To: John Swenson 
Cc: atm{at}shore.net
From: jsgrmfg{at}gracemfg.com
Reply-To: jsgrmfg{at}gracemfg.com



<<<<<<
   the photo LWT is the way I've been doing it for a while. When you are
   doing the test you don't have to visually be precise about anything.

   The one problem with this is you need to know the transverse measurement
     of the center of the mirror fairly accurately so you have the proper
   transverse offset. The way I do this is to take two images near the
   center (one on each side), then do an interpolation from their pixel
   positions and transverse measuremnts to find the transverse measurement
   of the center of the mirror.

   John S.
>>>>>>>>>

You have put your finger on the very difficulty I encountered when I
crunched the numbers on my first attempt at a digital LWT.  Where's the
durn center!  As I mulled it over in the shower this morning, I thought of
two possible fixes. 1.  The old center dot, just like the dot that many use
for collimation.  It might be a bit of a nuisance to put it on and clean it off
for every testing session but, as a solution, it has the virtue of being direct
and to the point.  2.  Pin stick.  In this case, the pins would simply be
known reference points in the image, simplifying the problems of
calibration
and measurement.  3.  Both of the above.  I already have a low power
viewing telescope that I made for phase contrast testing.  Maybe I could
use that to accurately set the wire at the center point with the aid of a
center dot or center pin.

Even with the center point a little ambiguous, the results of the test were
reasonably in line with other tests - I did a visual LWT and a digital
Foucault at the same time - and, with a little refinement, will probably
become my favorite test method.

Jim Sturtevant

--- BBBS/NT v4.00 MP