1. DRIVE ACCURACY
For ensuring that the RA axis drives to the Sidereal rate we have two methods: a Quartz Crystal oscillator offering the exact frequency or a Variable Frequency oscillator.
The Quartz oscillator is essential for photographic work and with a well aligned equatorial mounting will allow unguided exposures in excess of 30 minutes. These systems are more expensive as they require a unique crystal but offer the best performance. The products are detailed on the QUARTZ DRIVES SHEET. SYSTEM 3,4 and high performance SYSTEM 5.
The lower cost Variable Frequency oscillator (VFO) is adjusted by a knob on the hand control. The required rate must be adjusted by trial and error. The products are detailed on the VFO DRIVES SHEET. SYSTEM 1,2.
2. MOTOR TYPES
There are basically three motor types in normal use. These are STEPPER or SYNCHRONOUS or DC.
Synchronous motors have two wires (three wires for reversible types) and require operation using 240V ac at 50Hz. These plug directly into the mains. The synchronous motor based products and options have an inverter unit to produce the required voltages. The frequency of these motors can be quartz locked or variable by 20% maximum and are thus suitable for fine control only. There is no extra charge to provide 120V 60Hz based systems. The power is sufficient to drive the two motors in the old Celestron orange tube models.
Stepper motors have at least 5 wires and are driven by low voltages and are consequently safer in operation. They are suitable for fast movements, up to 50x sidereal rate depending on the version, without great expense. They also possess much higher torques for heavy telescopes. We offer 3 types for different loads.
These two motor types can be used on QUARTZ or VFO drives, and they can also be mixed on the same telescope. All motors have a connector to the drive box.
The DC motor is not capable of any constant speed unless it has position feedback with expensive electronics. This motor can be controlled by a "VFO" (or more accurately a variable speed control). We provide a declination option to drive such a motor.
3. HAND CONTROL
SYSTEMS 1 and 2 come with a non-detachable handset.
SYSTEMS 3 and 4 have an optional handset which is either fixed or detachable. The Sidereal rate comes from the drivebox and the HANDSET (with options) determines the coarseness of the adjustments.
Operation with auto-guiders (SBIG ST4 CCD camera) is possible with all handsets, requiring OPT 2. Further information.
These systems have the ability to drive Synchronous or Stepper motor for the RA drive. It can drive our stepper motors (STEP2,3) or synchronous motors (STEP6,7). Power is required at 12V DC up to 0.6 amp. This can be supplied from the BATPACK for single axis use. The handset contains the rate adjustment and the FAST / SLOW scan rates and is permanently fixed to the drive box by a 2 metre lead. In common with our other drive boxes the RA motor is always driven and the handset only provides the adjustments. There are two SYSTEMS based around these products with options available for Dual axis. All motors plug into the drive box so the electronics can be removed between sessions. The synchronous motors are supplied with a safe round 3 pin connector and we have an additional adapter (ADAP240) allowing you to convert this into a mains plug.
SYNCHRONOUS motor RA drive. (OPT 8 is not available with this). If you have an existing mains operated motor then a 50Hz supply is required which we supply as standard. USA motors require 120V 60Hz (specify on order form). If you are buying a synchronous motor then we need to know the slow motion reduction ratio. A specific gearbox will have to be calculated allowing the motor to work in the 42Hz to 80Hz range to give the correct reduction. Easy ratios are 96:1 144:1 288:1 360:1
STEPPER motor RA drive. Slow motion reduction ratio is required so that we can work out correct gearbox and drive frequency.
Here are the descriptions of SYSTEM 3 and 4 although all remarks regarding driving stepper motors apply to all systems using stepper motors.
For synchronous motor (RA axis) with or without a handset but exact crystal controlled drive rate. The drive box can be fitted with options 7 or 9 and 10. The HC20 handset is compatible with options 2,3,11.
For stepper motor (RA axis) with or without a handset but exact crystal controlled drive rate. The drive box can be fitted with options 7 or 9 and 10. All three handsets are compatible and all options 2,3,8,11 are available.
The DRIVE BOX construction is waterproof (designed to work in exposed environments) and the stepper motor connections are made on a flying lead and connector. 12V DC is required at up to 1.2 Amps depending on the options fitted.
The HANDSETS are ergonomically designed to allow real control at the telescope. A red LED in the centre of the four buttons allows finding the unit again when you have put it down in the dark. the front panel is sealed with raised buttons that you can feel under gloves. The normal leadlength supplied is two metres but can be supplied at different lengths if required. The function of the handset is to adjust the tracking rate when the button is pressed (or switch in a VFO function for LUNAR and SOLAR tracking) and the various handsets offer various levels of ability in altering the rates.
This is the basic handset that can be supplied. It is offered as the lowest cost version for budget systems, ideal for driving synchronous motors. A VFO option (OPT 3) is available when used with SYSTEM 3 or 4. For controlling stepper motors, an option giving FINE or COARSE movements when the button is pressed (OPT 8) is available.
It is also possible to offer the SYSTEM 5 handset separately. This has the VFO option included.
OPTION 11, the waterproof handset connector allows keeping the integrity of the drive box and disconnecting the handset. Dust caps cover both parts of the connector when not in use to prevent physical damage. The alternative is for the handset to be not removable
OPTION 2, the SBIG remote control is a plug coming out the top of the handset allowing connection of the SBIG in autoguider mode. The opposite half of the connector and pin wiring information is provided. Several CCD cameras have an autoguider output, check with your supplier. The CCD's supplied by SBIG have such an output. Further information.
