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Equipment Discussions >> Reflectors

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tag1260
sage

Reged: 10/07/12

Loc: Ohio, USA

Blackening mirror edges?
#5711573 - 03/03/13 09:53 PM

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

Post Extras:

Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Blackening mirror edges? new

[Re: tag1260]
#5711612 - 03/03/13 10:11 PM

Quote:

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

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GlennLeDrew
Postmaster

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada

Re: Blackening mirror edges? new

[Re: Jason D]
#5711756 - 03/03/13 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.

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Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Blackening mirror edges? new

[Re: GlennLeDrew]
#5711772 - 03/03/13 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

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Jarad
Postmaster

Reged: 04/28/03

Loc: Atlanta, GA

Re: Blackening mirror edges? new

[Re: Jason D]
#5712021 - 03/04/13 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

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Mirzam
Post Laureate

Reged: 04/01/08

Loc: Lovettsville, VA

Re: Blackening mirror edges? new

[Re: Jason D]
#5712025 - 03/04/13 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

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FirstSight
Postmaster

Reged: 12/26/05

Loc: Raleigh, NC

Re: Blackening mirror edges? new

[Re: Mirzam]
#5712046 - 03/04/13 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.

Post Extras:

dcoyle
professor emeritus

Reged: 10/11/05

Loc: Turbulent but dark skies, N.M.

Re: Blackening mirror edges? new

[Re: FirstSight]
#5712265 - 03/04/13 10:45 AM

+1 on felt tip smear.

Black pigmented 5 minute epoxy is more resistant to solvents.

Dan

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Sarkikos
Postmaster

Reged: 12/18/07

Loc: Suburban Maryland, USA

Re: Blackening mirror edges? new

[Re: dcoyle]
#5712348 - 03/04/13 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.

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

Mike

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles

Re: Blackening mirror edges? new

[Re: Jarad]
#5712770 - 03/04/13 03:46 PM

Quote:

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.

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careysub
Pooh-Bah

Reged: 02/18/11

Loc: Rancho Cucamonga, CA

Re: Blackening mirror edges? new

[Re: Starman1]
#5712835 - 03/04/13 04:20 PM

Quote:

...
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).

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GlennLeDrew
Postmaster

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada

Re: Blackening mirror edges? new

[Re: careysub]
#5712965 - 03/04/13 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.

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precaud
sage

Reged: 12/05/12

Loc: Sunny New Mexico

Re: Blackening mirror edges? new

[Re: GlennLeDrew]
#5714433 - 03/05/13 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.

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Binojunky
Carpal Tunnel

Reged: 12/25/10

Re: Blackening mirror edges? new

[Re: precaud]
#5714629 - 03/05/13 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.

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mark cowan
Vendor (Veritas Optics)

Reged: 06/03/05

Loc: salem, OR

Re: Blackening mirror edges? new

[Re: Binojunky]
#5714858 - 03/05/13 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.

Best,
Mark

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tag1260
sage

Reged: 10/07/12

Loc: Ohio, USA

Re: Blackening mirror edges? new

[Re: mark cowan]
#5714865 - 03/05/13 04:47 PM

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

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GlennLeDrew
Postmaster

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada

Re: Blackening mirror edges? new

[Re: tag1260]
#5715263 - 03/05/13 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.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada

Re: Blackening mirror edges? [Re: GlennLeDrew]
#5715744 - 03/06/13 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.

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dave brock
sage

Reged: 06/06/08

Loc: Hamilton, New Zealand

Re: Blackening mirror edges? [Re: GlennLeDrew]
#5715747 - 03/06/13 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

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Jason D
Postmaster

Reged: 10/21/06

Loc: California

Re: Blackening mirror edges? [Re: GlennLeDrew]
#5715753 - 03/06/13 02:13 AM Attachment (4 downloads)

Quote:

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

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