Re: [ATM] squaring truss tubes and collimation

[Dave Smith <smithersscope@yahoo.com>]

--- Jay Kirkland <jaykirk2@compusmart.ab.ca> wrote:

> the light still has to travel down exactly the 
> same path from the eye/laser/crosshair to and from
> the primary. But there's something there that still
> nags at me - perhaps the fact that a 
> crosshair is not all that sensitive (compare the
> results of crosshair vs 
> laser vs barlowed laser collimating for sensitivity)
> and that the laser 
> only illuminates the base of the laser itself, that
> the beam doesn't 
> travel all the way thru the optical axis. But then
> it doesn't have to, 
> does it? If it travels part of the way, back to its
> source, it has to be 
> following the exact same path, doesn't it? it's not
> going to be crossing 
> over on itself anywhere. No, it can't.

Mind if I comment here?
I cast my laser image out the end of the OTA onto a
wall (preferably something bright or white). I bought
one of those funky laser things off ebay that has 50
different image attachments for the laser. I picked
the soccer ball--it's spherical and has a repeating
pattern that helps me center the shadow of the
secondary in the center of the soccerball. Adding my
2x barlow to this collimation protocol CONSIDERABLY
improves collimation. It doubles the soccerball image
on the wall which in turn helps me resolve what is
centered and what's not. I've seen laser collimators
that are designed with a 45 degree window at the
source of emission. From the primary
mirror/collimation screws you view (and center) the
returning laser beam at/in this window. So yes the
beam can impinge and return on itself, no problem. The
photons going to and bouncing off of our primary
mirrors pass each other all the time before hitting
our retinas.


>     Which I guess just leave me with my original
> thought, that, 
> especially for big scopes, or weird scopes, the
> directions in 
> Kriege/Berry aren't sufficient to ensure that the
> mechanical center of 
> the spider cage is co-incident or even reasonably
> close, with the 
> mechanical centre of the mirror box.
 
I've run into this problem with my tridob. If I break
down my scope completely for travel and then set it
back up, on occassion I've found that the endring is
WAY off (mechanically AND optically), even though I've
numbered and marked my trusses. I suspect my truss
clamps are shifting. And you're right, it doesn't take
much to be way off at the endring! I've learned to use
my laser collimator to line up the endring and mirror
box. It has to be dark enough to see the beam, but
what I do is the following. Place the collimator in
the corner (or on the outside) of the mirror box,
lining it up with the mechanical "up" direction of the
mirror box. Then I adjust my truss pole lengths so
that the laser beam strikes the endring at the correct
position. Sometimes it's tricky finding the laser
spot. It's definitely a 2 man job. But it works.

HTH,
Dave


	
		
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