To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 22:47 2003-03-27 -0400, Alfredo Neves wrote:

>I use the Hartmann formula to calculate the ROC of the zone.
>
>I use the following formula:
>     ROC = (X1 * Yd) / (X1 + X2)
>
>Where X1 First reading
>       X2 Second reading
>       Yd Delta Y between readings
>
>After having the ROC of each zone I use a Foucault data reduction program
>to get my answers.

I think the problem here is semantics.  First, remember that the zones'
centers of curvature don't lie on the optical axis - they are on the
"caustic horn" (technically, on the evolute of the mirror's
profile) while the reading in the Foucault test is the longitudinal
distance to the intersection of the rays from two mask holes (usually close
to or on the optical axis).  Alfredo's formula also gives that distance,
but, due to the the lateral measurements, it won't be as accurate as the
Foucault readings.

Even the classical Gaviola caustic test doesn't measure the zone RoC.  For
each zone, you set the KE's longitudinal distance to the zone's RoC
(assuming the mirror's a parabola), then measure the lateral distance to
the image.  The main advantage of the somewhat arbitrary longitudinal
setting is, as Nils Olof says, to make the image as narrow as possible.

So all the amateur tests, except one, work with mirror slopes.  The
exception is the Poor-man's Caustic test which gets the mirror profile by
tracing out the caustic horn.

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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