From: "Dan Chaffee" 
To: 
Reply-To: "Dan Chaffee" 




>Small amounts of turned edge are not so easy to see but
>with experience can be seen. I'm talking here about having
>no more than one
>line visible at focus with say a 100 lpi grating.

At that point it's practically a 'null wire' test. I too have found that if
one is patient, ronchi nulls on a star can be more revealing than often
thought by amateurs. What works best for me is to orient the lines nearly
parallel to the movement of right ascention, so that equatorial tracking
will cause the lines to remain almost stationary at the focuser. Actually,
a slow steady movement of the line(s) across the field aids the
detectablity of subtle zones in less than ideal seeing; something to do
with repetitive patterns relative to motion.

Back to the comments on the star test: I think the point Bill was making
about star testing the near-spherical mirror in question was that the best
way to begin to understand the relationship of the caustic "horn"
and the out of focus diffraction patterns is to examine an optic when you
in fact know what's wrong with it. Approching parabolization by cross
referencing the foucault with the star test will show how your mirror will
perform under actual viewing conditions relative to what the foucault says
in the often unrealistic world of indoor bench testing. I see no reason
this would be out of reach for a first time mirror maker.One *may* find
that a slightly  undercorrected (foucault) medium to large primary tests
dangerously close to perfection in the star test, where temps are slowly
falling throughout the observing period. I've made mistakes in interpreting
the star test, but I always learn something I wind up using to my
advantage. It's not easy and takes alot of patience, but IMO there is no
substitute for testing an optic during figuring in at least some of the
actual conditions for which it is intended.

If the star test is so unclear at telling the optician what to do, why
bother with it at all as a confirmation right before aluminizing? After
all, what are you going to do if the foucault says you RMS at .02 w.f.
smoothly undercorrected and repetative star testing says you are a bit over
and  have a tad of a zone at 60%? Chances are, the mirror is very good,
but....it could be better:-)

Dan Chaffee

--- BBBS/NT v4.00 MP