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Re: [ATM] correction -+
A clarification. Conic constant and Strehl are two
different things.
http://mathworld.wolfram.com/ConicConstant.html
The conic constant for a parabola is -1.
The Strehl number is the ratio of how well your mirror
focuses light to a point compared to a perfect mirror.
The ideal Strehl number is 1 but anything over .8 is
good enough as described here:
http://grus.berkeley.edu/~jrg/Aberrations/node14.html
Ignore the math if you want but find the following at
the bottom:
"The Marechal criterion states that a system is
regarded as well corrected if the normalized intensity
(Strehl number) at diffraction focus is greater than
or equal to 0.8 , which corresponds to an rms
wavefront error of 1/14 wavelength."
Foucault and even interferometry can not measure with
the precision necessary to find the true Strehl. For
example neither can measure the fine surface roughness
than may be large enough to cause scattering. If the
mirror is smooth then they will get very close.
One way to check for smoothnes is to use the Foucault
or ronchi image. In the Foucault look for shadows the
show bumpyness. In the ronch look for ragged stripes.
If you see any then the mirror may not be smooth
enough.
The ROC is also not the same as the conic constant and
how close it is to your desired ROC is not a measure
of the correction.
FigureXP can show you the best fit conic but use the
reported RMS, and Strehl number to decide if you are
done. They are the best indicators of how good the
correction is.
Dale Eason
--- KR <kreid@primus.ca> wrote:
> Thanks Dale
> I'm using Dave Rowes Figure XP and foucault
> instrument. I had a difficult
> time getting the centre down until i made a small
> tool. The centre is rough
> but covered by the secondary. So it's relatively
> smooth.
> The foucault results say the closest ROC is
> +.005. So I'm thinking that
> conic must be Strehl=1. If I live another century, I
> might even try
> interferometry.
> Thanks Dale, jerry.
> Ken
> ----- Original Message -----
> From: "Dale Eason" <atmpob@yahoo.com>
> To: "KR" <kreid@primus.ca>; <atm@atmlist.net>
> Sent: Friday, June 22, 2007 8:38 PM
> Subject: Re: [ATM] correction -+
>
>
> > For a 10 inch F5 assuming only spherical error a
> CC of
> > -1.065 is 1/29 RMS and -1.060 is 1/30 RMS on the
> > wavefront. Not much difference.
> >
> > I computed this using my program OpenFringe and
> tried
> > different spherical zernike terms until I got a
> conic
> > of -1.060 and -1.065.
> >
> > So if the mirror is smooth and has no zones then
> it is
> > good enough assuming the Strehl and RMS of the
> > Foucault data reduction agree.
> >
> > Dale Eason
> >
> > --- KR <kreid@primus.ca> wrote:
> >
> >> Hello list,
> >>
> >> Twice I've asked myself, what value of
> >> correction is on the level of a
> >> RMS value of 6, a PV of 1/13, strel .988?
> >> I don't recall any discussion on the matter. When
> >> you approach very good
> >> readings, the temptation arises; I can't make it
> >> around the barrel again
> >> without loosing good Foucault results, so the
> >> good-enough becomes relevant.
> >> This is plate glass, f/6. If I do a set of
> >> readings in the morning when
> >> it is warming up, they are different than an
> evening
> >> set when cooling. I
> >> will continue to average readings, so far -1.06,
> but
> >> they have ranged
> >> from -1.065 down to -1.06 (not really an
> average).
> >> If plate glass gives such
> >> wide ranging results, I ask what value of
> correction
> >> weighs in with good
> >> Foucault readings?
> >> Star test will tell in the end. Does anyone
> have
> >> thoughts on this?
> >> Thanks KMR
> >>
> >> _______________________________________________
> >> ATM mailing list http://www.atmlist.net/
> >>
> >
> >
> >
> >
> >
>
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>
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