Re: ATM Fouchalt tester

["Frédéric Géa" <geaf@unionfinancieredefrance.fr>]

Hi,

Some friends has made a great web site on mirror testing :

http://www.astrosurf.com/tests/


for special details on how to see roughness :

http://www.astrosurf.com/tests/contrast/contrast.htm#haut

Sorry, again a french we site but that's a really high quality job and worth
spend some time on it.

Hope this help.

Frédéric Géa
http://www.astrosurf.com/altaz
-----Message d'origine-----
De : lowther@att.net <lowther@att.net>
À : Frank Ward <thewards@mindspring.com>
Cc : Dave Sopchak <dsopchak@mac.com>; ATM@shore.net <ATM@shore.net>
Date : lundi 26 mars 2001 05:17
Objet : Re: ATM Fouchalt tester


>
>Frank.  Different things here.  Foucault is measuring zones.  In that,
>color shouldn't make a difference. What ever you got and can best see.
>I see roughness most easily with my film based Ronchi grating.  Someone
>once said that one of the tests for surface roughness, don't remember
>which one, was simply a 'partially lamp blacked slide'.  I have wondered
>if my non opaque Ronchi grating was kind of doing double duty in that
>respect.
>
>Ken Lowther
>
>
>Frank Ward wrote:
>>
>> Dave,
>> > If I'm wrong, I hope someone out there will correct me!
>>
>> I wouldn't think of correcting you.....but I get different results.
>>
>> Up until this weekend I have used a white light from a MagLight.
>>
>> The surface I was working on was beautifully smooth and the
>> shadows easily defined.
>>
>> Then I rebuilt my tester and using a slitless source and a green
>> LED.
>>
>> The same surface I was working on became rough and the shadows
>> themselves had jagged edges.  When I switched to a full sized lap
>> and used Black Rouge (sloppy but known to give an excellent surface)
>> the same mirror suddenly became smooth again and the edges became
>> more distinct.
>>
>> I am convinced that for my use the Green LED has improved my view of
>> the surface and allowed me to make changes I didn't know it needed when
>> I was using white light.
>>
>> What bothers me today is the knowledge that I don't know what I don't
know
>> and that's beginning to bother me...but then I am trying to strech my
>> abilities
>> and figure mirrors into the teens so there are a >lot< of skills I have
to
>> learn...
>> and a lot of changes I have to make to my previous techniques.
>>
>> This green LED has helped a lot.
>>
>> In friendship,
>>
>> Frank Ward
>> Atlanta, GA
>>
>> ----- Original Message -----
>> From: Dave Sopchak <dsopchak@mac.com>
>> To: Tom and Beth Simmons <tomandbeth@theriver.com>
>> Cc: <atm@shore.net>
>> Sent: Sunday, March 25, 2001 11:21 AM
>> Subject: Re: ATM Fouchalt tester
>>
>> >
>> > >Why do we want to test with RED light? >IF< I understand correctly:
>> > >* Foucault testing is comparing relative intensity of light at X/-X
from
>> > >center
>> > >* The determination of where these two points equal is a (the?) weak
link
>> > >* Our eyes are more sensitive at higher frequency
>> >
>> > to an extent- they start losing out when you hit ultraviolet ;)
>> >
>> > >* The wavelength is part of the equation that determines accuracy
>> > >     (1.22 lambda F/D)
>> > >
>> > >Am I missing something here? At least with my, ummm, mature eyes, red
>> light
>> > >causes my focus to wander. Wasn't this the basis of a discussion last
>> year
>> > >about
>> > >what color light is "best" for low light vision?
>> >
>> >
>> > Tom,
>> > I'm pretty sure that the red vs green or blue only applies to
>> > observing objects with the mirror, not testing it(with the exception
>> > of seeing fringes for TDE with the knife edge far from focus). All we
>> > care about with the Foucault test is something that your eye can see,
>> > as all radiation that hits the mirror and reflects does so the same
>> > way, regardless of its frequency.
>> >
>> > If I'm wrong, I hope someone out there will correct me!
>> >
>> > Dave
>> >
>> > --
>> > _______________________________________________________
>> > "How would you like it if somebody picked your apples?"
>> > -irritated tree in The Wizard of Oz
>> >
>> > 28d 13m 56s N
>> > 82d 40m 54s W
>> >
>