Affordable astronomy

Affordable equipment makes telescope control easier.

In the early days of astronomy, telescopes took hours to set up and necessitated complex clockwork mechanisms to keep them pointed towards a particular star or planet while the Earth rotates, otherwise known as the sideral rate. The earliest types of mounts were altitude-azimuth systems. Altitude is the angle an object appears to be above the horizon and azimuth is the angle along the horizon of the object. The problem is that the telescope's field of view will then rotate at a varying speed while the telescope tracks an object across the sky.

Equatorial mounts are more suitable for telescopes because by tilting the horizontal base of an altitude-azimuth mount up until it is parallel to Earth's equatorial plane, the azimuth rotation then swings the telescope in an arc that follows the stars as they move across the sky. Affixing a clockwork mechanism to this axis allowed long observations on early equatorial systems.

One such clockwork mechanism may be seen at Airdrie Public Observatory, owned and funded by North Lanarkshire Council and operated on their behalf by ASTRA. The observatory houses a 6 inch refractor telescope made by Cooke of York, who was one of the country's foremost telescope makers. The telescope is mounted equatorially and has a clockwork drive. The observatory's dome is rotated to the required position using an electrically driven mechanism.

The introduction of electricity ushered the use of motors. Then the computer age improved accuracy, making it easier not only to track an astronomical object but to locate it too. This means that mechanical power can be applied precisely to the task, and some amateur astronomers have shown remarkable ingenuity in such applications.

Take the Croydon Astronomical Society's Kenley Observatory for example. This was built in 1979 and houses a Meade 14in LX200 telescope. The parents of the late Alan Treayn recently donated a Fullerscopes 10in Newtonian telescope to the society.

"The telescope had been unused for a while and was in a state of disrepair," recalls society member and embedded systems engineer Steve Sumner.

"We took it apart, cleaned it and repainted it. At that stage, the system was manually driven so I thought it would be a good idea to fit a dual-axis motor drive system for right ascension and declination on equatorial mounts.

Stepper motors

"We used 0.2A stepper motors geared down 50:1 to provide the necessary torque. The steppers move 7.5 degrees per step, or 48 steps per rotation, which after the 50:1 reduction produces a one degree rotation at the gearbox output shaft."

Sumner used a half step sequence on the motors. This gave 96 steps per revolution of the motor, then through a 50:1 gearbox to 4,800 steps per revolution to the output shaft. This then drove a 360:1 worm and wheel resulting in a total step count for a complete revolution of the scope to 1,728,000 steps. Siderial drive rate is 23 hours 56 minutes and four seconds or 86,164 seconds so drive speed is 20.0547 steps per second which gives a step resolution of 0.75 arc/seconds.

"We needed to do that because otherwise you would see a star moving out of view and then jumping back," Sumner continues. At about 20 steps per second sidereal drive rate, the motors keep the star stationary in the centre of the eyepiece. Therefore, the telescope is constantly in motion as the star moves across the sky as a result of the Earth's rotation. The idea is to keep the step resolution below that of the theoretical resolution of the telescope."

But moving from one star to another presented different problems. Although this was also done manually originally, Sumner wanted to drive it automatically by motor. If the object was difficult to find, he would manually move the telescope to a nearby bright star, synchronise the system on it, and then instruct the system to move to the object from a keypad with an LCD display.

"You just tap in the destination and the system software calculates the number of steps required to move to the new position," Sumner continues. "This was all designed around an 8-bit PIC microcontroller. The reason I used this was that I learnt Microchip PIC coding for a computer systems engineering course at college and had gone out and bought a PICstart Plus programmer.

"I incorporated a small object library which was stored on an external I2C serial EEPROM. This contained all the messier objects (from a catalogue that was compiled by French astronomer Charles Messier in the 18th century) and Caldwell objects (a catalogue of 109 bright star clusters, nebulae, and galaxies compiled by Patrick Caldwell-Moore, the famous television astronomer, as a complement to the Messier catalogue).

"Due to the inherent problem with stepper motors and lack of torque at high speed, the slew rate was very slow, maximum around 32x Siderial so I needed to sync on a bright near by object. I did plan to build a much better drive stage for the system using bipolar chopper current control and possibly microstepping the motor but I never seemed to get around to it.

"Although this system was fairly basic I did learn a great deal, it worked well, and I used it on many an observing session. You just had to be patient."

Accuracy and gearing

Accuracy hinges a lot on gearing says Ninian Boyle, a keen amateur astronomer with many years of experience. A Fellow of the Royal Astronomical Society and contributing consultant to the BBC Sky at Night magazine, Boyle also runs a specialist astronomical equipment company called Venturescope.

"The better the gearbox, the better the accuracy of the system will be," he argues. "Some modern telescopes use servo motors to improve torque on heavier telescopes and mounts. Servo motors have smoother motion as well, compared to stepper motors, which move in discrete steps, although some telescopes use microstep drives to overcome this, providing accuracy of less than half an arc second. Many manufacturers will offer both types, often as retrofit mounts to existing telescopes.

"Tracking stars is relatively easy because their motion relative to Earth will be along the Earth's single axis. Tracking planets introduces complications because they orbit the Sun, and moons orbit planets. In fact, the word planet originates from the Greek term 'wandering star'.

"Looking at the future, direct drive motors will be incorporated into telescope mechanisms, so that the mount itself is the motor, avoiding the use of gears. The price will drop, putting the technology within the reach of universities and research establishments.

"By losing the gearing, you eliminate a potential source of error - both backlash and periodic. With periodic error, gears will glitch, say, every third turn of the worm, by slowing down or speeding up. Computer systems can read that and compensate, but it is never perfect."

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