Re: [ATM] Mirror question

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

I agree with Jim that you can polish your coating away. 

However, before you do that, take a look at your mirror. Does it appear smooth and uniform all the way across the surface to your naked, careful eye? Any lack of polishing will be very, very obvious to the naked eye after aluminization. Many newbies end up polishing the center part much better than the edge, which appears rough after aluminizing. If you can get one of those 60 to 100 power Radio Shack hand-held microscopes (something like $15 or so), you can see the roughness very easily in some parts of the mirror (typically the outer 1 cm or so) and not in others. I've never tried this myself with an aluminized mirror, and would be reluctant to use the microscope on a coated mirror, because you pretty much have to touch the mirror surface to look at anything. However, if you can tell that part of your mirror is rough with your naked eye, then you probably will want to re-polish it anyway. I find that with an uncoated mirror, those microscopes give a very nice, very
 objective way of telling whether the mirror is fully polished out. If you can't see ANYTHING under the microscope on the face of the mirror, then it's polished fully right there. If you see ANYTHING that's not dust particles, then that part needs to be polished some more. Focus the microscope first on a piece of paper of some sort (a printed newspaper works quite well).
I find the other methods (such as the one Jim describes) much more difficult to interpret objectively. YMMV.
Guy

Jim McPherson <jim@eflightproducts.com> wrote: Hi John,
        You can just use your pitch lap with some polishing agent to 
remove the coating. Your going to have to refigure anyway so just polish 
it like your going for a sphere and the coating will come right off. 
Determining when your done polishing is hotly debated by even the top 
professional mirror makers. Here is my way. I take a red laser and shine it on the surface. If I can still easily see the beam on the surface of  the mirror, it's not done polishing. Upon very close inspection you can  see an extremely faint beam on the surface of a polished mirror, it's due to the properties of glass and is about 4% of laser beams intensity. You can also use this method to test the different zones of the mirror  for completion. I'd bet that the center of your mirror will show less reflection than the edge does.

-Jim McPherson

John wrote:
> Hello All,
>
> I bought an 8" mirror grinding kit awhile back. I read up on techniques
> for grinding and I figured I would take a chance. I ground it down and
> polished it to a point I "thought" was good, I sent it off for coating
> and it came back coated but they also put a note in the box saying the
> mirror was not polished completely.
>
> So, being new at this, what is my next step? Do I re-polish it with the
> coating on? or, do you somehow remove the coating, then polish. Then,
> the most important question of all, how do I know when it's polished
> completely?
>
> Thanks in advance   
>
> John




Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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