Re: Star Test Question
Bratislav Curcic (epabcc@epa.ericsson.se)
Tue, 2 May 95 09:42:58 EST
Maybe I should have made myself more clear.
All the time I was talking about NULL Ronchi testing. Not at radius of
curvature, not just testing the primary. I am talking about using
Ronchi grating AT THE FOCUS OF A COMPLETE TELESCOPE. A perfect optics
will show straight bands. Any departure from a straight shows itself
easily visible. You might even use one line of the grating as a Fucault
knife, but atmosphere usually plays havoc in a telescope (good to see
how much your cherished mirror capable of 1/20 wavefront performance
gets thrashed by atmosphere). But Ronchi lines, even if they get
deformed and distorted by atmosphere/tube currents, are quite easy to
interpret, and our eyes/brain are very capable of "integrating" the
effects. Turned down edge, zones, over/undercorrection, all are easily
visible, easy to interpret and (relatively) easy to quantify.
The only evil which might escape this test is astigmatism, which is
on the other hand quite detectable in the eyepiece/star test.
Bratislav
PS to increase sensitivity at the focal point, the grating has to
be quite dense; 100 lpm I consider as a minimum, better still
is 200 lpm or more