Re: ATM Design advice for tracking Dobs

[andydtg@phoenix.net]

Jim Lawrence wrote:

>2. I hope to use Mel Bartels' motorized dob design.  I am looking for
>other ways to interface the steppers to the altitude and azimuth
>bearings without using the gearboxes that Mel specifies.  Mel told me
>there are many other ways to do it and some builders have done so.  If
>anyone has, how did you do it?  I have been thinking of using timing or
>cog belts, but how do I calculate the wheel sizes?  Mel's design uses
>60:1 gear reduction, so I am assuming I would have to calculate the
>same.  Also, doesn't the 28 inch wheel that the motor turns constitute
>another reduction I have to account for?  Dan Gray thinks I will have to
>stick with gear reduction boxes.  Does anybody have any input on this? 
>I am assuming that I have to keep the same gear reduction ratios
>described by Mel so the scope will track properly.  I did not see any
>description In the documentation about whether the microsteps could be
>changed to allow for different gear ratios.  Maybe I missed it.  If it's
>there, can someone point it out to me?

I also build alt-az drive systems, mostly for people who don't have the
skill, resources, & time to do their own either with Mel's system or
with my components.

An important consideration is whether you plan to image with the system
or just use it visually. Imaging systems are a *lot* harder to build 
unless you use an autoguider. They also need focal plane derotation
for long exposures. Visual systems are easier because they do
not need the exteme precision or the fpr.

The reason for the large total gear ratios (hence gearboxes) is that
you want to keep the output step size (as seen thru the eyepiece) to
less than 1 arc sec to have the tracking appear smooth and continuous
rather than moving in visible steps. This ratio can be easily found
by calculating the arc sec / step at the motor, i.e. for a 200/step/rev
motor the step size is 1.8 deg or 6480 arc sec, so the gear ratio
needed is 6480:1 to get 1 arc sec/step, or 12960:1 for 1/2 arc sec etc.

This much reduction is difficult to obtain in a single stage so
if you have, say, a 30:1 roller for the final stage, you still need
216:1 between the roller ansd stepper. This can be had in a worm gear
or with gearboxes with multiple sets of spur gears.

You can use microstepping to reduce the initial step size & thus the
total reduction, but remember that microstepping does not improve
the precision of stepper motors, ie if the original 1.8 deg steps
are +/- 5% of a step repeatable or linear, the microsteps will not
improve this, each one could be off the 5% of 1.8 deg.

Some steppers, like the ones I use, have a gearbox built in, thus
simulating microstepping but with much better accuracy & linearity.
You give up the ability to rapidly slew by switching to full steps
with this type of stepper.

Drive types are several:

- worm gears (expensive & can have periodic errors)

- roller/disk drives, like Mel's and on my photo quality mounts.
  These if well made have virtually no periodic errors & are 
  very linear. Ratios up to 100:1 possible with *big* disks.
  Getting enough friction can be a problem.

- Roller chain drives, the big disk has a smooth metal surface &
  a small sprocket drives a roller chain. An idler sprocket
  provides chain tensioning and variable  friction so
  the scope can be moved independent of the drive. Very smooth
  but have long period periodic error due to how the chain
  meshes with the sprocket. I use these for visual drives, very
  strong & reliable. Ratios up to 30:1 or so.

- Belt/timing sprocket drives - potentially better than chain
  drives but can be pricey in larger sizes & won't deliver as
  much torque w/o stretching. Usually use a smooth disk (can be
  non metallic) and a small toothed drive sprocket. Ratios 
  up to 50:1 or so. 

The last 3 have to be calibrated for actual drive ratio, this is
fairly simple. Mark the big disk so you can tell when it's rotated 
by some exact known amount, like 360, 180, or 90 deg. Put a graduated
dial on the sprocket or roller & just count the # of turns & fraction
of a turn it takes to rotate the big disk by the known amount. For
360 deg the # of turns+fraction is the ratio, also do this in the
other direction & for multiple turns to evaluate the repeatability 
& improve the accuracy.

Hope this helps!


Andy Saulietis / DTG Alt-Az-Fp Drive Systems
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax