Telescope Reviews: KE testing accuracy- notes.

A "discussion" with a gentleman over the sensitivity & accuracy of the standard knife edge test, known as Foucault led me to believe he was in gross error. I recently completed a 6" F/13.63 mirror, that I tested thoroughly 4 times with naked eye, and 4.5x50mm scope attached to the test stand. More of this later.
The disagreement was, he insisted that Foucault was not accurate to closer than .010" accuracy at the ROC with reads. I felt that the consensus was this equated to about 1/10th wave as approximate (swa) guess.
I knew this was, if not grossly incorrect, was too fast a reply. F/D plays a large part in assessing tolerance of mirrors. Also, .010" was, and has been bantered around both in literature and in various places as "accepted norm". This may appear to be true if some details are accepted as "good enough". Those being-
Construction of toy testers as wooden blocks sliding on rods, with poor or springy KE arms with various loose tolerances.
Testers as a unit, being fairly light, and too easily moved by careless testing.
Accepting everything as read, being"the final word". I call this "the earth is flat" syndrome. I also believe that various pieces quoted as sources, are "cross referenced" and so have an inevitable bias as to what's possible. In particular, the .010" accuracy limit seems to be 5th hand and is repeated ad nauseum.
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On to the test of that long mirror. Through data obtained on hundreds of mirrors, I have the following range of accuracy of readings with -MY- tester.( It consists of a lathe cross slide complete with adjustable gibs, on a flat steel top, which is welded to a column affixed to a transmission bell housing. To that, there is welded a tripod. Total weight around 100# or more. It doesn't move easily, nor flex when pushed on hard)
ROC range Tolerance (max + - value, typically -within- said value)
to 60" .001"
60-120" .002"
120-160" .003"
200-300" .005-.015" respectively

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The "nominal" test of this mirror revealed the efforts at making predominantly a sphere from the start. There exists a roll off at the edge, which equates to +.002" at the 4th zone, and .012" at the outer zone. Approximately 3/4" ~ of the outer part of the mirror is outside of the nominal sphere in the central 4.5"
It has to be remembered that this spherical section is a null with the KE test, and has all the inherent characteristics of a null. You see all the surface at once, astigmatism is glaringly obvious if present, and I had repeatability in this area down to .001" as long as I stuck with either eye, or scope throughout the test sequence.
The outer 3/4" of mirror, I bagged as +.002", and +.012" respectively in the outer 2 zones. Repeatability was within .0025" when average of all 4 tests were reduced.
Shadowing was very delicate and with just a cursory glance, the mirror appeared to be a perfect sphere. Only under close scrutiny over the period of an hour and a half could all the finer points outlined above be ascertained. Data on following post later.

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So many projects, so little time!

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