The STEPPER MOTOR assembly comprises a motor, gearbox, lead and electrical connector. There are two motor types to cover various performance requirements and they can be driven by any AWR SYSTEM with stepper motor output. STEP2 is suggested for telescopes up to 10" aperture, STEP3 for bigger ones, or very heavy ones. The overall performance is determined by the electronics driver stage and this appears as an output torque. The maximum torque ratings are gearbox limited, but then there is not much dynamic range available. If the torque required to turn the RA axis of the telescope is known then the system is designed to produce this with a safety factor and so maximise the dynamic range.
The worm set ratio on the telescope defines the gearbox ratio (this information is required on the order form) so that normal Sidereal Rate drive is between 10 and 50 steps per second. For SYSTEMS 4 and 5 the precise drive frequency is calculated individually and if necessary a custom crystal is purchased. The drive rate is slightly modified from Sidereal to give the best rate over several hours for photography - the King rate - which is refraction corrected.
When driven by a VFO unit the frequency is adjustable but we still need to know the slow motion reduction ratio so that we can get the correct gearbox.
Rotation direction of the motors is altered electrically within the drive unit or handset. The standard lead lengths are 0.5m (RA), 1m (DEC). SYSTEMS 1 and 2 plug directly into the drive box and SYSTEMS 3 and 4 have an in-line lead connector.
The normal gearbox has a typical backlash of 2 degrees which translates to an amount in time determined by the slow motion reduction ratio. See article "UNDERSTANDING TELESCOPE DRIVE SYSTEMS" for further details. OPTION 4 is a low backlash gearbox but only available in 25:1 and 50:1 ratios so will not normally be available.
A mechanical coupling is available (of the OLDHAM type) as an extra. This connects the motion between two shafts, both of which have two bearings. The coupling allows the shafts to be slightly mis-aligned. The alternative is binding and possible damage. If this coupling is required specify the shaft diameter on your slow motion and we will then drill it out to the correct size. If you have a lathe then you could probably get more accuracy than we can.
TORQUE: To get an idea, a 10Ncm torque is that of a 1kg weight hanging on string wrapped around a 2cm diameter horizontal shaft. Typical drive systems would have torques in the range 5Ncm to 100Ncm.
The design process involves feeding the telescope parameters into a programme and by selecting the Sidereal rate steps per second at around 20 then we know the gearbox and the torque at low speed. From your measurements or requirements the maximum speed can be looked up on a graph to give this minimum torque. Hence we know the dynamic range of the drive.
SYNCHRONOUS MOTOR ASEMBLIES
We can supply synchronous motor assemblies but there are problems. A two wire motor will only work in one direction and you must be able to attach it at either end of the slow motion shaft to get the direction correct. We can get some gearbox ratios meaning that only certain worm reductions are suitable. These would include 96:1 144:1 288:1 360:1. Others would be possible but then must be driven from other than 50Hz, so changing the adjustment range available to the handset. It is possible to drive most synchronous motors in the range 42Hz to 80Hz. The torque does drop off at the extremes of frequency but there is usually more than enough torque to cover. The drive box has recently been uprated to supply 9 watts of electrical power which is sufficient for most two motor systems.
Synchronous motor assemblies for all the normal gear ratios will provide gearbox limited torque of either 50Ncm for the standard type of instrument gearbox. We can however get a larger gearbox with all metal gears and this gives 200Ncm torque, again gearbox limited.
A three wire reversible motor for declination is more intricate. There must be a phasing capacitor and the 240V is applied to one end or the other end of this. Two relays are provided so that the power is applied under handset control. This has been done on systems supplied.
The synchronous motor assembly includes a 1 metre lead and large round 3 pin plug. This plugs into any AWR synchronous motor drive boxes. We also supply an adapter box (ADAP240) enabling one of our motors to be plugged into the U.K. mains with the standard 13A plug (fused 3Amp).
WARNING: When using mains directly to drive these synchronous motors the units should be plugged into a RCCB Circuit Breaker. Observatories can be damp places and this is dangerous with voltages above 50V and so the correct precautions must be taken.
DC MOTOR OPTION
We provide a variable speed option to drive a 12V DC motor for Declination. The output is adjusted to between 3V and 12V by rotary knob on the handset. It will drive a DC motor up to 0.5 Amp. It is activated only when the button (UP or DOWN) is pressed. This is OPTION 13.
It can also be ordered as a "DC focussing motor control" in which case a switch for direction (centre OFF) and a speed adjust knob is provided.
Article - UNDERSTANDING DRIVE SYSTEMS
This one explains the basics of telescope mountings, how they are driven and what motors can be used for driving. Backlash and drive rates are explained. The paper copy (see price list) has drawings in it!
Article - TELESCOPE POINTING ERRORS AND CORRECTIONS (pdf format)
This one is fairly technical but explains how the various types of mechanical errors in the telescope construction contribute to errors in pointing and what can be done to compensate.
Article - INSTALLING TELESCOPE DRIVE SYSTEMS
Advice on motor mounting, balancing, power supplies and troubleshooting problems.