"First--I would like to thank Nils Olof for taking the considerable
time and making the magnanimous effort to provide us with a
quantitative analysis of the autocollimator reflections. I would also
like to thank Jim Fly for creating the Infinity II autocollimator. With
its wider field of view and highly reflective first surface mirror,
this tool enabled us to observe and verify the rather complicated
theoretical analysis that Nils Olof has now finally completed.

Nils Olof commented that his autocollimator web page "...is no doubt
controversial."

I don't think so, if you stick to the analysis of the reflections as
they relate to the actual axial offsets.

What was proved:

1.) The autocollimator is the only (currently) available multi-pass
collimation tool for both the focuser axis and the primary mirror axis.

2.) When ALL of the reflections are precisely stacked on top of each
other, ALL axial defects are eliminated.

3.) Using the "carefully decollimated primary mirror" protocol (which
I strongly urge if you are using the 2-inch autocollimator), the
focuser axial error can be easily removed revealing the primary mirror
axial error visually magnified eight times! This protocol is described
on my web site, (scroll down to the autocollimator sections.)
http://homepage.mac.com/vicmenard/telescopes/NPaddend.html

4.) Using the iterative protocol (which Nils Olof recommends) is in
fact the same thing that is suggested in "New Perspectives..." Of
course, with the recent availability of the 2-inch autocollimator, the
fifth edition will encompass the newest protocols!

5.) Using the new 2-inch autocollimator is easy, predictable, and
repeatable. If you're using the 1.25-inch autocollimator (with its
limited fov), I would suggest that you skip the math and use the
iterative procedure--it still works as advertised!

6.) As Nils Olof suggests, "Enjoy, but don't choke!" Even the
"warnings" are a bit overstated--if you use the suggested protocol(s)
it just works! And although the camera lens does indeed show
significant defocusing of some of the reflections, in actual use the
eye accommodates the differential focusing and even easily resolves the
subtle parallax issues. Hopefully Nils Olof will soon have the
opportunity to actually use an autocollimator so he can see for himself
how effective the tool is!

Finally, the autocollimator is not absolutely necessary for every
Newtonian telescope or every Newtonian telescope owner! Some people
have a knack for getting the collimation right with minimal tool
support, others need help (or assurance), to finish the job. I don't
think I can stress enough how much easier it is to use the new 2-inch
Infinity II! If you've had problems with the 1.25-inch autocollimator,
you will quickly become proficient with the 2-inch (and then fully
understand what you were seeing "just off the edge of the field" in
the 1.25!)"