Re: ATM A cage as secondary holder

[Mel Bartels <mbartels@efn.org>]

I too have been doing some thinking on this subject - how to build a dob
mount that is adjustable optically (for collimation) and mechanically
(to align the mechical axis to the optical axis for more accurate
pointing and tracking).  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.  I took a couple of small wood dowel pieces, bolted them
together, and swung them on pivots, and watched the end of the dowel
that simulated the tube assembly make arcs, not lines, in the air.  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.  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!

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.

--
Clear skies, Mel Bartels // Programmer/Analyst, amateur astronomer
Springfield, Oregon,     // USA homepage: http://www.efn.org/~mbartels
mailto:mbartels@efn.org  // atm, atm-digest list-owner
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html