From: John Murray 
To: Jon Bishop 
Cc: Amateur Telescope Makers 
Reply-To: John Murray 


Jon,
        I have just uploaded some pics and an explanation of how I go about
getting
the pics and the setup I use. Go to the latest ronchigrams link in my sig
and you'll see what I do.
        To answer your question though, the knife edge works by slowly entering
the
return beam. As it does, the light doesn't suddenly cutoff. It will go from
a light gray (or colour depending on light source) through to a dark and
then darker gray. The area that we are interested in is what is called
"NULL".
        If a mirror is perfectly spherical then the entire mirror will go
slowly
darker as the knife edge enters the return beam. This is because the light
from the source expands in a spherical front outwards from the source, hits
the spherical mirror surface and returns at the same time. As the edge goes
in further to the beam it will cut off all the light.
        If a mirror is other than spherical however, the spherical wavefront
will
hit the mirror at different times and the result will be shadows on the
mirror. The end result will be that only (hopefully circular) rings or
zones on the mirror will go null at the same time. By using a measure of
some description we can measure the position of each null zone and
translate that into the mirrors shape.

BTW, I hope I got that explanation right?

Cheers

John Murray
Whyalla
South Australia
Check our web page at http://astronomy.sa86net.com ATM and M-O-M stuff.
Latest Ronchigrams only at http://www.users.bigpond.com/imasloth/


> -----Original Message-----
> From: Jon Bishop [mailto:jbishop{at}sequencedesign.com]
> Sent: Friday, 15 August 2003 9:55 AM
> To: John Murray
> Cc: Amateur Telescope Makers
> Subject: Re: ATM Foucaul pictures using digital camera.
>
>
> But if you are using the knife edge test then doesn't the knive
> edge need to be
> in the path of the returning rays?
>
> Jon
>
> > G'day John,
> >     I use a JVC digital camera with auto focus, auto exposure to take my
> > foucault and ronchi pics. There's no black art to it. Just have
> the camera
> > supported in one spot so it doesn't move and then adjust the
> tester to feed
> > the return beam into the lens. See my page on ronchigrams in my
> sig to see
> > the results..
> >
> > Cheers
> >
> > John Murray
> > Whyalla
> > South Australia
> > Check our web page at http://astronomy.sa86net.com ATM and M-O-M stuff.
> > Latest Ronchigrams only at http://www.users.bigpond.com/imasloth/
> >
> > -----Original Message-----
> > From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Jon
> > Bishop
> > Sent: Friday, 15 August 2003 9:11 AM
> > To: ATM
> > Subject: ATM Foucaul pictures using digital camera.
> >
> > Hi,
> > I am in the process of polishing and figuring an 8" F5.25
> mirror. I would
> > like some advice on figuring it, but before I do that it would
> be nice to
> > take some pictures of the mirror under test.
> > I have a Canon A40 digital camera, but when I try to use it it
> does not pick
> > up the mirror disk. Has anyone suceeded in using an autofocus
> digital camera
> > to do this?
> > Thanks,
>

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