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MAINTENANCE of SCOPES |
Problems can manifest in several ways:
DC Motors - with two wire connection - can burn out if they stall if they are not driven by a current limited circuit. Most Meade, Celestron and modern Vixen scopes have these types of motors fitted because they offer large torque and can rotate at a high rate of knotts. The designers problem is getting a steady slow running speed and so they all have encoder wheels to provide about 5 pulses per second of feedback when going at sidereal rate. All the scopes I have examined involving small DC motors also have fairly flimsy mechanics and gearboxes and can suffer from an uneven torque requirement over one revolution. This makes the motor very uneven in its speed and the servo control loop can make the instantaneous speed between zero and twice sidereal rate. The average is right but not good for smooth following. The rotation can be observed by drawing a line across the end of a shaft that rotates, then you can see how smoothly it rotates,
A general check on motor operation is to swap over the electronic drive cables to the two motors. The fault transfers if it is in the drivebox or stays where it is if it is the motor. Certain mounts do not have this flexibility - all the cables are internal and not possible to swap them over.
GEARS
When the motor is removed you can rotate the shaft by hand. This is an extremely sensitive test for torque variations in the rest of the gear train. If there are tight spots these need investigating. All unevenness will result in wobbles in following stars.
Worm and wheel sets are the normal gears in most telescopes mounts. Things to watch out for are the closeness of the fit of the worm to the wheel (backlash) and the worm shaft able to shift along its length (endfloat which causes backlash). If you grab the telescope and rock it backwards and forwards then you may be able to see or feel any movement in the wormwheel or worm axle. These need to be fixed. In normal use the forces on these components are considerable.
A common problem with endfloat is a lack of thrust bearings or washers at the ends of the slow motion shaft. If there is a roller bearing secured in place with a grub screw then in normal use the roller bearing will get pushed outwards, the forces are that much. The only solution is to add an end plate onto the worm bracket (both ends) with washers to bear down onto the roller bearing. An example is required - if the endfloat is 10 thou (0.25mm) and the wheel is 6 inch diameter then the endfloat movement corresponds to 11.5 arc minutes in the sky. The plates area arranged so their mounting screws control the endfloat and so it is adjusted until the rotational force just increases. Feel with the fingers to get the right condition.
The fit of the worm against the wheel is also critical. There should be fine adjustments to gradually bring it in closer. Again the best position for it is when the rotational force on the slow motion axle increases. After the adjustment has been made a locking mechanism should be used. This can be by the provision of grub screws locking the adjusting parts together.
Lubrication - A common problem with grease lubrication is that it can pick up dust or grit, or in extreme cases metal swarf. I have seen all of these, even on freshly manufactured mounts. The effect is to cause sudden binding or grinding down of worm teeth. The only cure is to dismantle and thoroughly de-grease. Worm and wheel sets do need lubrication otherwise the torque required to overcome the friction can be considerable and enough to stall the motor at low speeds. A Teflon spray can be used for open gears or a Lithium based grease for enclosed worm wheels.
Gearboxes used on telescopes tend to be run at their maximum torque limits and so they exhibit abnormal wear in their internal bearings quite quickly. Cogs can also loose teeth, so jamming up the gearbox completely. The Russian TAL telescopes have a metal geared gearbox but the shaft bearings are all nylon and after a time these get very sloppy resulting in increased backlash out of the gearbox. We can repair such items.
Loud clicking noises coming from exnclosed worm wheel or spur gear sets when operated at slew speed need to be investigated at once. It could be that there is so much backlash that the gears are nearly dis-engaged and at high speed they become disengaged. This can cause horrendous damage to the shape ofthe worm wheel teeth, especially if this component is made of brass.
Meade equipemnt has a particular problem with the sprung loaded worms. In one direction the worm is actually forced out of mesh with the wheel when it turns in one direction only. Turning in the other direction it screws into the wheel. The problem causes a gradual movement in that axis over a period of 30 seconds when the motor has stopped. The spring forces the worm gradually into the wheel so causing the axis to rotate about 1/4 degree! There is a mechanical stop to prevent the worm coming all the way out but all these adjustments are prone to wander so they may need attention from time to time.
MOUNTING BRACKETS
Motors, worms and gears need to be held in precise positions even when the yare driving the telescope round. The forces on the telescope wheel can be about 200 Newton-metre. Motors can generate between 0.5 and 2 Newton-metre. 2Nm is the rotational force achieved when a weight of 20kg is hanging off a string wrapped round a 2cm diameter horizontal shaft.
With these forces involved it is apparent that brackets need to be made of substantial material otherwise they will flex. Items which are also bolted together can move, all upsetting backlash adjustments. If your mount suffers from these effects then the metal work needs to be strengthened and bracket parts need to be locked together by the use of grub screws.
Finally the mechanical arrangement should be tested. An ideal device to do this is the AWR SEEKER which plugs into the autoguide socket of nearly all drive systems. This plots the motion of a star with a given fixed angular movement in all four directions. The variations from a true matrix of points can be analysed to determine periodic errors and backlash components in both axes.
SAFETY ISSUES
Mechanical components need a safety review. Motors and gearboxes generate considerable torque and if there are no safety guards then clothing can get trapped. The motors will not stop rotating so BEWARE! If there are no safety guards then see if you can fit some. Although a worm wheel may be rotating ever so slowly it can cause huge problems. If gueards cannot be fitted then consider emergency cutout switches conveniently placed around the telescope to cut the power.