Re: [ATM] squaring truss tubes and collimation

[Daniel Fundo <fundo@bellsouth.net>]

<<   Here's something I've never seen discussed:  the importance of 
making sure your truss tube telescope components are square.....>>

Having a square, parallel assembly is a sign of good workmanship.  The
only thing that is absolutely necessary is that the optical axis is
aligned. You can do this with a Whoscope (re: Dr, Seuss) if you like. 

The mirror can be wedged, the secondary can be at an angle other than 45
degrees.  As long as the surface of the mirror is "square" to the
optical axis, and as long as the focuser operates along the secondary
optical axis, the telescope will work.  

As long as the truss poles are of equal length, the static alignment of
the components will always be the same when reassembled.

That said........the rotating secondary cage requires that the plane of
rotation of the cage is perpendicular to the primary optical axis.....to
a rather precise 90 degrees.  Otherwise, alignment will disappear with
any change, even the slightest rotation.  For a Dobsonian mounting,
there will be binding if the altitude bearings on the scope aren't on
the same axis.  It would help if the (alt) bearings on the rocker box
are parallel, square and at the same height too, but not necessary.  As
long as you stick to 3 pads and a flat bearing plate on the bottom of
the rocker box, it will work in azimuth.

There was a beautiful piece of craftsmanship at the Stellafane meeting
in 1970 or 72.  It was an all aluminum telescope on a fork mounting. 
The secondary cage could be rotated without the object drifting off
center.  This was demonstrated one night at something like 150x!  There
were slow motion and slewing controls and a two speed focuser that could
bring an object into focus even at 600x.  Dampening on the whole
assembly was phoenominal.

As has been pointed out on this list a number of times, a telescope
doesn't have to be aesthetically beautiful, or have engineering
perfection, to be functional.  That is part of the beauty of the
Dobsonian mounting.  It is inherently stable, and doesn't need a high
degree of accuracy to work acceptably well.

*****-----
Dan F.
Former Secretary/MAS
Memphis, TN
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