|
Actually, I didn't state the diffraction rings outside focus are more
well defined because the mirror hasn't reached thermal equalibrium. In my
estimation that would be nonsense. What I said was interpretation of the
diffraction rings inside focus was a virtual impossibility until my mirror cools
down. Frankly, you'd be lucky to see diffraction rings at all.
However, once it cools off it yields sharply defined diffraction
rings. My ultimate reasons for responding to the
original post are as follows:
a) For a long time I was baffled(no pun intended) by the
apparent inconsistency of my telescope to perform in the star test and in
focus even when the seeing was "good". As I experiemented
and gradually increased the amount of air flow on the optic, it performed
better and better to the point where it now performs at a level
consistent with the excellent test data derived from the
foucault tests. An otherwise "excellent" mirror like this
one could easily have been erroneously declared "bad" by
the uninitiated because of it. Therefore, based on my experience, its
possible a large mirror like the one in question could very well
be troubled by the thermal problems I experienced.
b) I've come to strongly believe that thermal issues(especially with
large thick mirrors) are an absolute VAMPIRE to these Newtonian's. Furthermore,
I think the severity of thermal problems is largely underestimated by the
majority of the people that own these large instuments, and for the most
part those who attempt to abate the problem are not aggressive enough.
Alan Garcia |