Smoothest transmission is a true star

You might not expect that a company whose main activity is designing electronic test meters and equipment for US and UK companies would also be responsible for a series of drives that allows telescopes with mirrors up to 50in in diameter to accurately track the sky as the Earth rotates. Yet this is the case for Buckman Hardy Associates. The company has developed a geared motor transmission which offers exceptionally smooth running because it is evenly loaded during each step and microstep produced by the stepping motor, while the gears are specially designed and manufactured. An ingenious logging and analysis system, meanwhile, identifies any intermittent or periodic movement errors so they can be rectified. While the drives for the larger telescopes are designed and manufactured in response to particular customer requests, the technology could, in principle, be applied to any drive train and for any purpose if the goal is to improve precision, smoothness and quietness of running. Alan Buckman, one of the proprietors of the consultancy, explained the requirements: "The stars are whizzing past at 15 arc seconds per second and, if you are out by a few arc seconds per hour, people complain." This is because even ambitious amateurs now use long time exposures to allow digital cameras to build up images of distant objects. The first crucial breakthrough that enables the high degree of precision, said Buckman, is the use of 'our own PWM [pulse width modulated] circuitry'. "This delivers a constant torque throughout each step. Normally, when a stepper motor is unloaded, the demanded position is what you get. But to get out any torque, you need an offset angle. So, to achieve a constant angle of offset, you have to have a constant torque." The motors, made by Sanyo Denki, produce 12,800 microsteps per revolution and were selected following a long search. Buckman said: "This means we can do without a gear train and, because they run the freeest of any available, the evenness is what I need." The motor shafts typically go through a 3:1 pulley reduction, then to a worm reduction that turns the telescope. Buckman said a lot of technology goes into the worm reduction to make the backlash minimal. "There needs to be a tiny amount of backlash – a few arc seconds in the sky is ideal – which translates into a few tens of microns of mechanical movement. If normal tooth shapes are used, you get non involute coupling, which results in a periodic error of a few arc seconds in addition to periodic errors produced by the main worm and wheel. "We know so much about this," he continued, "because our customers use CCD cameras with autoguiding, which produces a log of the error trace. If you follow a star for 20 minutes, you can put a Fast Fourier Transform of the guiding trace into a computer and produce a spectral analysis." Buckman showed an example of such an analysis, with peaks corresponding to very small periodic errors. These could be related to mechanical perturbations within the gear train so precisely that, according to Buckman, 'you can see the effect of one gear tooth moving against another gear tooth and the step to step sequence within the motor'. For fast traverse, the drives can be made to slew at up to 'one or two degrees per second, even on the largest telescopes'. The controllers use dual processors – one looks after the two stepper motors and their acceleration and deceleration, while the other is a command processor, converting pulse counts into astronomical units and maintaining star time. Many of the drives are used to retrofit old telescopes, including an instrument formerly owned and used by Sir Patrick Moore, which has been restored and improved by Bruce Kingsley, whose photographs have featured in 'The Sky at Night' magazine and tv programme. Buckman gave, as an example, a system retrofitted to a telescope owned by Canterbury High School. This uses a friction drive. "A friction drive is a different approach, with its own problems," he observed. "The reduction ratio is usually not unity, but we can program that in. The great benefit, however, is that there is no backlash, so the only error is the angular deviation associated with the motor." Most customers are what Buckman described as 'well funded amateurs' – both individual and clubs. But the drives have also been applied to telescopes owned by schools and colleges, including a Victorian refracting telescope at Marlborough College and one of the telescopes at the ex-Royal Greenwich Observatory at Herstmonceux Castle, now moved to Cambridge. Active mirrors A pneumatic active mirror support system supplied by AWR Technology uses three load cells around the mirror's periphery, three servo valves supplied by Asco Joucomatic and 18 Bellofram rolling diaphragm actuators. The working fluid is dry nitrogen gas. The actuators are pumped up until there is the same load at each cell, so the mirror maintains the shape it had when it came out of the optical workshop, regardless of its orientation. Pointers * Transmission is smooth and precise to much better than a few arc seconds per hour * Constant torque is delivered during the duration of each step and microstep * Special gear tooth profiles avoid inconvolute coupling and consequential periodic errors * Feedback from perceived star positions followed by Fast Fourier Transform analysis identifies other possible periodic faults and indicates the need for maintenance Amateur astronomers and how they might save the planet The universe is vast and the resources available to professional astronomers are limited, so it is not possible to maintain a professional watch on everything in the sky. This means the vast network of amateur astronomers usually notices anything unusual –particularly comets. With NASA and ESA believing it may be possible to 'nudge' a potentially dangerous comet into a slightly different path where it could be made to miss the Earth, it is worth detecting them as early as possible. Amateur astronomers also fulfil an essential role by getting kids 'hooked' on science. If kids don't study science at school, they cannot go on to study engineering later, and more than a few engineers admit their journey started when they were shown 'something amazing' through a telescope.