Re: ATM First Light ... uh, kinda...

["Dave McCarter" <dmccarter@sympatico.ca>]

I think that one of the difficulties you may be having is with the light
source on your tester. Yes, in theory a blue LED might give better results
due to the shorter wavelength, but that is negated by the simple fact that
the human eye is very insensitive to blue light, diffraction errors will be
much worse, and the eye has difficulty focusing blue.

It also sounds like this is your first experience with mirror testing.
Trying to master a single test method can be a steep learning curve, because
you not only have to learn that you are seeing the slope of the galss, but
also how a particular polishing stroke will alter that slope, and what the
resulting change to the test will look like. Trying to master multiple
methods at the same time is bound to create confusion and frustration in the
extreme.

Pick a test and learn how to perform and read it. I suggest a Ronchi test
with green or red LED, because it is so darn easy to set up. Play with it
for a while and don't give a darn about the mirror figure. Try to get the
mirror back to a sphere from edge to edge, which Ronchi is very good at
testing. Then start back to a parabola, and when you think you are there,
star test. Time spent really learning to test is invaluable.

Also, place a piece of Ronchi grid in a film can lid in which you cave cut a
1/4 inch central hole. Hot glue works well to hold it there. Cut the bottom
off the film can and snap the lid on. Now you can point any scope at a star,
replace the eyepiece with the film can and rack the Ronchi grid either side
of focus. If the scope is well corrected the bands will be straight. This is
probably, IMHO, the easiest test to do on scope optics, and although it does
not generate numbers to brag about, at least you know what the mirror is
doing.

Dave