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Re: [ATM] 20" F4.5 Done
MitchB Wrote
>> Geogre --- What is Pegasus's explaination?
Well here is the explanations from Pegasus Optics
---------------------------------------------------------------------------
All other mirror makers include only one measurement, which is what the test
setup yields, and which I refer to as an "indicated" value. Having a
metrology background, I'm acutely aware that every piece of test equipment
has an expected error of measurement, and that other test variables affect
the accuracy thereof. It is not realistic to assume that a measurement has
zero tolerance and is absolutely accurate. In the Quality Control/Quality
Assurance world, we assume that quality measurements have expectation of
this variation, and quantify it in a bell-shaped curve. Basically I've done
the same thing, and I add 1/20th wave to the indicated p-v wavefront, and
1/60th wave to the RMS error, to more accurately depict expected accuracy,
and that allows for a permissible .01" knife-edge variation. One error,
which is pretty small, is the repeatability factor for individual
measurements, which I have quantified at +/- .001", and a secondary error
source is the sampling nature of the test, since measurements between zones
have an expected variation from the zonal centers depicted, and this is also
quantifiable at about +/-.0015" of knife-edge movement.
--And this is a part of the document accompanying the mirror measured data--
The Wavefront Rating Summary, contains a summary of the numeric wavefront
values for the mirror.
It is divided into two parts, Indicated Error, and Worst Case Error.
All test methods test are subject to variables of sampling,
setup, test equipment or reference/null element inaccuracy;
therefore, we add the known error from our measurement process to the raw,
indicated results.
(The Foucault Knife Edge test uses no reference or null elements, hence no
error from these sources applies.)
The result is a conservatively overstated error which assumes the mirror
is afflicted with the maximum error factors to which the measurement process
could be subjected.
In reality, the true error has a probability of being somewhat better than
the worst case error.
But using this weighting process, it cannot be as good as, or better than,
the indicated error.
Our warranty of < 1/20 RMS wavefront for F/4.2 or slower F/ratios is based
on the Worst Case Error.
F/ratios <F/4.2 are guaranteed to <1/15 RMS Worst Case.
As far as we know, we are the only company that makes this worst case
assumption
and applies a corresponding correction to allow for its degrading effect.
Failure to apply a realistic correction factor results in an over-optimistic
assessment of the mirror's precision and expected performance.
----------------------------------------------------------------------------
George Nikolidakis
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Mitch_Bisnauth@Cknainc.com
Sent: Thursday, January 31, 2008 4:06 PM
To: Mitchell R
Cc: atm@atmlist.net; atm-bounces@atmlist.net
Subject: Re: [ATM] 20" F4.5 Done
MitchR.....I don't know anything about testing mirrors - but that would
make Pegasus's residual "system" error is bigger than the actual measured
item, (in rough numbers 1/20 = measured item & 1/10 = "system
error")...seems reverse of what you would expect, (or have I got this
entire statistics/metrology thing wrong w.r.t mirrors....??)
Geogre --- What is Pegasus's explaination?
(--- is it a "localized" w/front reading that was generated with the
mirror in a specific orientation? )
(That scope looks like a work of art...)
MitchB
"Mitchell R" <funnybone101@embarqmail.com>
Sent by: atm-bounces@atmlist.net
01/30/2008 06:43 PM
To
<atm@atmlist.net>
cc
Subject
Re: [ATM] 20" F4.5 Done
Hi all
Finally I finished the construction of my telescope.
A 20" with F4.5 Primary Mirror from the Pegasus Optics
and a 4" secondary from the Antares Optics.
It took me 4 months with very afternoon and evening job,
too running for materials and highly improvisational ...
Details of the construction finished and standing, you can see
http://geonik.homeip.net/home/complete/tabid/106/Default.aspx
I received measurement data from John Hall director of Pegasus optics,
Listed below.
There seems a Wavefront PV Error 1/19.71 Wave
but below there is a Worst Case P-V Error 1/9.93 Wave
What I finally can not understand is what is my mirror Wavefront error?
I bet that there is 1/9.93 Wave, but the 1/19.71 is confusing me.
Can someone explain that to me?
***
Let me first say what a nice looking scope! Very nicely done!
My *guess* would be the company has a certain amount of determined max
error
in their testing that they find from mirror to mirror or set or readings
to
set of readings and factoring in the "error factor" if you will they are
stating that even if their testing was off by their determined max error
it
would still be 1/9.9 wave.
Hope my wording is understandable, and again that's my idea, not really
sure.
Anywho, Great scope!
Mitchell
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