Speaking of mounts...

"Thad Floryan" wrote in message
om...
(Jon Isaacs) wrote in message
...
I stumbled across this very interesting page on Alt-Alt-Az
mounts. The idea is a dob-style mount that does away with eq
platforms and field derotation.

I wonder how long it would take to actually align the mount
accurately enough to achieve reasonable accuracy necessary to image.

Looks to me like the motion is really quite complex and that while
in concept it may be interesting, small alignment errors and
mechanical imprecision would
make it very difficult to actually use in the real world.

Why is that? You've been around Dobs too long?

In the real world, mass-produced and inexpensive stuff with precision
greater than 10 arcseconds is readily and commonly available.

As just one for-instance (a supplier from whom I've equipped a good
portion of
my home machine shop):

http://www.grizzly.com/catalog/2004/475.cfm?

referencing the rotary table(s). Sub-arcsecond accuracy is also
readily available (just not at a price I'm willing to pay :-)
and commonly found which
is why I've no respect for slipshod astro junk from the likes of,
say, Synta.
Beware.
Don't confuse 'precison', with 'accuracy'. They are two distinct ratings.
Precision, is used in engineering sales, for a measure that is essentially
'repeatable' to this level of alignment, but _not_ for an absolute
'measure'. A rotary table with 1 arc second 'precision', _does not_
guarantee, that if you turn it through the number of turns on the handle,
corresponding to 10 degrees, that the output shaft will actually have
turned through 10 degrees. The error in the figure here, is the 'accuracy'
of the system.
The engineering company you quote, is at times misusing the two terms, and
(for instance), the fact that the inclinable rotary table only guarantees
concentricity, to within 0.0008", implies an inherent angular pointing
error, of up to 0.6 arc minutes, ignoring any other surface errors on the
gears. Even quite cheap scopes require _accuracy_ to a level that is
significantly above such machine tools. The 'Synta' components you are so
sconful of, have _accuracies_, that are as good as the engineering tooling
you are pointing to...

If you haven't visited a modern machine shop in the past several
decades you'd be astounded at what can be easily and inexpensively
performed today. There is absolutely NO reason for mechanical
slop in any astro gear we buy except for
sheer laziness and/or incompetence and/or a "who cares?" attitude.
Incorrect. Try specifying finished _precision_, rather than accuracy to
the level required for a good scope mount, and even with modern CNC
equipment, you will be adding a couple of zeros to the end of the price.
As for 'no reason for mechanical slop', getting rid of backlash
completely, requires doubling every gear and linkage in the system, and
then spring biasing these systems, with sufficient force to overcome every
likely torque loading, increasing the drive forces required, and
massiveley increasing the cost. Even a simple 'zero backlash' coupling,
typically costs more than most basic scope mounts...

Best Wishes