[Ayoh]

And an expensive failure of one at that...

 

Here they have published a new article on the sony A99 mirror:

 

http://www.lenstip.com/137.1-article-Eve...rrors.html

 

They go into several areas they would have been all the wiser to stay out of... first they have a test flat, about 6" diameter I would wager, and they ruin it for lab use by placing something directly on top of it.  This is a $10,000 part...  Certainly I assume they will continue to use it, but no good lab would ever use it because it is no longer flat due to scratches on its surface which will perturb the fringes.

 

They also go on to note fringes seen in the mirror with the flat... yes they would exist!  You can place two test flats on top of each other and get such fringes even if they were polished to 1/500th wave roughness.  The minor changes in the airspace produce fringes regardless of how flat the surfaces are.  There is spherical aberration as well as astigmatism in the fringes of the mirror placed on the flat... it is clearly at least somewhat warped and also not perfectly flat on the test flat.  In essense, that observation says nothing about the roughness and it certainly is not on the order of 1um - such a high value would appear very noticeably matte/opaque and not at all "clean" or sharp as the mirror actually does.  1um roughness corresponds to ground, not polished glass.

 

Their (homemade!) interferometer appears to be on a sheet of plywood, thus any alignment is not scientifically valid at all.  It also is not in a controlled room and vibrations as well as air currents will disturb the results. 

 

Their reference field contains fringes and very few of them, so the interferometer is useless for precise measurement.  Anything finer than 5 waves or so I would say is invalid using it. 

 

The disagreement between horizontal and vertical tilt in their data also would invalidate their findings.  Any axis of tilt should produce equal results for a flat, certainly not a difference of a factor of 2. 

 

I would also question who and how designed their diffraction limited ultrafast lens that covers such a large field.  Such a feat is not impossible, but is very difficult.

 

The article is useful for generalizations only, it is not scientifically accurate.  Please tell anyone who shares it with you to not take it as gospel, the test is magnifiscently flawed.

The test may have flaws but it is a hobby article, not a journal publication so I think you are being overly critical. For one, looking at the photos of their rig the interferometer setup is placed on what looks like a glass plate, supported by a wood sheet (A ground stone table may be underneath for all we know). Rather than directly placed on the wood itself as you suggest. Also the hardness of the glass test flat should be significantly higher than the plastic frame of the sony mirror so unless there are some abrasive particles in between, I don't think introducing meaningful scratches to the flat would be that likely. Additionally, the flat and rig may also be from a decommissioned lab which they use for hobby work so I doubt they spent or intended on spending much money as you suggest, or be overly concerned about slight equipment misuse. It is possibly also written by an enthusiastic student (in which case i'm sure criticism will be appreciated). Relax and enjoy the free entertainment, your technical specialty in this field is noted and I'm sure you could do better for a nice fee.