Re: ATM astigmatism with a sling?

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

Before you really go off on blaming the mirror when startesting, I suggest
that you do a Ronchi and Foucault test on the mirror to see what it really
is like.  You don't have to pull the mirror from the cell (and I suggest
that you don't to insure that the figure is what you are thinking that it
is) and see what those tests show.  My suspicion is that you either aren't
getting a good enough sky for startesting or you aren't seeing things
correctly.  I might also add that a poor secondary will produce problems
that you may be thinking are in your primary.  You can even test the primary
with the spider and secondary in place although you won't get a good center
in the Foucault test but that can be simulated on the test data by bringing
it to the correct position by changing the value you put in for the center
zone.  If you have a truss tube type, just don't assemble the top part and
you will be able to see all of the mirror in the tests.  I'd also suggest
that you do the test at about 30deg. of elevation.