RE: ATM 10" Problems

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

Hello Dale,
 
It sounds to me like you need to become more certain of 1) the collimation
of the scope and 2) 
the quality of the primary.  If you can get a copy of Suiter's book on star
testing, it will help
you very much in evaluating both.  There was a thread just a month or so ago
on the list
concerning collimation.  I don't have all the messages anymore, but here's
the one I submitted:
 
<snip> 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).

<snip>

- KenB

-----Original Message-----
From: Dale Schmitz [mailto:aetheria@concentric.net]
Sent: Friday, July 30, 1999 1:44 PM
To: ATM List
Subject: ATM 10" Problems
 
<snip>
 

Alignment of the mirrors was done with a collimation tool from Orion
Optical.  Looking through the peephole you see crosshairs over a ring of
light reflected in from the side.  Manual adjustment lines up the
crosshairs, light ring, and both mirrors as good as can be expected
(everything appears to be in line, right down the center).  Also, higher
magnifications turn everything distinctly yellowish.
 
 <snip>
 
 Is there further alignment that needs to be done?  Is there something I
overlooked?  I've read and reread Texereau's book and don't think I've
missed anything (I could be wrong).  Is this as good a performance that I
can expect out of my 10"?