From: DROWESMI{at}aol.com
To: atm{at}shore.net
Reply-To: DROWESMI{at}aol.com


Some other possibilities:

Make an 11" very long radius sphere and use the inner 8 inches.

Why?  Because autocollimation flats don't have to be exactly flat.  A
sphere with a few waves of sagitta will do the job just fine, especially
when testing a relatively slow scope.  The "flat" must be free of
fourth-order surface structure, i.e., spherical aberration.  This will
likely be true if you use the inner 8" of an 11 inch "flat".
 The beauty of this method is that you don't have to work so hard to get
your "flat" flat.  Just use your spherometer to grind the best
flat you can, polish it and test it.

Test the "flat" in autocollimation mode using a known-good
Newtonian telescope, preferably with a long f/ratio and small central
obstruction.  Be careful of off-axis light sources when testing in the
autocollimation configuration.  Understand what you're doing.

Another approach:  If you have that handy, known-good Newtonian, use it as
a collimator and test your refractor that way.  The trick is to figure out
how to insert the light source at the infinity focal position of this test
telescope.  This method works very well, but you loose the ability to see
the central part of the refractor objective due to the central obstruction.
 Not really a big deal.  Collimation of the Newtonian and the refractor can
be challenging, but doable with patience.  A laser can be very useful for
this. If the central obstruction really bothers you, figure out how to
place the very small light source exactly on-axis at the infinity focal
position without the use of a secondary mirror and focuser.  When you
figure it out, tell me how it's done....

-Dave Rowe-

> For testing my 8" lenses, I'll need an optical flat, I
>  figure 8 or 9".  I have a rough understanding of the
>  method for making optical flats with three blanks, but
>  unfortunately have found a shortage of web pages in
>  this area.  I have texereau's book which I'll read up
>  on.

--- BBBS/NT v4.00 MP