RE: ATM Collimation Question

["Bertapelle, Ken" <Bertapelle.Ken@tci.com>]

This is a question  that I'm also interested in.  I've been fussing with 
the collimation of my 10" dob for longer than I'd like to admit, but I 
think I now know how to do it.  This is what I do, all comments greatly
appreciated:

   1) Make sure the focuser points at a spot on the opposite side of the
      secondary cage, i.e., 180 degrees away and at the same height as 
      the focuser.  This is only done once a season, under ordinary 
      circumstances.  The tool I use is a non-magnifying eyepiece that
      has a centered cross hair in it.
   2) Center the secondary mirror in the eyepiece used in step one.
      I do this by retracting the eyepiece until the secondary just about
      fills the view.  Then orient the secondary by adjusting the spider
      and center screw so that the secondary forms a circle (not
      an ellipse) that evenly fills the eyepiece view.
   3) Center the primary in its mirror box by adjusting the sling that
      supports it (ordinarily only necessary once a season), and nudging
      it to center it left or right in the box (necessary once a night,
      unless someone trips over the telescope).
   4) Center the image from the secondary to the primary on the primary.
      I use a laser collimator to do this by adjusting the four screws on
      the secondary until the laser beam hits the center of the primary.
   5) Center the reflected laser beam so that it retraces the the same
      path from primary to secondary to the laser source.  This is done
      by adjusting the screws on the primary.

It's my understanding that the steps should be done in the above order. 
Any other sequence causes problems, although I'm not sure I can explain
why.

The final collimation is against a centered star (polaris is best, since
I don't yet have a tracker), which should show the secondary shadow centered
in the diffraction rings on both sides of focus.  If the shadow isn't 
centered (it should be close after the above procedure), then it's direction
from center gives an indication as to how to make the final adjustment.
The final adjustment is made with the primary screws.  If, for example,
the secondary shadow is leaning toward 12:00 in the view, then it can be
brought back to center by adjusting the 6:00 screw.  (The screws on my 
primary are oriented at 2:00, 6:00 and 10:00).

Like I said, this is what I do, but it's still pretty new to me.  It seems
to provide good results.  If anyone out there notices anything wrong, or
that could be improved, I'd appreciate your comments.  Thanks.

- KenB



> -----Original Message-----
> From: Bradley Davy [mailto:bdavy@fred.net]
> Sent: Tuesday, July 06, 1999 11:25 AM
> To: atm@shore.net
> Subject: ATM Collimation Question
> 
> 
> 
> I've read several web pages, Mels included, searched the archives and
> not been able to answer this seemingly simple question about 
> collimating
> a Newtonian.
> 
> I finally had a clear night to star test my as yet uncoated 
> 7" f4.3.  It
> looks like there might be a slight turned edge not visible with the
> Ronchi test, but I can't be sure until I correct what appears to be a
> significant collimation problem.  In the severely defocused image, the
> secondary shadow is not centered, and the slightly defocused 
> image shows
> the bright center way off center.  Focused stars at high 
> power show coma
> that is not significantly reduced and points in the same direction at
> all points in the visible field, confirming a collimation problem.
> 
> Questions:
> 1)  To center the secondary shadow, do I first adjust the spider
> position in the OTA, the primary, or the secondary tilt?
> 
> 2)  To center the bright spot in the diffraction pattern, do I adjust
> the primary or secondary tilt?
> 
> Thanks.
> -- 
> Brad
> bdavy@fred.net               http://www.fred.net/bdavy/home.html
> bdavy@nox.cs.du.edu
>