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


>>>>>
First off, can someone recommend some good references for either/both of these
tests?

Secondly, am I correect in assuming that the LWT, being essentially a "single
wire ronchi grating", one is measuring/comparing the shape of the
image of the wire, or just its position?

--
Michael Lindner
<<<<<

Francis wrote a description of the LWT in ATMJ.  Sorry, I don't remember
which issue.  However, if you scrounge in the archives, you will find at
least one contribution from Nils Olof that is much more helpful and
practical than the Francis article.

In the LWT, the wire is placed well inside the RoC (about .5"). To an
observer behind the wire, the wire appears to cast a shadow on the mirror. 
If the wire is moved transversely, the shadow moves across the face of the
mirror.  A pin stick to mark radial zones is placed in front of the mirror
and the wire is moved to match the center of the shadow with the tip of
each pin.  The amount of wire movement (read on a micrometer) needed to
move the shadow from the center of the mirror to each pin is recorded and
entered in Sixtests as a caustic test.

As it happens, last night I attempted a photo-digital version of the LWT. 
Instead of moving the shadow to predetermined radial positions, I moved the
wire in constant increments, digitally photographing the shadow after each
move.  The potential benefit of this is that my aging, myopic eyes won't
have to try to determine the position of a fuzzy shadow and a tiny nail at
a distance of 8 feet.  I can enlarge a digital photo on the computer to any
degree I please and count pixels. I made and measured the photos last night
but I haven't had time to crunch the numbers, so I don't know how well it
worked.

Jim Sturtevant

--- BBBS/NT v4.00 MP