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FlorinAndrei
scholastic sledgehammer

Reged: 09/28/10

Loc: California

polishing the back of mirror - any reasons not to? new

new

#5426625 - 09/18/12 01:10 AM

Doing a 12.5" f/4.7 parabola, Corning Pyrex, full thickness.

I intend to fine grind the back of the mirror all the way down to 5 micron AlOx, but not as thoroughly as the front. Then I'll give it a quick dummy polish with the pads.

I am going to go ahead and fully admit I want to do this just because it looks cool. There might also be a tiny advantage in the way it settles in the mirror cell, or not.

Any reason why I should not do this? One thing I could think of is ghost reflections off the back. But those should be very very faint. Light should pass twice through the metal layer, plus a reflection by the non-metallic back in between, then come out as a divergent beam (passing twice through the concave refractive surface).

I'm having a bit of trouble estimating the worst case scenario for the intensity of the beam reflected off the back after it comes out of the mirror. The unknown is the transmission coefficient of the metal layer. If it's 90% reflective, I don't expect the transmission to be 10%, since some of it must be absorbed, no?

If the main reflected beam is at intensity = 1, I'm guessing the ghost beam will be less than 1/1000, perhaps much less.

Anyway, I assume someone else did this before, so if you have any feedback please educate me. If it's a bad idea, I'll just fine grind it a little bit until it looks nice, but no polishing.

Thanks!

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Bart Wide
sage

Reged: 05/16/08

Loc: Europe

Re: polishing the back of mirror - any reasons not to? new

new

[Re: FlorinAndrei]
#5426651 - 09/18/12 01:53 AM

Any reason why I should not do this? One thing I could think of is ghost reflections off the back. > Exactly, little light will pass through the coating, but it might interfere (passing through the surface, refraction, then reflection off the bottom face) with the reflected light off the coated surface. I would leave it at 5 micron.

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dave brock
sage

Reged: 06/06/08

Loc: Hamilton, New Zealand

Re: polishing the back of mirror - any reasons not to? new

new

[Re: FlorinAndrei]
#5426652 - 09/18/12 01:55 AM

I've had a mirror come back from the coater with the back side coated by mistake even though it had scratches all over it. When I rang him he said he thought it was the worst mirror for scratches he'd ever seen (don't ask me why it never occured to him something was amiss).

I would be very nervous sending a mirror to the coaters with a nicely polished back.

Dave

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laserrain
member

Reged: 09/07/12

Re: polishing the back of mirror - any reasons not to? new

new

[Re: dave brock]
#5426683 - 09/18/12 03:14 AM

Quote:

I would be very nervous sending a mirror to the coaters with a nicely polished back.

What exactly happened to me also, despite all my recommendations, marks on the mirror's edge an so on.
Because I found easier inspecting for best contact on the pitch tool looking through the back with no need to keep it wet.
Believe it won't happen to you, after the "coater" reads this post

.
Greetings
Anthony

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Mark Harry
Vendor

Reged: 09/05/05

Loc: Northeast USA

Re: polishing the back of mirror - any reasons not to? new

new

[Re: laserrain]
#5426854 - 09/18/12 07:58 AM

Any reason -NOT- to....
Well, generally it's more work, and the real "horsepower" is on the flip side.
But it's always a good idea to grind it thru some of the fines at least to make sure it's flat, and with minimal wedge.
When testing, having a polished back results in a tiny bright dot right in the center. I'm thinking it -should- be obscured by the secondary mirror shadow, so I think the worries about reflectance isn't necessary.
I have also heard of a mirror or two getting coated on the wrong side. I generally make an engraving, or leave blanchard artifacts plainly visible to negate this from happening!
M.

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Mike I. Jones
Carpal Tunnel

Reged: 07/02/06

Loc: Fort Worth TX

Re: polishing the back of mirror - any reasons not to? new

new

[Re: Mark Harry]
#5426949 - 09/18/12 09:29 AM

I like dummy-shining mirror backs - gets the scrungy-looking mold marks out, leaves it nice and mechanically flat, and gets the wedge out of the blank (more of an OCD thing with me than of any importance), and makes it a lot easier to see how the pitch lap is contacting. The little back-reflection is handy for lining up to the Foucault/caustic tester, and after a few mirrors you learn to ignore it in testing.

Regarding which side to aluminize, a heavy black Sharpie note labeling the "DON'T COAT THIS SIDE!" side should help prevent that. Once dry, the Sharpie note shouldn't outgas anything into the vacuum chamber.

Mike

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EddWen
scholastic sledgehammer

Reged: 04/26/08

Loc: Here or There

Re: polishing the back of mirror - any reasons not to? new

new

[Re: Mike I. Jones]
#5427125 - 09/18/12 11:07 AM

FYI, the Astro-Physics 10" Maks have the back of the mirror polished, as does my APM Mak. Supposedly it allows for more rapid cooling when fans are directed at it.

Note, neither my Mak or the A-Ps have a flat back. The have a conical cross-section like Celestrom and Meade SCTs.

Also, they are masked from coatings.

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mark cowan
Vendor (Veritas Optics)

Reged: 06/03/05

Loc: salem, OR

Re: polishing the back of mirror - any reasons not to? new

new

[Re: FlorinAndrei]
#5427518 - 09/18/12 02:16 PM

Quote:

Any reason why I should not do this? One thing I could think of is ghost reflections off the back. But those should be very very faint. Light should pass twice through the metal layer, plus a reflection by the non-metallic back in between, then come out as a divergent beam (passing twice through the concave refractive surface).

Unless you're leaving it uncoated this is not a factor. 4% reflection off the back internally, less than 4% transmission to the back, and again less than 4% transmission through the front coating - that adds up to approximately nothing.

Polishing the back is excellent, more work though. I finish them to 25micron unless the back is already polished.

Best,
Mark

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Aaron Turner
sage

Reged: 03/18/08

Loc: Southbury, CT

Re: polishing the back of mirror - any reasons not to? new

new

[Re: mark cowan]
#5428144 - 09/18/12 07:53 PM

I have only one recommendation - don't work on the back at all once you have the front surface figured. Change in the back surface will change the surface stress. This can cause the mirror to bend slightly, mostly a change in power, but you can easily get spherical as well, and astigmatism if the back polish is at all uneven. A partial polish can produce more stress than no polish at all; a full polish (no visible grey) will leave that surface stress-free. But if you work the front surface while the back is stressed, and then relieve the stress by polishing the back, the figure can change.

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laserrain
member

Reged: 09/07/12

Re: polishing the back of mirror - any reasons not to? new

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[Re: Aaron Turner]
#5429458 - 09/19/12 03:49 PM

Quote:

But if you work the front surface while the back is stressed, and then relieve the stress by polishing the back, the figure can change.

I do quote all.
Anthony

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SACK
member

Reged: 08/11/11

Loc: Texas, Yahooo!

Re: polishing the back of mirror - any reasons not to? new

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[Re: laserrain]
#5429575 - 09/19/12 05:04 PM

@Mike and Mark,
What is "wedge"? Thanks for educating me.
Jonathan

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laserrain
member

Reged: 09/07/12

Re: polishing the back of mirror - any reasons not to? [Re: SACK]
#5430337 - 09/20/12 04:31 AM

Two not parallel flat surfaces make a wedge

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kfrederick
Carpal Tunnel

Reged: 02/01/08

Re: polishing the back of mirror - any reasons not to? new

new

[Re: laserrain]
#5430432 - 09/20/12 07:19 AM

Wedge is very bad for lens not so much for a mirror . I had a quartz mirror the back was polished I got messed up and put the good side down on the table .After that I marked the bottom .I do like a fine ground edge .Fine grinding the back makes the glass stronger a deep scratch makes stress like a glass cutter does . All good .Should be a nice size .Next one maybe try a CHief .

