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MAINTENANCE of SCOPES


MECHANICS
In order to keep the scope tracking at optimum performance it is necessary to inspect and adjust mechanical alignments periodically. GOTO scopes have the additional requirements of acquiring objects, so the pointing accuracy needs to be repeatable and any mechanical mis-alignments need to be fixed in order to be calibrated out or reduced.

Problems can manifest in several ways:

MOTORS
Nothing much can go wrong with motors! The sounds they make can indicate electrical or mechanical problems. A stepper motor can 'rustle' at slow speeds. This is a normal for a chopper mode current source drive electronics. A loud buzzing sound at high speed actually means the motor has stalled. A stepper motor is not harmed by such behaviour but the vibration and sound can be very loud. This only indicates that the load is too heavy and could be caused by an out of balance telescope or the gear reductions requiring more torque than when it was set up. A buzzing sound from a stepper motor at low speed can indicate that one phase is missing. There is generally a loss of power and the motor may go in unpredictable directions. Check the wiring - plugs and connectors are the usual problems. Occasionally the drivebox may have blown up. These faults can usually be repaired (by us).

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.

ELECTRONICS
FRAGILITY OF EQUIPMENT
In my experience there are many commercial systems that suffer from fragility, that is they refuse to work when the user has done something wrong, like connecting up a power supply the wrong way round. Come On! In the dark Astronomers will do anything and everything!!! If it is possible to do something wrong then it will be done sometime. It is an unfortunate fact that equipment designed for astronomers continues to suffer from bad design - I have seen it all, and tried to repair said units. It is not too difficult to make diode protection for low operating current units. Power supplies CAN be made so that they won't destroy the equipment. Even if the manufacturer fails to do it YOU can do it by suitable choice of connectors.

Conecting signal leads when powered. This is another common failing of commercial electronics! Equipment can and does break if serial leads are plugged or unplugged when the units are powered. It does not have to be like this! The interface can be designed so that HOT CONNECTING is possible. Unfortunately there is nothing you can do about this, except to ensure that leads do not come out when they are not meant to. AWR Technology equipment does not suffer from this defect. The alternative is blown up electronics and an unusable telescope.

CONNECTORS
Connectors are prone to problems, expecially when you need to keep undoing them. Pulling on wires or flexing can result in cables breaking by pulling out of the connector housings. The sheath of a cable should always be secured by clamps otherwise pulling forces are exerted on the soldered joints which can then fail.

The small telephone / modem style connectors (RJ12, RJ45 to those in the trade) are very fragile. The wires make connection by an insulation displacement style of contact and it is extremely easy for the wires to pull out. The only real solution is to replace the complete cable. Luckily they are very cheap and easily available. The RJ45 style is an 8 way cable commonly called a 'patch cord' used in local areas networks (computers). RJ12 style is a 6 way connector which may have a 6 way cable or a 4 way cable inserted into the contacts. These cables are commonly seen on cheap motor drive units (Synta equipment) right up to Meade LX200 telescopes.

Periodically connectors and cables should be inspected for damage or evidence of being pulled. Corrective action should be taken to stop gradual degredation and then catastrophic failure.

CABLES
Cables can be nicked, crushed, pulled and cut completely in two! Obviously you need to look after cables!. If the cable does not need to move then make sure it is secured, or run it within trunking which gives a much better mechanical solution. Cable ties can be placed every six inches.

Sometimes faults can be caused by cross-talk between cables if they run appreciable distances tied together in the same bundle. Wires carrying mains should be routed separately from signal wires. The problem comes with transients. The mains supply can carry 1000 Volt transients (thunder storms) and with a small amount of capacitive coupling to a signal cable there can be enough volts induced to destroy the signal circuits, even if the circuits are unpowered. Poorly designed handsets can cause the other motor to give a kick to the one that you have pressed the button for. This is also cross-talk. This type of fault can be cured by means of small decoupling capacitors on the signal wires down to the common zero volt or return wire.

ELECTRICAL INTERFERENCE
We all know the effects of interference - an unsuppressed motor can cause crackles on the Radio and break up the picture on a Television Set. It may be that your equipment is fairly immune to electrical radiation of this type but it can bother other people. Nowadays the regulations in Europe are reasonably stringent in that all equipment sold must have passed a CE test for Electromagnetic Compatibility. We have found that if the equipment radiates badly it is also likely to be more susceptable to this and other forms of electrical damage, such as Electro Static Discharge. A poorly designed unit can be quite difficult in these respects.

Improving the immunity to interference is a matter of separating cables, adding in decoupling capacitors and perhaps tackling the source of the electrical noise. We have had cross-talk from high power motor drives affecting a CCD camera when taking pictures. This showed up as flecks arranged in a regular pattern across the frame. One way of keeping RF noise in is to add an RF choke to the offending wires coming out of the unit, The component used is a ferrite inductor which is clamped along the cable - this adds a higher resistance to high frequencies. The other solution is to use screened cable for the sensitive wires and make sure the metal components of the sensitive equipment are earthed. If the source of the RF can be short circuited by decoupling capacitors, but the earth at that point must be good, or it will just find another path to follow. In practise anything and everything is tried to reduce the interference as much as possible.

DC motors are another cause for interference and small suppression capacitors are required, soldered at the motor, to reduce this interference. It shows up as white lines on TV's or crackle on radios.

OTHER
Odd motion problems can be tracked down to the handset on certain designs. We have seen an interaction between the axes so that when one direction button is pushed the mount actually blips the motor on the other axis as well. This was shown up by using the SEEKER. This was not a cross-talk problem but a software problem and the only solution was a complete Factory Reset of the handset. It is a fact that popular ranges of telescopes can and do suffer from bugs like this from time to time which can only be shown up by using the proper tools.

As mentionned previously, electronic drive circuit faults can make motors buzz strangely. Be aware of the normal sounds your system makes when operating through all its normal modes. Then when it changes you will know that something is wrong. It is best to examine whatever it is at that time rather than waiting for it to develop into something more serious. It could be indicitave of mechanical or electrical problems.

SAFETY ISSUES
If your observatory has mains laid in then periodic safety checks should be carried out to make sure that the mains wiring is still safe. As well as the obvious like checking the insulation and the armouring of the extension cable is still intact; the circuit must be fitted with breakers which are tested periodically. All the mains wiring in the observatory needs checking. Mice have been known to do strange things!

An RCD type of breaker should be fitted back in the house to turn off the whole observatory circuit in the event of a fault. This type of breaker measures the imbalance between the current going out on the LIVE wire and the current returning on the NEUTRAL wire. If it is out of balance by more than 15mA then it should trip (This is a 30mA RCD breaker). This will have a TEST and RESET button. The TEST button needs to be operated periodically to make sure it is still functioning.

In case of any uncertainty call in a qualified electrician.

OPTICS
CLEANING
ALIGNMENT


May 2005 AWR Technology 01304 365918 www.awrtech.co.uk