Re: [ATM] ATM Digest, Vol 31, Issue 3

[Bill Thomas <bthomas32000@yahoo.com>]

> > From: Scott Carson <scottcarson@austin.rr.com>
> To: ATM <atm@atmlist.net>
> Date: Sat, 01 Jul 2006 22:28:44 -0500
> Subject: [ATM] Small Dob Question: Alignment
> features needed for accurate
> 	encoder GOTO's?
> 
> 
> I'm building a relatively small 8 inch Dob (truss
> tube) to gain 
> experience and am planning to add encoders.  Hoping
> to get some design 
> advice / recommendations regarding mount "alignment"
> features to ensure 
> accurate encoder GOTOs.
> 

> For example, are the altitude bearings typically 
> accurate enough 
> (perpendicular to the azimuth axis) with careful
> building (approx. 1/32" 
> tolerance?) for successful encoder use?

The best way to have insight to the adjustments you
will need is to describe the one time setup of the
DOB?s GOTO:

On a level floor, setup the spider as you would as the
first step of the optically alignment i.e., center of
the top of DOB.  Clamp an arm (a stick) to a
stepladder (or something else that will hold the arm
over the top of the DOB).   Move the arm over the top
of the spider.   At the center of the spider, mark the
arm. Rotate the azimuth bearing and note the wobble of
the center of the spider with respect to the marked
arm.  Adjust the altitude, and altitude bearings axis
(up/down ? that is the adjustment you will need) to
minimize the wobble.  You will also need to mount the
altitude optical encoder right on the altitude axis,
and the azimuth optical encoder at the center of the
azimuth bearing.  You may also have to fudge by moving
the spider center a bit off set with respect to the
center of the top of the DOB.

Now make some type of physical stops (e.g., marks,
bolt and hole, blocks) to be able return the DOB to
exactly to the altitude and altitude bearings axis
that you just found minimized the wobble.

>From then on (in the field) - align the altitude and
altitude bearings to the physical stops and indicate
to the GOTO electronics that the DOB is now setup
vertically. Then remove the physical stops. Locate the
two stars and make sure the alignment error is less
than the max allowed by your GOTO.




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