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laserrain
member

Reged: 09/07/12

Re: polishing the back of mirror - any reasons not to? new

new

[Re: kfrederick]
#5430474 - 09/20/12 08:21 AM

Quote:

Next one maybe try a CHief .

Right, with a built-in astigmatism...It could turn even useful

...

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kfrederick
Carpal Tunnel

Reged: 02/01/08

Re: polishing the back of mirror - any reasons not to? new

new

[Re: laserrain]
#5430492 - 09/20/12 08:41 AM

The CHief can be better corrected than any design .Off topic I know .

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Mike I. Jones
Carpal Tunnel

Reged: 07/02/06

Loc: Fort Worth TX

Re: polishing the back of mirror - any reasons not to? new

new

[Re: kfrederick]
#5430519 - 09/20/12 09:02 AM

Grinding wedge to zero in a mirror substrate simply makes it independent of rotation angle in the cell.

Mike

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ed_turco
Pooh-Bah

Reged: 08/29/09

Loc: Lincoln, RI

Re: polishing the back of mirror - any reasons not to? new

new

[Re: Mike I. Jones]
#5431037 - 09/20/12 02:09 PM

Frankly speaking, (and when did I ever not speak frankly?), I can hardly imagine a bigger waste of time polishing the back of a mirror.

The newer blanks are regular enough for an ATM to surmise the contact between the mirror and pitch lap. And a polished backside is not going to cause you to have even better contact.

Temperature effects are another non issue; this better cooling is going to improve your image how much?

Spending the time to improve the figure of a mirror is by far the best bang for the buck.

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FlorinAndrei
scholastic sledgehammer

Reged: 09/28/10

Loc: California

Re: polishing the back of mirror - any reasons not to? new

new

[Re: FlorinAndrei]
#5432030 - 09/21/12 01:28 AM

Oh, like I said, it's partially a vanity thing. I'm also curious to look through the giant divergent lens that the mirror will become once it's polished on both sides; no particular reason why, I just want to see how it looks like. If I was a cat, I'd be dead already for a long time now.

I didn't think about the pitch contact issue, but it's a great point.

I find it hard to visualize how it makes a difference for the heat transfer. Perhaps there's slightly better laminar flow near the polished surface, on a very very small scale. Interesting. The difference can't be huge, I think.

It's more work, indeed, but that's not important. This is again a project I do mostly to educate myself, so trying various things is kinda the whole point. It will probably suffer the same fate like its predecessors, will end up being more of a test subject for various techniques, rather than a simple project moving linearly towards the goal. But I noticed I learn a lot this way, as long as I do end up meeting the goal eventually.

The front and the back are going to be processed in parallel, step by step. When I move to #120 SiC on the front, I'll do the same on the back. And yeah, once figuring begins, I'll stop messing with it altogether.

Well, it looks like it's been done before with no ill effects, so I'm going to go ahead and do it. Thanks everyone!

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mark cowan
Vendor (Veritas Optics)

Reged: 06/03/05

Loc: salem, OR

Re: polishing the back of mirror - any reasons not to? new

new

[Re: FlorinAndrei]
#5432067 - 09/21/12 02:13 AM

Quote:

I didn't think about the pitch contact issue, but it's a great point.

No, it's a moot point. Any water on the fine-ground back of a mirror blank shows you the pitch contact very easily.

The thermal comments were all somewhat vague - there's no mechanism there to improve cooling. Radiative cooling is in the IF and transparency of the surface to optical wavelengths isn't going to be a factor. Convective cooling just operates by contact with the atmosphere. A fan might be more effective due to a smoother finish, again it seems very minor IMHO.

The only real reason to go to the trouble is because you think it's cool.

And that's totally sufficient.

Best,
Mark

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kfrederick
Carpal Tunnel

Reged: 02/01/08

Re: polishing the back of mirror - any reasons not to? new

new

[Re: mark cowan]
#5432317 - 09/21/12 09:17 AM

You might use your turntable top to grind your back on that way they both match .Having a full thickness blank it might not be needed .On thin blanks that is what I do .

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