ATM (no subject)

[David Eric Allen <ericallen@worldnet.att.net>]

For the benefit of those who are having trouble collimating the Newtonian secondary, or 
having a discrepancy between eyeball and laser 
collimation, I would like to point out what is apparently a little known problem.  This 
is described in detail in my article on collimation in 
TM#22, but for those who don't have access to antique publications, it goes like this:  
The secondary is tilted to the optical axis; for our 
purposes, that part of the axis between the secondary and the eye.  This places one edge 
of the secondary closer to the eye than the other 
edge.  This in turn causes the closer half of the secondary to appear larger than the 
more distant half.  Closer is bigger, right?  The net 
result of all this is to cause an optical illusion which (this is the tricky part) makes 
a CENTERED secondary appear DEcentered, and a 
PROPERLY DEcentered secondary to appear to be CENTERED.  Thoroughly confused?  It gave me 
fits at first.  Whether you center or decenter, and 
I don't really want to start an argument, the best way to position a secondary under the 
focuser is to put a spot centered (or offset, 
depending on how you're mounting it) on the secondary mirror, and position the secondary 
under the focuser by means of a collimation tube 
which has crosshairs in it.  Center the dot on the crosshairs (in either case), and don't 
worry about the edge.  The centered secondary will 
appear to be decentered in the direction of the sky, and the centered dot will appear to 
be off center.  On the other hand, the secondary 
which is physically decentered by the proper amount and has an offcenter dot on it will 
appear to be centered under the focuser, and the dot 
will appear to be in the center of the secondary mirror.  
        Still with me?  Just to complicate matters further, the REFLECTION of the 
secondary mirror in the primary is too far away to be 
affected by this optical illusion, so that it looks pretty much as it should, i.e., the 
centered secondary looks centered, and the decentered 
secondary looks decentered.  
        Incidentally, if you have an f/8 or slower system, you probably don't have to 
worry about any of this.  Sorry.  Should have said that 
up front, so you wouldn't have to read all this stuff.  For you, the usual "make 
everything look concentric" instructions actually work.  For 
the rest of us, they don't.  
        The laser collimator should work.  The only problems I have seen are minor 
construction (of the telescope) problems, and the problem 
of the laser collimator being so heavy as to distort the position of the focuser.
        It is possible to achieve optical collimation with the secondary in the wrong 
place, but you will not have mechanical collimation.  
The laser collimator will not work unless you have both optical and mechanical 
collimation.

Gotta Go.

Eric Allen
Edmond, OK