ATM More Collimation...HELP !

[cab <carl@pader-online.de>]

ATMers,

this is my second time typing this in. Last night after I performed a star
test I was eager to get it all written down so as not to forget anything. In
my rush (just when I was finished of course) I pressed the wrong key and
deleted everything. I was too tired to type it all in again so went to bed
in a foul mood, compounded by the fact that my star test was less than
stellar (for want of a better word). Here goes again....

First of all, many thanks to all who responded to my earlier requests for
help in collimation/star testing. I procured some higher powered eyepieces
(with my 50x I just wasnīt seeing any diffraction rings) Wow !! what  a
difference high magnification makes !! The diffraction rings were there
clearly to be seen. Now I was getting places. I spent a lot of time star
testing on real stars (took me some time to get used to the high
magnification as well) Turbulance was a problem throughout the testing.
There were very few nights of even reasonable seeing conditions. But I had
gotten a feel for the star test.

Two day ago I decided I would construct an artifical star and try and star
test with that (btw. the scope is a 10inch f4.5). I was hoping for less
turbulent conditions and also avoid the need to constantly track the star at
high power thereby being able to concentrate more on the star test. So I set
it up last night and tested.

First of all, the results were the same as the īrealī star test. At least I
had set up the artificial source correctly. As the image wasnīt constantly
drifting out of view I could relax and concentrate more on testing. Hereīs
what I saw....

Outside of focus the diffraction rings were clearly delineated. I tweaked
the adjustment screws on the back of the mirror and finally got all the
diffraction rings nicely centered (using my highest magnification x300 + ).
The outer ring was the brightest ring and wider that the rest. The outer
edge of this ring was slightly jagged.Then came a (relatively) dark band and
then 3 or 4 more rings. Each of these rings were separated by dark bands
which were not as dark as the first dark band (moving from outer to inner).
Then came the central abstruction (diagonal) and its associated finer rings
which surrounded a central point. It looked promising to my uneducated eye.

Inside of focus - well what a different story. It was a mess. It was a blur,
but behind this blur I could (barely) make out the diffraction rings which
were similiar to those outside of focus. It looked like turbulance was
having a party in the tube ! I snapped back to outside of focus - relatively
steady. Back to inside of focus- the same mess. It was as if the clean
picture I had outside of focus had been somehow badly smeared. I couldnīt
understand it. I still canīt. Radiating outwards in all directions were
īspikesī to add to the confusion. And to add even more confusion the central
part of the image was off centre - contrary to what I saw outside of focus.
How could that be ??? Looking again through Dick Suiters book as I lay in
bed last night I was unable to find anything that approached what I had
seen. Thereīs a fair amount of head scratching been done today !

Have I made a fundamental error somewhere or do I have a turkey ??

Ideas anyone ???

Carl Brennan
Paderborn,Germany
carl@pader-online.de