Re: ATM 1st vs 2nd edition Texereau & 3rd edition Berry

["Bob May" <bobmay@nethere.com>]

As to tools for measuring the mirror, there are several websites that have
good drawings on them to help you in your difficulties.  There are several
different basic designs and you need to understand them first:

Original design: A lamp with a slit in front of it and a traveling Knife
Edge (KE) with measuring of the travel.  Difficulties are that the slit can
be hard to find and the KE needs to be parallel to the slit so you can't
just rock the KE support to get it into proper alignment with the return
beam.

Moving Source:  Divided into several designs.

Seperate light source with slit.  This also has the problem of finding the
fine slit on occasion but has the advantage of the lamp and KE needs to be
aligned only once as both are on the platform which can rock back and forth
rather than slide back and forth.

Slitless: A single KE provides the one edge required by the test (remember
that the light is inverted when it returns from the mirror!) and is
automatically self aligning as the light source is usually located
underneath the area used for looking at the mirror and, since the light
source is much larger in size, it's a lot easier to see when the test isn't
fully aligned.  The test does have the problem in that the light return is
high and not defined until the conditions for showing the shadows on the
mirror's surface become valid.

As to the bases, there are two basic varieties - the rocking version and the
sliding version - and three measuring versions that are common - the
multiplied scale, the micrometer screw indicator and the direct dial
indicator reading.
The rocking version uses a rod generally for a platform to ride on for one
side and this provides the pointing direction for the tester and the other
side of the platform is a screw to provide the side to side adjustment so
that you can do the proper amount of cutting the light off.  There is a
screw to adjust the distance from the mirror which can be a micrometer
thimble if so desired or can be a simple machine screw and a dial indicator
to indicate the distance movements.
The sliding version slides back and forth in various ways (linear stages, 4
rods holding balls between, dovetail slides and just straight slides) in
both directions that are required.  The travel may either be by the
particular platform being pulled to the adjuster by a spring (prefered as
the measurements don't have any slop in them) or directly moved by the
adjuster.  Only the direction towards and away from the mirror needs to be
measured as the other direction is just to make the picture pretty.
As to the measuring, the only measurement needed is to measure the relative
distance changes from the mirror - the absolute distance is measured with
something else and it is the relative changes that are most important when
doing the test - the absolute is also important but that's when setting up
the calcualtions for the test and that can be off by a percent or more
without any problems at all unless you need to work to 1/200th of a wave.
It's the relative changes in the ROC that are important to the test
measurements.
As to building the tester, I've built some really crude looking ones and
some nice looking ones and the amazing thing is that they all perform the
same.  You can hammer 2"x10"s together and get a good working tester while
the guy that built a fine looking one out of chrome steel and aluminum may
have some serious errors in the all important measuring and not do as
accurately as yours.
As you have seen from the above, there are a fair number of ways to do the
design and you can use a lot of different materials in doing each different
design so that's why you may be confused in some regards.
You may want to stop by my website and I have a bunch of references to a lot
of things that are on the web in my mirror making article.  Go explore!
Bob May
http://nav.to/bobmay
bobmay@nethere.com