UNDERSTANDING TELESCOPE DRIVE SYSTEMS SEP94 Alan Buckman B.Sc FRAS When purchasing a telescope, which is to be driven either manually or by motors, there are many factors affecting the decision and this can influence the type of telescope mounting and manufacturer to give you the best performance. The purpose of this article is to brief you of the factors and also what to ask for or specify if you are upgrading your system. Nowadays, manufacturers are making more extravagant claims for their own drive systems, and hopefully there is sufficient information here to pick out and understand the important points. TYPES OF MOUNTING A telescope can be mounted on an equatorial mount giving RA and DEC movements or on an alt-azimuth stand (pan and tilt). The equatorial mount is the easiest to use as it can follow the stars by turning one axis only - the polar axis (so called because it points to the pole star). There are many varieties of equatorial mounting such as German, English, Yoke, Fork etc and a good book will be needed to show the merits of each type. The alt-azimuth mount is used for cheapness and is typically found on Dobsonian mountings. The drawback of this type of mounting is that both axes need to be driven to follow the stars. It can even be driven with slow motions but is difficult to motorise. The drive rate to each axis changes depending on the altitude of the object. It is interesting to note that the largest professional telescopes are arranged this way but computers are used to work out the tracking rates. For photographic use the equatorial mounting is essential. The alt- azimuth design suffers from field rotation when a time exposure is attempted. TYPES OF PRIMARY DRIVE The primary drive is the final drive reduction to smoothly turn the telescope drive shafts to follow the movement of the stars. This is usually by means of a large toothed gear wheel and worm gear with a typical reduction ratio of 144:1 to 360:1. If this is on the RA axis then these ratios correspond to one revolution of the worm in 10 minutes to one revolution in 4 minutes to follow the stars at the correct rate. These worm shafts are called the 'slow motions' but on low cost mountings these may not even be present. Guiding the telescope is best done by controlling the slow motions and these are essential if the telescope is to be motorised in the future. The toothed wheel should ideally be as large a diameter as the main mirror with more teeth giving a better performance. It should be closely meshed with the worm and there should be very little end play in the worm shaft bearings. All these factors affect 'backlash' which is present in any geared system when the direction is changed. Periodic errors will also be present with a period of one revolution of the worm. This will show up as positive and negative corrections being required to a steady drive rate when following the stars. This is purely due to mechanical machining tolerances of the worm and will be reduced if the worm is a larger diameter. Very expensive electronic drive systems can compensate for most of this error - called Periodic Error Correction or 'PEC' in the literature. It is better though to buy a good quality worm wheel set and housing with very low periodic errors. The large gear wheel can be present as a 'sector plate' and worm in which case the telescope can be driven for about one to two hours before the sector plate needs to be reset. This type of drive is much cheaper than a complete toothed gear wheel of the same radius and will give the same performance. The sector plate usually has a large radius and hence a large number of teeth if it were a complete circle. However it is difficult to get the exact reduction ratio when it needs to be driven precisely. The best way is to measure the time for exactly 10 revolutions of the worm whilst following a star at about 50 degrees in altitude. The reduction ratio can then be determined. A tangent arm drive is also the equivalent of a very large gear wheel, the radius of which is the length of the arm. A typical arm would have a reduction ratio of 1400:1. The same comments apply as for the sector plate drive. A larger diameter gear wheel has another benefit - it allows a heavier telescope to be driven with the same amount of torque. The torque requirements are interesting and discussed later. Alternatively a motor with a smaller output power can be used. It is possible with some types of mount, such as horse-shoe, yoke or the English type for the primary drive wheel to be a metre diameter on the RA axis, and such a wheel can be driven by friction or a chain belt. It would not have a big reduction ratio (under 100:1) but there can be no backlash at all if the mount is engineered correctly. The exotic mounts all tend to be home-made and the usual commercial drive arrangement would be by toothed gear wheel and worm set as described. Many Japanese mountings and equatorial heads use very small gear wheels (75mm diameter) with 144 teeth. Although not ideal they give satisfactory performance. BACKLASH Unless special steps are taken, all telescope drive systems suffer from backlash when the drive direction is changed. This is noticeable on declination when it may take several seconds for the motor to take up the slack in the gear train. The amount of backlash is purely a mechanical function of how closely the worm is coupled to the drive wheel and how much end play is present. An electric motor drive and gearbox will add backlash to the amount already present, but it is not serious if the worm ratio is large. The gearboxes used have a constant backlash amount of about 2ø and this translates into seconds of extra backlash shown in the graph below. This amount corresponds to reversing the RA motor, then driving forward at the sidereal rate. It will be a different time period if the motor is driven faster or slower. DRIVE RATE To follow the stars the telescope must be driven at the 'Sidereal rate' (1436.07 minutes per revolution). However this will produce appreciable errors when making long exposure photographs due to refraction in the Earth's atmosphere. This causes the apparent places of stars to be shifted towards the zenith. The graph below shows the effect of refraction on the rotation period for stars at different declinations. A better approximation than sidereal rate is to drive at the 'King rate' which is slightly slower than sidereal (1436.47 minutes per rev). For more information on this subject consult an article in Sky & Telescope November 1989. The normal RA drive