Description
The equatorial mount is constructed from aluminum except
for the gears and drive shafts. The Right Ascension (RA) and Declination
(DEC) axis drives are identical, 200 tooth, worm gear systems driven by
micro-stepping motors. The motors are open-loop steppers operating at
25,000 steps per revolution, giving .25 arc second per step resolution.
Each axis is driven through a slip clutch and the drive shaft is preloaded
against angular contact bearings for stable axial positioning. The slip
clutch is tightened with a bolt which protrudes through the drive box
cover. The RA drive is mounted on a large block, which pivots on a steel
shaft for latitude adjustment. Polar alignment of the mount is
accomplished by rotating the base and support column. The RA fork is a
solid piece of aluminum and is mounted to the drive shaft using a shrink
fit. The upper bearing of the RA shaft is installed before the shrink
fitting and is not removable. The telescope is mounted on the DEC axis
between the tines of the fork. Both axes are indexed using an optical
switch and metal encoding wheels. The indexing systems double as motion
limits.
The RA drive system and base of the equatorial fork
Maintenance
The gears and bearings require periodic lubrication.
Remove the gear box covers and wipe up contaminates and old grease inside
the box. Apply a good quality lithium or silicon grease, smearing a thin
layer on the worm wheel teeth and applying a thicker coating to the worm
gear. Force grease into the exposed faces of the bearings where possible.
Apply a thin oil to the shaft seal in the cover and reinstall.
The performance of the worm drives depends on the
tightness of the fit between the worm gear and worm wheel. If too tight,
the drive will be sluggish and may stall. If too loose, the backlash in
the system will be excessive causing the telescope to track poorly. The
backlash should be adjusted so there is less than .75 inch of wobble at
the rear of the telescope when pushing lightly on the body. In warm
weather the adjustment can be tighter than this without stalling the
drives. To adjust the backlash, loosen the ¼-20 cap screws that are
countersunk into the back of the drive box. It is not necessary to remove
the drive box cover but you may if you would like to better see what you
are doing. The space between the gears is changed by tightening or
loosening the set screws in the side of the box adjacent to the drive
motor. If loosening them, wiggle the motor a bit to assure that the motor
mount seats against the set screws. Test the backlash by wiggling the
scope and try slewing to check for stalling problems.
The DEC drive box. The backlash adjustment set screws
can be seen in the near wall.
The limit/index wheels can require adjustment,
particularly the DEC axis, to assure a proper operating range. Simply
loosen one of the locking screws in the edge of the ring and rotate to the
new position. Tighten and then rotate the scope carefully, by hand, to
check the fit of the wheel through the optical switch.
The limit wheel for the RA axis
The Dec axis limit wheel
Installation and Removal
IMPORTANT NOTE: The drive
systems cannot be disassembled with the telescope mounted. Only the drive
motors and couplings and the index wheels can be removed with the scope in
place.
Assembly and disassembly of the drives is straight
forward and should not require detailed explanation.
the telescope
Removing or installing the telescope is a simply
process, in principle, but the parts are heavy and awkward. It is best to
have three people with (2 with good backs) to handle this process. Rotate
the telescope over on its side with the drive side down. Remove the 8
bolts from the drive flange using an open end wrench. Steady the telescope
and loosen the 4 cap screws on the top of the fork arm opposite the drive
box. Lift the cap off the arm, the telescope is now free and can be lifted
out of the fork. The coaster shaft and bearing are left bolted to the side
of the scope during removal or installation. To install the scope, reverse
the procedure above.
The assembled system
Commercial Parts List
