Re: (ATM) Chasing Shadows

["Johnston, Steve (S.W.)@T.H." <SWJOHNST@tredegar.com>]

On 1995.10.20  Mark Suchting wrote:

> the ronchi grating is not so good for evaluating  TDEdge because of
> the inherent diffraction effects at the edge of the mirror.

   Ronchi gratings are difficult to use for detecting a narrow TDE. 
 However,
a variation of the Ronchi grating, the single line Ronchi grating is great 
for
this purpose.  In fact, this is the basis of the photo showing TDE in 
Texerau.

  What is a single line Ronchi grating? It is simply the Foucault knife-edge
placed somewhat inside or outside the radius of curvature of the mirror. The
knife-edge acts like a single line Ronchi grating.  Although the mirror edge
diffraction and multiple overlaping diffraction fringes of a real Ronchi 
grating
can make TDE difficult to judge, a single line Ronchi pattern avoids these
problems while retaining the simplicity of interpretation inherent to Ronchi
patterns.

                                                                              
     ---- Steve Johnston.
                                                                              
           SWJOHNST@tredegar.com