Re: SV: ATM Collimating Newtonians

[Greg Granville <greg@laser.arl.psu.edu>]

>Not so fast! There are two issues here... one is the alignment of the
>focuser, the second is the runout of the focuser. They are INDEPENDENT!
>You can have one without the other! The focuser can be pointed at 30
>degrees and if it has runout the dot will move! This is best seen on the
>opposite wall of the secondary support. This is ONLY corretable by a
>"better" focuser nand not by alignment issues.
>G.

I agree completely... that's why I said the laser is always parallel to the
focuser axis "in theory". If it's not parallel, then you have a REAL
problem, not just a condition that can set you up for problems. 
If the focuser is titled and the laser is parallel to it, as it should be,
then you still have a potential problem. As I've been saying, you can't do
an good collimation with a laser if the focuser it not perfectly square...
Although I've see lots of people trying.

As has been pointed out by others, an improperly adjusted secondary can also
get you into the same kind of trouble. Each adjustment is sort of relative
to the other. I notice that a lot of people go to great pains to get the
secondary at a 45 degree angle, a lot fewer seem to go to as much trouble to
get the focuser square.

With an out-of-square focuser, it is possible to get a perfect 45 degree
angle on the secondary, with respect to the primary optical axis and a
central point at the end of the focuser. Unfortunately, if you rack the
focuser in/out the angle will change because your "peep-hole" will move
laterally with respect to the optical axis.

~~~
Greg Granville  Applied Research Lab - PSU 
greg@laser.arl.psu.edu      greg@penn.com  
      http://users.penn.com/~greg