Re: ATM Replacing a Cassegrain Primary

[Aart Olsen <aolsen@prairienet.org>]

Chuck,

You say this is a classical Cass hyperboloid secondary so you don't need
to match the primary to the secondary in any special way except for
focal length, and that's just to fit the mechanical parts of the
telescope.  You need to establish where the secondary's two foci are
intended to be, and I think it's best to do that empirically.  Assuming
you can't borrow another large classical Cass primary, the easiest way
may be to borrow something like a 10 f/5 or f/6 inch Newtonian mirror. 
Build a rig to hold the secondary or in front of this mirror; the
contraption should allow axial movement of the secondary.  Place a
diagonal in front of the primary to redirect the optical path from the
secondary to a ronchi grating at the secondary focus, and point the rig
at Polaris.  Now move the secondary mirror back and forth until the
ronchi lines are straight.  The secondary focus will move back and forth
about 4-5 times as much.  At the point where the ronchi lines are
straight the secondary is where it is supposed to be, relative to the
prime focus, and the ronchi will be at the final focal plane.  The
secondary must be at this same distance inside the prime focus of the
mirror you're going to make.  The distance from the secondary to the
ronchi in your test rig will also be same in your final telescope.  Now
all you have to do is make your new primary's focal length such that
your secondary focus is at a handy place behind the primary.  Probably
the mechanical parts of your telescope will dictate where this should
be.

You can also do this testing with a biggish refractor lens.  The Cass
secondary doesn't care what creates the primary focus.

Aart

Chuck Klingel wrote:
> 
> A University has donated a large (1500 pound)  16" Cassegrain telescope to
> my astronomy club.  The telescope was sitting unused for many years and was
> in need of a complete restoration.  We have most of the mechanical parts and
> can fabricate the ones we do not have.  We do not have the primary mirror.
> We do have the hyperbolic secondary.  Would it be possible for us to grind a
> primary to match the secondary?  How would we calculate the primary focal
> length and how close would we have to get?  I do not want to touch the
> figure on the secondary.  Would it be in our better interest to obtain a new
> matched optics set?
> 
> Chuck Klingel
> 

-- 
Aart Olsen
aolsen@prairienet.org