What to do with my mirror?
Posted 02 February 2013 - 06:16 PM
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?
Posted 02 February 2013 - 07:02 PM
Posted 02 February 2013 - 07:21 PM
Posted 02 February 2013 - 07:41 PM
Posted 02 February 2013 - 11:12 PM
Posted 02 February 2013 - 11:36 PM
Posted 03 February 2013 - 07:47 AM
Posted 03 February 2013 - 07:54 AM
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).
Posted 03 February 2013 - 11:54 PM
Posted 04 February 2013 - 03:21 AM
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.
Posted 04 February 2013 - 03:59 PM
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.
Posted 05 February 2013 - 12:12 AM
Posted 05 February 2013 - 01:30 PM