[SENT] ATM Focal Length of first mir

[PFAFF2@delphi.com]

>I  am about to cut the hole for my focuser in my sonotube, but want to 
> make sure I have the focal length of my mirror exactly right.  In the 
> foucalt test, the light source and knife edge are separated by 5 
> inches.  Texereau suggests taking the average of the two distances as 
> the RC.   Is this an accurate enough measure?  It seems like even a 
> small error (1inch) would really screw things up.  Any suggestions 
> would be appreciated. 
> 
> David Sayah
> dsayah@ucla.edu
> 
That should be pretty close. The way I solved this problem is to
leave the tube a little long on the mirror end. That means the tube 
will extent somewhat past the mirror cell. Install the spider,diagonal
and focuser. Adjust the focuser out about a 1/2" to 3/4" an install an EP. Now 
 
move the mirror cell up and down the tube till you get
a focused image in the EP. Use some wedges to hold the cell in the tube.
Don't worry about collimation at this time. Next mark the cell position.
Square up the cell and bolt it to the tube. You can also cut off the extra
tube that extents past the cell if you wish. If you live where you have a
clear view of a terrestrial object a couple miles or more you can do this in
the daylight. 

The moon is great for this adjustment but add a temporary strap at the end
of the tube in case the wedges slip.

It works for me
Bob Pfaff