From: "James Lerch" 
To: "ATM List" 
Reply-To: "James Lerch" 


Hi Guys,

Trying to catch up after a 4 day vacation, so I hope I'm not repeating anything.

With regards to Camera Focus and Foucault testing, here's what I've observed.

In Theory I always focus the camera on the beveled edge of the mirror as
illuminated by ambient light.  In practice I often forget. :)  Fortunately
the Foucault-grams don't appear to be effected by the camera being
de-focused.  The only interesting effect of de-focus is the size of the
mirror in ambient light is different than fully illuminated Foucault-gram
of same mirror.  (Camera optics are a trip :)  As long as the mirror
diameter in Robo-Foucault is based on the fully illuminated Foucault-gram,
all should be ok.

With Regards to camera lens distortion and Robo-Foucault

I did as Mike Peck did and actually tried to measure the distortion,
finding little evidence of any.  However this doesn't rule out distortion
based on returning rays of light for the Foucault test, but still...

In any event, the most likely distortions would be pin-cushion or barrel
distortions.  Any of these distortions would only effect the 'Real' zone
radius center vs. the Imaged zone radius center.  Fortunately Foucault
testing appears to allow for significant room for error in the zone radius
centers before the test starts to break down.  So hopefully any of these
distortions would have limited impact on the final test results so long as
the longitudinal readings are accurate.

Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
----- Original Message -----
From: "Nils Olof Carlin" 
To: "Michael Peck" ; 
Sent: Wednesday, January 22, 2003 12:07 PM Subject: Re: ATM
Robo-foucalt...is it calibrated?


>
> hi Mike,
>
> > I'm pretty sure that according to the theory of geometric optics the
> > aberration of distortion is independent of and unaffected by the
> aberration
> > of defocus.
>
> I can't think of an easy way to explain, but it's certainly geometric
> optics. In normal use, the defocus that causes distortion will be
> readily apparent, but with this narrow geometry not necessarily. It
> doesn't have to do with the lens, but the geometry of the returning
> light rays. Even if you don't use a lens at all, you get an image,
> that is highly distorted but the focused lens will remove it.
>
> >  I've taken a lot of out of
> > focus pictures with a lot of lenses and have never observed this
> alleged
> > phenomenon though.
>
> Then keep on doing so, by all means. If you check that the focusing is
> on the irror, there won't be a problem, I think.
>
> Nils Olof
>
>
>
>
>

--- BBBS/NT v4.00 MP