From: "David Stevens" 
To: "ATM" 
Reply-To: "David Stevens" 


Thanks I will try that.

Dave
----- Original Message -----
From: CSC 
To: Atm 
Sent: Wednesday, December 11, 2002 12:57 AM Subject: RE: ATM My TDE


>
> 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