Re: ATM Colimnating a Newtonian with a perforated mirror?

[Dwight Elvey <elvey@hal.com>]

Nick Martin <bonnyton@ednet.co.uk> wrote:
> 
> Dear list,
> The astronomical society to which I belong has been given access to a very
> nice 12" Newtonian. The only problem is that the mirror is perforated,
> presumably for use as a Cassegrain or similar. It is optically good but the
> problem is how to colimnate it, other than doing a star test. A centre spot
> is clearly impossible.

Hi
 A cardboard cover with a spot on it would work. You could also
make a cross hair support, glued to the back of the mirror that
would mark the center of the mirror.
 If you wanted to go hi-tech, you could use one of the lasers
with the circular diffraction patterns. Many of these can be used with
a screen placed over the front of the telescope to get things
lines up. I also like making a disk ( about 1/2 diameter of the
mirror ) that you place on the secondary stock. If the disk
is made to mount on the axis of the secondary ( including offset ),
you can do good primary alignment by backing up until the
disk reflection fills the entire primary. Any annular ring that was
wider on one side but not the other, requires adjustment.
This can also be done inside-out, by placing a outside
mask that is concentric with the secondaries center ( the
method I use on a small telescope but should work as well
on any size ). This allows one to have a cross hair on the
mask to verify alignment to the secondary. That takes care
of the primary.
 When using a film can, one can make sure that the secondary
is pointed right by just looking at the framing of the primary.
you don't need a center mark for either of these setups.
 Also, there are many other traditional methods in the books
that would still work well.
 As far as star alignment. You don't need a point light source
placed at infinity to do final alignment. Doing a star test,
you do but alignment isn't a star test. It can be most any
small light placed at a great enough distance that the focus
travel is close to what you use at night. Almost any remote
light will make diffraction rings. In fact, because it may have
spherical errors from the shorter distance the diffraction
rings will be more visible. You just make them concentric just like
the star alignment.
Dwight