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Byers Mount Project
Update 08/31/06
We haven't posted an update on the Byers mount
lately, but work has been moving forward. A few more issues have
developed. We discovered that the RA gear was actually turning at 1/2
the sidereal rate, not great for tracking. The problem turned out to
be simple, but the solution caused a DEC problem. Other issues include
extremely slow guiding rates (partially a DEC motor problem and partially an
ALCOR issue), and there is a bit too much play in the RA and DEC assemblies,
complicating matters.
Most of these issues have been resolved now, and
we've had the scope over to CE a couple of times for viewing sessions.
We've used the mount to test both of the chapter's freshly refigured and
coated mirrors and the truss tube is still in the design stages.
We've invested a tremendous amount of time and effort in this mount, but we
are on the verge of creating a nice system for chapter members to use.
So, we are still on course and there is light at the end of the tunnel!
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The first issue to solve was the RA tracking rate. When the mount
was first tested under the stars, it didn't track well. We
primarily used low powers, so it wasn't a major problem but we did
notice a pronounced RA drift.
So after getting the mount back to the
garage, I marked the RA worm wheel and timed the rotation of both the
worm gear and worm wheel. I also counted the number of teeth on
the worm wheel and calculated the gear ratio from the 1 rpm motor to the
worm gear.
The results were interesting. The rate
was just under 48 hours. Either this was an alien mount from a
planet that rotates once on its axis every 47h 52m, or the 1 rpm motor
should have been a 2 rpm.
Assuming the latter, I realized that the DEC motor originally installed
on the mount was a 2 rpm. So I removed both motors, unsoldered
them from their wiring harnesses and installed the 2 rpm on the RA
assembly and the 1 rpm on the DEC assembly.
A quick garage test looked favorable, and a test of the mount under the
stars revealed that the mount tracked perfectly with the 2 rpm driving
the RA gear train.
Unfortunately though, the DEC guide rates were extremely slow.
Pressing the north or south buttons seemed fruitless. I verified
that the motor was turning, but no movement of the field of view.
The same occurred in RA, though I knew the rates would be very slow
since the ALCOR circuit was only increasing and reducing the RA rate by
about 1/5th sidereal.
This was actually unacceptable. We needed a fairly fast rate for
centering in both RA and DEC, but not so fast that the mount couldn't be
used for guiding during astrophotography.
So the problem was attacked from 2 directions. The ALCOR rates
needed to be increased and the DEC motor had to be changed. The
ideal rates would be + or - 1x sidereal on both RA and DEC.
So Michael Covington was contacted (Michael designed and donated the
ALCOR motor control circuit) and after a couple of tries we had new
firmware in our scope that would track 2x sidereal to the west, and
motors off to the east with the 2 rpm motor. This essentially
gives a 1x sidereal rate for guiding and centering in RA. Slow but
workable. Michael indicated that this is probably the best we can
do with synchronous motors. The next step would be to go with
stepper motors or servos, but for the money the synchronous motors
should do the trick.
The DEC issue though would need a new motor. After more timing,
gear ratio calculating and tooth counting, it was discovered that with a
1 rpm motor on the DEC assembly, the guiding rate is 1/4th sidereal, or
about what the ALCOR was giving us before the firmware changes. To
get the DEC up to speed would require a 4 rpm motor.
As you may recall, we replaced the original motors with new ones from
Hurst, so although we didn't have a 4 rpm motor, the old 2 rpm motor was
available. This would give us a 1/2 sidereal guide rate in DEC to
try until we can raise the funds for a new 4 rpm motor.
The 2 rpm motor worked well, though slow. Upon further inspection
it was discovered that both the RA and DEC assemblies have a
considerable amount of "play". With guide/centering rates this
slow, this could cause problems.
The source of the play in each assembly appears to be in how the spring
loaded worm assemblies are designed. We are currently designing an
anti play attachment that promises to eliminate most of it, while
allowing the spring mechanism to continue to function.
The 4 rpm DEC motor still needs to be purchased at this point. We
currently have the funds and anticipate purchasing the motor shortly.
With these last 2 problems solved, the mount should be available for
service. We'll have a party! (Star party)
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