[ATM] Collimation by concentric circles,

[artbianconi@blast.net]

5. ATM: Collimation by concentric circles, pr. 2 (David Harbour) 
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David wrote: The FIRST step is to square on the primary with the 
secondary. 



With an 8 foot tube (12.5" f-8) to contend with an being new to the 
game at that time, collimation was a trial and error (mostly error).
First I simply removed the secondary from the guide tube in the 
spider and naively thought "I'll 'bore-sight' it like we do with 
rifles in competition. I'll simply sight down the guide tube"

Close but no cigar. 

So I chucked a 6" long piece of 1"OD soft aluminum barstock in the 
small lathe and, using a long drill, carefully ran a 1/8 inch hole 
the length of the part. I then miked the OD of a laser pointer, and, 
with suitable letter drill, bored a slightly snug hole right down the 
center 3 inches deep.

I then turned the piece around and turned the other end down to snug 
fit in the guide tube of the spider.

I slid a laser pointer inside the device with the the light coming 
out the narrow end, mounted it in a suitable collet and into the 
business end of a milling machine to test it for concentricity. If 
the axis of the laser was co-axial with the center axis of the 
aluminum adapter, the light would stay in one spot. If not, it would 
describe a small circle. I lucked out!

Now after an overhaul or serious maintenance of the OTA, I adjust the 
screws in the primary cell so the mirror is square to the cell. 

I insert my laser pointer and adapter in the secondary guide tube and 
adjust the spiders until the red dot is in the exact center of the 
primary. Out comes the laser and adapter, in goes the secondary 
mirror and I then revert to the Orion laser collimator and the more 
conventional methods to fine tune everything.

If you don't have  a milling machine to test it with,  you can roll 
the laser pointer on a flat surface and watch the dot. It should move 
in a straight line.

Art Bianconi


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