

Best,
Mark

#16 tag1260

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Posted 05 March 2013 - 04:47 PM

Thanks guys. I've been talked out of it!!! Long winter with LOTS of cloudy nights to ponder upgrades and improvements!!

#17 GlennLeDrew

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Posted 05 March 2013 - 08:12 PM

Well, when I peer down an OTA and see a bright ring of scattered light (especially for what I consider abnormally wide bevels, or flats ground so as to be rid of turned edge), I want to be rid of it.

#18 GlennLeDrew

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Posted 06 March 2013 - 01:47 AM

Here's another way to look at it. A 2mm wide bevel on a 10" mirror presents an area of nearly 1,600mm^2, equal to a square patch of 40X40mm. Remember, that's a scattering, (usually) aluminized surface. This would be equivalent to at least 50 shiny, exposed screw/bolt heads on the inner tube wall, down near the primary. Would any discriminating amateur (or pro) tolerate that? We go through almost tortuous measures to lessen the impact of scattering sources less injurious than this.

#19 dave brock

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Posted 06 March 2013 - 01:59 AM

The screw heads typically do not have a ground surface, are not angled away from the light path and definitely do not have to get realuminised one day. :)

Dave

#20 Jason D

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Posted 06 March 2013 - 02:13 AM

Here's another way to look at it. A 2mm wide bevel on a 10" mirror presents an area of nearly 1,600mm^2, equal to a square patch of 40X40mm. Remember, that's a scattering, (usually) aluminized surface. This would be equivalent to at least 50 shiny, exposed screw/bolt heads on the inner tube wall, down near the primary. Would any discriminating amateur (or pro) tolerate that? We go through almost tortuous measures to lessen the impact of scattering sources less injurious than this.


Glen, I have suggested to the OP to place a temporary front aperture mask to mask the bevel and check a shiny star with and without the mask to see if any difference can be observed at the eyepiece. If your 50 shiny screw heads claim has merits then the OP should see a difference. If a difference is observed then the OP has a choice of darkening the bevel or installing a permanent mask closer to the mirror as shown in the attachment.

Jason

Attached Files



#21 beatlejuice

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Posted 06 March 2013 - 03:29 AM

Sorry, off topic just for one question.
Jason, what is that mask made of? Looks like a wider version would make a good ring baffle.

Eric

#22 Jason D

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Posted 06 March 2013 - 03:50 AM

I cut a ring from MDF backboard material then wrapped it with flocking paper. I drilled holes matching existing mirror cell bolts to place it securely in place.

I used the mask with my old stock mirror to cover a mild TDE then I kept it with my premium mirror to redirect air flow across the mirror to reduce thermal boundary layer but I found out that the mask was creating too much turbulence and decided to remove it. Now my scope performs better without it.

Jason

#23 nevy

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Posted 06 March 2013 - 06:58 AM

I cut a ring from MDF backboard material then wrapped it with flocking paper. I drilled holes matching existing mirror cell bolts to place it securely in place.

I used the mask with my old stock mirror to cover a mild TDE then I kept it with my premium mirror to redirect air flow across the mirror to reduce thermal boundary layer but I found out that the mask was creating too much turbulence and decided to remove it. Now my scope performs better without it.

Jason

Hello Jason , what do you mean by turbulence ? I was about to make one of them there rings for my 12" solid tube dob, would I be wasting my time & would I be better off just baffling the fan and leaving it at that?

#24 Jason D

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Posted 06 March 2013 - 11:44 AM

I will share my experience but let me clarify from the onset that I am not making any recommendation since I do not know how much of my experience and preference applies to other scopes.

In my case, I did install a fan baffle and the ring mask I have shown in the earlier post in this thread. I used this setup for a while but I have not been happy with it. When I did some analysis, here is what I noticed:

1- I noticed some air was escaping around the fan. I did my best to cut a baffle with an almost perfect fit but the pressure building between the mirror and the fan was too high and managed to push air through crevices between the baffle and the scope.

2- As far as the air that was redirected across the mirror by the ring mask, it was causing too much air turbulence above the mirror surface. I can see that clearly by defocusing a bright star. When the fan in turned on then the “boiling” effect got too severe.

I ended up removing the baffle and the ring – back to the original scope setup – and things got better. The turbulence was minimal when the fan was turned on.
These days I hardly use the fan but I live in California and nights temperature here is not too bad.

Jason

#25 Binojunky

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Posted 06 March 2013 - 12:13 PM

One thing worth remembering is that any time you remove and replace optics in a scope you run the chance of an accident resulting in damage, for what is a pointless upgrade(edge blackening) why take the risk?,DA.






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