Monthly Archives: September 2009


Bob, Glen and Rusty met at the observatory for regular maintenance of the 24″ scope — for both the drive system and the optical alignment.

Drive System

In order for the drive system to work properly, it is necessary for the scope to be balanced.


Declination balance:

  1. We backed off the locking screws on the worm gear (wheel) so that the shaft was effectively disengaged from the worm gear mechanism.
  2. We then tested for balance dynamically — that is, get the scope moving in one direction and check the stopping time and whether it ‘bounces back’ with motion in the other direction.
  3. We then adjusted the trimmer on the secondary axis to achieve balance, and reattached the worm wheel to the shaft.

RA balance:

  1. We disengaged the worm drive by loosening the three 1/4-20 bolts holding the worm in, and moving out the latter.
  2. We then tested for balance as before, this time on the RA axis.
  3. We adjusted the large trimmer (on the Dec axis) to achieve dynamic balance as before.
  4. We then carefully and firmly pushed in the RA worm assembly (against the tensioning springs) and tightened the 1/4-20 bolts. [Care was taken to make sure that the teeth of the worm wheel did not scrape on the stationary block at each end of the worm.]

Drive Motors:

  1. Glen checked the operation of both drive motors in a variety of telescope positions, all over the sky.
  2. With the Dec drive, it was found that during some down movements, the system would oscillate, galloping fast and then slow. The period of oscillation was around 1 second. During this oscillation, the spring-loaded worm assembly would move in and out against the tensioning screws. Although the tensioning screws are intended to act as a safety valve, and no damage seems to be occurring during these oscillations, it is thought that the latter are undesirable as it might place excessive stress on the drive gear-motors and their internal gears. (Anyway, it sound bad.)
  3. Accordingly, adjustments were made to attempt to eliminate the oscillations. It was found that the oscillations were eliminated by making the telescope slightly off balance in declination in the sense that the top of the telescope tube would want to ascend if the locking screws were backed out. (In other words, the oscillations ceased if the telescope were ‘going downhill’ as it were.) No oscillations were ever detected in the other direction.
  4. In addition, it was found that backing off the tensioning screws (thereby relaxing the tensioning springs) seemed to help the situation.
  5. With the RA drive, some small oscillations were detected in westward direction. This time, a slight (i.e., smaller than for the Dec axis) imbalance solved the problem. This imbalance was in the sense that the telescope would move west if the worm were disengaged.

The members present can think of no explanation for the asymmetry in these symptoms
(i.e., why oscillations would occur in one direction but not the other — unless it had to do
with the tension on the backing plate … too complex to explain here).

Anyway, the drive system is working fine now.

Optical Alignment

Next, we examined the optical alignment. When the Hotech laser collimator (privately owned by Bob) was placed in the top focusser, the beam hit the centre of the secondary perfectly. Only a minor adjustment of the secondary adjustment screws was necessary to make the reflected beam line up with the incident beam.

As the sky was not clear, we were unable to check the alignment of the primary mirror.

In conclusion, the 24″ scope should be ready to go. It is unlikely that the primary needs to be adjusted, but on the first available clear night, its alignment will be checked on a bright star.

Respectfully submitted,
Bob Nelson


Wednesday evening, Gil and I remounted the refurbished RA drive assembly back on the telescope. Although I can’t seem to seat it the way I want, and the springs are not fully operational, the drive seems to work satisfactorily. I’ll take another look at it next time I’m out there (taking my digital calipers with me) and see if I can make it better. I’m off to Vancouver for the weekend, back Sunday night, I believe.