Re: ATM Another poor man collimating

[doug kniffen <dk@usmo.com>]

At 12:07 AM 10/5/00 +0200, you wrote:
>
>I'm not familiar with the Orion device. 

If you saw one you would recognize it. The unit is just a cheshire with a
cross haired sight tube stuck on the end.

>A laser is really great when you adjust the secondary. To adjust the
primary, however, it can't do the job like a good Cheshire can. And this
is, after all, the critical part. So, for the job a laser collimator *can*
do, you need no fantastic precision.
>
>Nils Olof
>
Here we go again; That is exactly the opposite of the way I see it. The
laser isn't that good for lining up the secondary unless the focuser has
been squared, the main benefit of a laser is more precise primary mirror
tilt - beyond what can be visually dectected with a star test.  I tried to
point this out with the sketch a while back. 

The other night I went to help a friend whose 'scope went airborne in a
wind storm. The instrument was an Orion Dob with the metal tube. Four feet
down, and six feet over, it landed on the focuser (tough 'scope!). The
focuser was fine but the tube was badly dented around the mounting
location. It was necessary also to remove the secondary and spider so that
we could hammer the tube. Shims were required to properly square the
focuser, but the pattern didn't seem to fit the damage so I wonder if the
focuser was ever properly squared before impact. After re-assembly, we used
his orion tool for indoor alignment and then took the repaired 'scope
outside for the star test using Polaris. Since this particular individual
never understood my ranting about collimation, we took the opportunity to
view a number of objects with a range of magnifications both before and
after tweaking the primary with a laser.  To sum up the results; below 20X
per inch there was no detectable difference, while at 40X there was a
significant difference in the quality of the image. 

Doug Kniffen