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What to do with my mirror?

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#1 thetortoise

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Posted 02 February 2013 - 06:16 PM

I sent my 10" Orion stock mirror to be tested and refigured. The test showed that the mirror was of surprisingly high quality with a very smooth surface, no zones, good edge, and dead on correction. The coating is in good shape, so basically drop the mirror back in and view with confidence right!

Well, I received another message that states the mirror has fine pitting when viewed under a microscope, most likely from rushing the manufacture on the production line, the pitting is also apparent when inspecting the mirror.

My question is, with a mirror of otherwise "spectacular quality", would "fine pitting" be something that I should have corrected? Basically, the optician is ready to drop the mirror back in the mail and is not worried about the pitting as any decrease in contrast would be tiny. To the mirror experts - could these pits be smoothed out of the mirror and would it make sense to do so? If I was going for a mirror to enjoy the best views from for a lifetime, should I invest in perfecting an already great mirror, or just appreciate that I won the Orion mirror lottery as this is as good as this mirror can be considering its origins?

#2 Ed D

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Posted 02 February 2013 - 07:02 PM

I would tend to agree with the optician and just put it back in the scope and enjoy it. As you describe it, I would think that the central obstruction alone would have more impact on the image than the microscopic pits. I'm not a mirror expert, but I have seen quite a few, some which could give most of us nightmares for a month, and yet the views were very good. I think you won the luck of the draw with your primary.

Ed D

#3 stratocaster

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Posted 02 February 2013 - 07:21 PM

I'm not a mirror expert at all. But how can a mirror have a very smooth surface, yet have pits in it?

#4 Mirzam

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Posted 02 February 2013 - 07:41 PM

A mirror is considered smooth if it does not have much wavelength scale roughness, which is known as "dog biscuit". To see this roughness requires sensitive testing methods, such as a foucault test. Any pits visible under a microscope are left over from the grinding process and are much much bigger. These will slightly increase scattering from the reflective surface, which will slightly degrade contrast. However, unless you plan on never having a spec of dust on your mirror, there is no reason to worry about the pits. Unless they are extremely numerous the pits will have effects similar to a comparable amount of dust.

JimC

#5 thetortoise

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Posted 02 February 2013 - 11:12 PM

Thanks Ed, I appreciate the input, I should probably just call well enough alone and not worry about what the microscopic view. Stratocaster - I had the same question, there were actually several seeming logical inconsistencies between the two messages but I was sure they had to do with my lack of knowledge on the subject so thank you for bringing that up. Also, thank you for clarifying that Mirzam. I do believe a Foucault test and Ronchi test and maybe a Ross-Null as well were used and then the next day he looked at it under the microscope so the different methodologies created the different seemingly contradictory reports. Also, I can't even get my mirror from the sink back into the holder without dust starting to collect here in Colorado so a great perspective to consider.

#6 azure1961p

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Posted 02 February 2013 - 11:36 PM

Id let it slide if it were me and I tend to get a little neurotic about perfection. That your mirror is that good and those pits, such a nano fraction of contribution or some such is beside the point. I'd enjoy it and move on. And congrats on a great optic.

Pete

#7 yonkrz

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Posted 03 February 2013 - 07:47 AM

Sounds like you have a good piece of glass,i guess i would just put it back. :penny: :penny:

#8 bob midiri

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Posted 03 February 2013 - 07:54 AM

If the pits were detrimental that would have been reflected (no pun intended) in the optical testing. Drop the mirror back in and enjoy your fine mirror. Bob

#9 KerryR

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Posted 03 February 2013 - 01:02 PM

I'm not a mirror expert at all. But how can a mirror have a very smooth surface, yet have pits in it?


Here's the skinny version:

Mirrors are ground using successive stages of finer grits, similar to what you'd do if you were sanding furniture. The big grits take out big chunks, the spaces where the chunks were are called pits. Each grit size, moving towards smaller sizes, is used to remove the bigger pits from the previous size grit, and replace them with smaller pits. When all the larger pits have been replaced with the new smaller size, the next smaller grit is used and the process is repeated to the final smallest grit. If, at any time, the optician changes grit sizes too soon, some of the larger pits from the previous grit size may remain on the finished optic.

After grinding, the glass is polished out. Polishing out is a different process from grinding, using very, very fine abrasives and surfaces that both remove glass at the near molecular level, as well as 'smear' near-liquid glass around the surface of the glass. This process produces the glass-smooth surface you think of when looking at a lens. This process can't 'fill in' or grind out the larger accidental pits, so those pits remain. Pits from the finest grits can remain if the glass isn't polished out long enough, even if the proper progression of grits was used.