rate corresponds to 15 arcsec per second. A handset that is marked at 2x (or 100%) will drive at 30 arcsec per second in FAST and stop the motor in SLOW when the corrector button is pressed. This is a relative motion of +/-15 arcsec per second compared to the stars. A fine setting would adjust the rate by about 30% (+/-5 arcsec per second) and this is ideal when guiding for a photograph. Note that the RA motor is still being driven in the same direction when FAST or SLOW is pressed in this instance and so there is no backlash problem. Ideally the Declination adjust should also move the telescope at the same rates so that when both RA and DEC adjust are pressed together the star should go at 45ø. This means that it may be necessary to have different motors / gearbox and drive frequencies to the two axes depending on the telescope reduction arrangements. A motorised system using stepper motors should be designed to provide an RA rate of about 20 steps per second to follow the stars. Slower than this there is a danger of blurring the image due to vibration and the larger step size required. ALIGNMENT To get the best out of your mounting it needs to be aligned correctly. The optical axis needs to be co-incident with the rotation axis of the telescope; the two drive axes also need to be perpendicular and it needs to be adjusted carefully (if equatorial type) so that the polar axis points to the exact rotation axis in the heavens. Please consult other books to find the correct procedure for accurate alignment. Only by carrying out the alignment procedure properly will you be able to follow the stars without adjustments being made to the declination axis. Then you should be able to take unguided photographs with the drive turned on for 15 minutes. TORQUE REQUIREMENTS The torque required to turn the slow motion shaft is to overcome friction and to move the weight of the telescope around the axis. Once the weight is rotating it takes some stopping as it has appreciable inertia. The inertial mass also means that extra torque is required from the motor to accelerate the load and so turn the telescope faster. When attachments are added to the telescope, such as a camera or heavy eyepiece, as well as putting the telescope out of balance it also increases the inertial mass. If the telescope is not balanced then gravity acts on the out of balance weight to produce an additional torque which adds or subtracts to the total torque meaning that the total load may be too much for the motor. This effect shows up if the motor can drive in one direction but stalls when driven in the other direction. If the telescope has slow motions then this will have been noticeable. The torque required to turn an axis can be measured by the following procedure. Arrange to get the axis horizontal with the full load attached, wrap string around the shaft and attach a weight to the end of the string. Add weight until the shaft just rotates. The torque is the weight times the radius of the shaft. Torque requirements for small telescopes are very low, larger telescopes on very heavy mountings could require 50Nm of torque. MOTOR DRIVES There are basically three types of motor drive, Synchronous, DC or Stepper and these are described below. The motor must supply enough power to turn and accelerate the telescope but must also provide a good sidereal (King) rate, and ideally a means of adjustment from a handset. The accuracy required in the drive rate is dependant on the use to which you are putting the telescope. For visual observing an error of 1 arcminute in 15 minutes of time may not be serious. This corresponds to about 0.5% error. For photographic use at the focal plane of the telescope may require 1 arcsecond over 15 minutes of following. This is less than 0.01% error and can only be achieved with a quartz crystal oscillator. All motors will drive the slow motion shaft through a gearbox. The function of this is to multiply the small torque available from the motor to make it useable to drive systems requiring appreciable torque. STEPPER MOTOR The stepper motor is a low voltage drive system and is easy to drive. It can be driven down to zero steps per second, reversed, and up to a high speed in the order of 300 steps per second using simple drive electronics. Other stepper motor types can be driven to 10000 steps per second using more sophisticated drive electronics. Stepper motors are the usual motors on nearly all drive systems. The torque characteristic of a small stepper motor driven in a certain way is shown below. It is possible to alter the drive electronics to produce a completely different curve. SYNCHRONOUS MOTOR The synchronous motor is designed to be driven by mains 240V 50Hz voltages and will track the mains frequency in synchronism. There are several drawbacks to this motor: - Mains at the telescope at night in a damp atmosphere is dangerous. An isolating transformer and RCCB circuit breaker should be used. - The mains frequency cannot be varied unless an invertor is used to generate 240V ac from a car battery. - Even with an invertor it is not possible to vary the frequency supplied to the motor much beyond the range 40 to 80 Hz as it loses torque. - It is only suitable for certain telescope reduction ratios (144:1 is one of them). This type of motor is usually supplied on the lower cost commercial systems. DC MOTOR A DC motor system has higher torque than stepper motors and can run at faster speeds. However, it is more difficult to provide control for driving at a known steady rate and a servo must be attached. Its use is rare and is only known on the MEADE LX200 telescope systems. HIGH SPEED DRIVES If the torque characteristic of the motor / electronics is flat to very high step rates then it is possible to drive the telescope very fast. It is not possible to move the telescope from rest immediately to the high step rates as the telescope has inertia and must be accelerated. The electronic drive control should take about 0.5 minutes to reach full speed. Conversely it also cannot be stopped immediately. If the system has been designed correctly the sidereal rate will be about 20 steps per second. To reach 1ø per second requires 240 times this rate or 4800 steps per second. This rate is about right for following the telescope through the finderscope. Higher slew rates than this are a gimmick. The objects to be observed in an observing session should be planned and should all be near culmination to give the best views and so will not be at great distances from each other. ---oooOooo---