Re: [ATM] Faucault testing questions

["vladimir sacek" <vlad@copper.net>]

Ken Reid wrote:

> This time I  did
>nine readings the average of  which I plugged into sixtests, A Reijke, and
>figure xp. They concurred that b was now overcorrected at  b=-.8   (have
>trouble saving and loading frontsix)
>    So my question :  In view of the recent controversy over Faucault
>testing. Is it necessary to input a minimum quantity of data to get
>accuracy? If not do I overshoot and go for a -1.25 hyperboloid?

If your present conic is -0.8 the mirror is undercorrected (ellipsoid),
not overcorrected. In general, the more readings the better, but nine
should be considered more than sufficient for this mirror.

I'm not sure which Foucault controversy you are talking about. If it is
the recent thread at the atm_free, about what seems to be empirical
evidence of Foucalt's bias toward undercorrection, you probably don't
need to care much about, since your 10" f/6  is neither large nor fast.
But since you seem to be getting quite a bit of undercorrection anyway,
maybe there is something in the way the measurements are done that
promotes this kind of bias.

One of factors that could possibly cause bias towards undercorrection
is the uneven structure of defocused image in a typical Foucault test.
Parabolic mirrors have significant overcorection at the c.o.c., the 
larger/faster
the more so. It raises exponentially towards the edge, going from very low
for the most inner zones, to most often significant for the most outer 
zones.
When combined with an effective huge c.obstruction (looking at a pair of 
zonal
openings as a section of a highly obstructed aperture), the extra-focal 
pattern
defocuses into a pattern with bright core not significantly wider than the 
focused image,
with significantly fainter wide halo. On the other hand, the intra-focus 
defocuses into a
spreading pattern brightest at the outskirts, while the inner core quickly 
disintegrates.

I don't know at what point the transverse move of the KE results in null 
appearance,
but seems logical that it occurs when it hits the bright, compact core of 
light in the focus zone.
If so, since this bright core noticeably extends extrafocally, while very 
quickly disintegrating
intrafocally, readings done from outside in would tend to be farther away 
from the mid point
(median radius) than redings done from inside out. In effect, average value 
for the two readings
will tend to give longer than actual median zonal radius. In other words, 
the mirror will look
flatter than it really is. And it would look near-perfect when somewhat more 
strongly curved
than that (undercorrected).

If readings are done only from outside in, the bias would be more 
pronounced.

A 10" f/6 has ~1 wave of overcorrection at the c.o.c. and the most outer 
zone probably have
~1/3 wave (guess), more than enough to result in defocus asymmetry. Still, 
it's hard to tell
if yuor significant undercorrection has something to do with it, or not. At 
least something
to think about...

Vlad








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