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Re: [ATM] correction -+
Ken
I have only worked Pyrex glass and my opinion of what is a reasonable target
quality would have an adjustment factor for the increased coefficient of
expansion (COE) of the plate glass. The adjustment I would make would be
based what I think the heating effects might be as I work the glass, and on
the added difficulty of testing. The work and testing area temperature
control would have to be tighter. As you have noticed you have one shape in
the rising temperatures of the morning and another shape in the falling
temperatures of the evening. I see that with Pyrex. I can imagine it is
worse with plate glass having about 2.5 times the COE of Pyrex.
I would choose to measure with stable temperature. If I couldn't do that I
would choose the conditions most like the expected conditions of use. Most
people would say that would be falling temperature. Many would choose to
bias toward the falling temperature. Few would choose the rising temperature
condition.
The level of quality that is reasonable to accept depends mostly on the
makers skill level and how tightly you are able to control the working and
testing environment. And then maybe how much you want to learn on one
mirror. If you set your goal to far above your skill level you could be
working a long time on one mirror.
I'm rather old school and my Foucault program only tells me PV error. So
that is what I go by. I usually do well enough at that I could accept the PV
number if it was RMS.
There is a lot of room for other opinions on this so take it with a grain of
salt. For a 10 inch mirror on Pyrex 1/20 wave PV on the wavefront is a
reasonable target for an experienced worker. By that I mean he could do
better if he wanted but the effort beyond that is rather difficult. It would
require better temperature control in working and testing than most ATMs
have readily available in their garage shops.
On plate glass, I'm not sure. I'm not sure 2.5 times the COE translates into
2.5 the difficulty or that my "reasonable target" / 2.5 would be the
reasonable target for plate.
Having said all of that, my conclusion is....
I'm reasonably sure that 1/13 PV is an acceptable mirror and that you should
consider yourself done.
I'll spare you my method of fixing center roughness. It's a secret. And you
are finished anyway.
Jerry
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
KR
Sent: Saturday, June 23, 2007 12:23 AM
To: Dale Eason; atm@atmlist.net
Subject: Re: [ATM] correction -+
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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