Re: ATM Concerning ronchi star test

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

Rick, it's simpler than that!  Rather than destroy a star party EP, grab a
chunk of anything, wood even, and carve it so that you have a fair
approximation of a 1.25" OD (even go and buy a piece of the plumbing that
slides inside the 1.25"  hole) and attach the Ronchi Grating so that you can
put the mess into the focuser.
When you get near the focus on a star, you will start seeing the Ronchi
Lines just like you should see with a spherical surface at the bench.
Seeing inside of Focus lines is just like seeing the Outside of focus images
as they will both be straight lines if your mirror is correct.  The lines
will tend to waver a bit due to seeing conditions and on a really bad night,
they can really be an interesting thing to see.   Remember that the test at
the bench is a ROC test where the spherical surface will provide straight
lines as the light comes to an exact focus.  The Paraboloid surface with a
light source at infinity will also have that same condition of a perfect
focus and thus you should have straight lines.  I might note that the Ronchi
Grating was the method that the SureSharp camera focuser worked.  When you
got to focus, the star would blink in and out as it went across the grating
and that was the focused condition.  Inside or outside of focus, you got the
Ronchi lines.
Also note that when you get the Ronchi Grating at focus, it will do the real
Foucault Test (Leon Foucault never did the work with the Couder Mask or the
stuff with all of the calculating of the differences between a paraboloid
and spherical surfaces - he just made an elliptical test with the light
source at the near focus and the KE at the far focus and went for a null
with a final on a distant light source and the secondary in place to bring
the image out) if the edge of the grating lines are good enough.  Film
gratings sometimes are a bit fuzzy on the edges and that doesn't work as
well as a sharp, straight edge does.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net