Figuring is the last process, where the same abrasive and surface is used to accurately shape the surface of the glass to the correct curve necessary to focus the incoming light to the correct location. If the curve is very accurate, the curve is said to be 'smooth'. If the curve is wavy, the curve is said to be 'rough', or 'zony' (zony--certain circumferences focus the light at the right place, other circumferences focus the light in the wrong place, usually alternating as you progress outward from the center of the glass to the edge). It's possible to have a smooth curve or a rough (zony) curve, and have 'lumpiness' on top of that (dog biscuit). What we want is a smooth curve with no lumpiness. On top of all of the above, there's also the issue of correct curve-- you can have a smooth curve with no zones, but the curve is the wrong curve, deviating from the ideal paraboloidal surface either towards a hyperbaloid (over corrected) or more towards the spheroid (under corrected).The degree to which those things occurs determines the quality of the optic. Zones, and particularly lumpiness, scatter light around and outside of the image, reducing contrast sharpness. The degree to which the surface approaches the ideal paraboloidal surface affect the ability to attain a sharp focus. When things are really bad, the scope won't seem to have an exact point of focus-- the image 'smushes' through a region of best focus without ever getting particularly sharp or contrasty. When it's good (no zones, no lumpiness, correct curve) contrast is high, and the image 'snaps' to focus at a very definite and repeatable location.

So, you can easily have any combination of the above. In the OP's case, he has a very accurate curve from center to edge (no zones, correct curve), and no lumpiness (roughness). But, the optic does have pits that are left over from either an improper grit progression or from too short a time polishing. In his case, they're difficult to detect, so the negative impact to the image is likely not enough to offest the accuracy of the curve. When it's bad, the aluminized mirror will look 'smokey' right out of the aluminzing chamber (not to be confused with the smokiness of old and degraded coatings).

#10 thetortoise

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Posted 03 February 2013 - 11:54 PM

Azure - neurotic definitely describes my recent obsession with getting a perfect view - this is a dangerous hobby for a perfectionist. Also great point Bob - the viewing tests trump the microscope view. Kerry - thank you very much for writing out that detailed explanation - I am understanding what is being tested and why it matters much better now. So much still to learn.

#11 bob midiri

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Posted 04 February 2013 - 03:21 AM

BTW how does the coating look? Ive seen coatings on mirrors not fully polished, and the aluminum looks hazy. Does yours? Bob

#12 dan_h

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Posted 04 February 2013 - 09:53 AM

My question is, with a mirror of otherwise "spectacular quality", would "fine pitting" be something that I should have corrected? .........To the mirror experts - could these pits be smoothed out of the mirror and would it make sense to do so?


I'm not an expert by any means but I have enough experience to know that you cannot get rid of the pits without also getting rid of the fine figure. Sure it is possible to put the figure back in once the pits are gone but will the new figure be as good as the original? There is also a question of how deep are the pits? While it may be possible to reduce the size and number of pits by polishing the existing surface, it may be that the pits cannot be completely removed with fine grinding. You could repolish and refigure and still have pits.

Keep it as is.

dan

#13 thetortoise

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Posted 04 February 2013 - 03:59 PM

Bob - the coating looked good to me - nice and shiny - not hazy - there were little spots that I assumed were water spots but now I'm wondering if they are actually spots from the pitting. I will have to take a look with a more critical eye with all of the new found knowledge everyone has given me when the mirror comes back. Optician said coating looks good and advised not to re-coat at this time.

Actually this morning I had sent an email to the optician saying to just pack it up after everyone's advice here. He wrote back a little while later and stated exactly what you are saying Dan, that it would need to be refigured after polishing and it would be difficult to polish out the pitting because he would have to try to match the grinding tool they used for one thing, and then have to find the right grits etc., at the end it would be only slightly easier than starting from a blank. So keeping it as, as per the consensus here, wins the day.

Thanks again everyone.

#14 bob midiri

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Posted 05 February 2013 - 12:12 AM

If the spots were caused by pitting it would be more universal like the outer zone or the outer two zones around the whole mirror at those points., it wouldnt be indiscriminate round spots in certain areas. bob

#15 Binojunky

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Posted 05 February 2013 - 01:30 PM

Considering what these Orion scopes cost, the fact they seem to offer consistantly good mirrors is amazing, if you want optical perfection then cough up the dough? , its a simple as that,DA.

#16 ed_turco

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Posted 05 February 2013 - 07:46 PM

You will enjoy that scope; trust everyone here. :)


Ed






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