From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


I think a knife edge will show all you need to know re: TDE.  The Ronchi
line image that people rely on to detect TDE is about the same as the knife
edge.  The edge of the line behaves exactly like the edge of any solid
object.  If you see the  edge of the shadow hook in opposite directions
when you're well inside and well outside the focus area, there is a
different radius there.

Ronchi gratings can cause multiple images through diffraction, and the
displaced images sometimes look like a hook in the band.  I think one atmer
was trying to correct these fringe effects, and I have been confused by
them also , as they obscure the real shape.

Put the KE about a half inch inside focus and the shadow about 25-30%
across the disk and see what's there.

Star testing will detect it.

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of David
Stevens
Sent: Monday, December 09, 2002 1:06 PM To: ATM
Subject: Re: ATM My TDE



Hi Herb,

I don't have a Ronchi grating to check the edge but I have heard this is an
excellent test. I decided to go back to a sphere and get the edge first.
This is my 3rd attempt at parabolising the mirror, on my first attempt I
did have a diffraction ring all the way around but my edge zone was high. I
tried TOT center of tool over the edge of the mirror 1X around and the ring
was gone and my edge turned. This tells me the ring is possible to get on a
mirror this fast but its very difficult to get the edge and have a fully
corrected center.

Do you know where I can get a Ronchi grating without having to pay more
than twenty dollars??

Thanks,
Dave

----- Original Message -----
From: Herb Kasler 
To: ; 
Sent: Monday, December 09, 2002 12:05 PM Subject: Re: ATM My TDE


>
>
>    Dave,
>
> Can you see any hook at all on a Ronchi test? At F/3.9, the Ronchi lines
> should be straightening out quite a bit towards the very edge of the
mirror,
> so a really significant TDE should give you a pretty easily noticable
hook.
> Alternately, a star test on a bright star with the uncoated optic should
> give you a characteristic pattern if the very edge is siginificantly long.
> If these two tests don't indicate any problems, then I don't think that I
> personally would get too worried about not seeing a diffraction ring all
the
> way around the mirror on the Focault rig. I certainly never saw such a
ring
> with my 16" f/3.9 mirror, even when I masked off the edge to a spot where
> the figure looked perfect. Perhaps the ring gets harder to see if the
focal
> length of the optic is very short. Experts?.....
>
>
> Good luck,
> Herb.
>
>
>

--- BBBS/NT v4.00 MP