Re: ATM A cage as secondary holder

[iss@pvtnetworks.net]

Mel writes:

>I see no way around it: we must be able to move
>one side of the altitude bearing up and down, and forward and
>backwards. 

Moving the bearings fwd/backward would have the effect of rotating the OTA
~around the OA, I'm not sure this is required unless you also want to adjust
the alt bearings to be concentric.

>In addition, the rocker needs to have one side where both bearing points
>are adjustable in terms of sliding them back and forth.  The upper end
>needs to be adjustable where the truss tubes attach, and the spider
>needs to be adjustable in its centering in the upper ring.  So, one
>collimates optically first, getting the focuser squared on and the
>diagonal and primary lined up.  Then the mechanical collimation is done
>to match the optical collimation.  This may have to be iterative if the
>optical collimation is not centered on the mechanical collimation,
>because adjusting the truss tubes and spider will necessitate optical
>realignment.

My seqence is to adjust the mount axes orthogonal first by adjusting the
height of the elevation rollers & using a dial gage to measure the heights
of the elev trunninons above the ground, rotating 180 in az inbetween.

The OA (after collimation)is set orthogonal to elev using the spin alinement
technique I developed and Chuck Shaw describes on his web page

<http://www.ghgcorp.com/cshaw/>

One of the trunnions has to be 
adjustable vertically to do this. A screw adjustment for this makes
it a *lot* easier to do. I have not found it neccessary to readjust
the mount axes when the alinement is done this way.

This procedure also works for eq fork mounts but GEM's require 
something different.

>I think a reasonable goal is one arcminute accuracy - if
>for no other reason that a host of other problems crop up when we move
>into the arcsecond realm.  Then, we can let software do its magic and
>take the raw arcminute accuracy and bring it down by an order of
>magnitude.  This coupled with high precision offsets from nearby known
>star positions should get us down to arcsecond accuracy.  Mission
>accomplished!

The home built (teflon/formica bearing) dobs I've aligned showed
wobble in the alt/az axes of several arc minutes, doing better than
0.1 deg would be difficult. The mounts I've built with ground
steel tracks can be alined to <<1 arc min, flexure becomes the
main problem rather than bearing quality. S/W compensation works
*great* as long as the errors are smooth and predictable.

>Dob builders are going to have to switch gears because precision work
>and adjustable setups are not a part of our consciousness quite yet, but
>we are getting there.

It's a dirty job but *someone* has to do it...:)





Andy Saulietis / ISS Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designs & Machine work
29 Serenity St  Mayhill NM 88339
505-687-3067 Voice
505-687-3021   Fax
e-mail: iss@pvtnetworks.net
32 54 13 N  105 31 44 W 7300' elev

"The journey of a thousand miles begins with a
broken fan belt and a leaky tire."