Encoders keep telescope on track to the stars

Amateur astronomer John McKay has used Sick-Stegmann ARS60 encoders to control the azimuth and altitude positioning of his home-grown 12-foot radiotelescope.

Should you happen to come across John McKay's house, high in the hills above Settle, North Yorkshire, you might think that you had stumbled into a scene from "Last of the summer wine".

In a field behind the house you will find a strange, dish-shaped construction and perhaps, a retired gentleman or two working on it.

On closer examination it turns out to be a fully functional homemade radio telescope.

This is not a prank for the amusement of BBC viewers, but, in fact, a serious amateur astronomer's very accurate tool of the trade.

And, thanks to encoder technology from Sick-Stegmann, it is capable of accuracy down to mere seconds of arc.

Builder and operator McKay retired from the merchant navy in 1991 and decided soon after that if he had time, after playing crown green bowls every day, he would like to build a six-foot telescope to help him with his other hobby, astronomy.

He designed and built it himself and it was completed in 1996.

After seven years of very successful operation McKay wanted to look even further into the galaxy and so decided to build a 12-foot telescope.

He enlisted the help of Roger Pilkington at Roll-Tec Safety in Heysham which, as an expert in roll-over protection equipment, was well qualified to help with the mechanical construction.

He also received much welcome assistance from Jimmy Rattray on the electronic side of things and work on the new telescope commenced in 2003.

One of the first problems to overcome was that of ensuring that the neighbours continued to have an uninterrupted view of the nearby quarry.

This meant that the telescope could not be designed with the standard altitude/azimuth axis rotation.

Although the azimuth axis was thought to pose no problem, the altitude axis would have to be redesigned.

Eventually after much deliberation, an offset altitude axis was agreed on.

Most of the problems were solved during construction, especially some concerns regarding the efficiency of windscreen wiper motors being able to lift the radio telescope on a five-foot arm.

However, there were still misgivings about the movement of the plane of the antenna whilst observing in tracking mode.

The high torque of the motors has meant that the concerns about lifting the telescope were unfounded but the accuracy of the plane will only really be tested when McKay works in collaboration with another telescope on even more exciting projects.

The accuracy of both the azimuth and altitude positioning owes much to the "top of the range" Sick-Stegmann ARS60 encoders that are used.

15bit shaft encoders producing 32,768 steps per turn are fitted to the final drive of both the azimuth and altitude gearboxes.

Theoretically, the ARS60 will give a pointing accuracy of 40arc-sec (approximately 0.01 degrees).

This is a far greater accuracy than will be needed for the theoretical 3.5 degree beam width of the radio telescope, but will allow for future increases in capabilities by using higher frequencies.

Sick-Stegmann's CoreTech technology enables the ARS60 to offer tailor-made solutions for many applications, due to its modular design; any desired number of steps between two and 32,768 can be realised.

Furthermore, the use of interchangeable collets with hollow shaft variants provides great flexibility.

Simply by fitting a different collet the same encoder can achieve hollow shaft diameters from 6 to 15mm and 0.25, 0.375 and 0.5in.

Additional highlights include: simple zero adjustment via pushbutton or signal line; excellent price/performance ratio; maximum reliability, as a result of opto-ASICs with chip-onboard technology.

McKay explains: "I found the encoders I needed on the Sick UK website, a comprehensive site that gave me most of the information I required".

"Any further assistance I needed was then provided by the Sick applications engineers over the telephone, who were more than helpful".

Initial results are very promising and in drift mode, the Moon shows up very well.

The results show great promise for future moon-bounce experiments and the possibility of receiving the Seti League beacon from America on 1269MHz, via our moon, appear to be well within the telescope's capabilities.

McKay's particular interest is in the most abundant element in our universe, hydrogen.

He is planning a hydrogen line survey along galactic longitudes of approximately 30 to 230 degrees, encompassing the galactic anti-centre and approximately ten degrees north and south of the Galactic equator, with a high theoretical definition.

This will be well within the capabilities of the radio telescope and the ensuing data will be mapped in such a way as to show the structure of our galaxy.

When used as a radio telescope in drift scan mode, pointing at the correct declination along the meridian and using a wide bandwidth, radiation from galactic and extra galactic sources can be observed.

It is hoped that the radio telescope, with suitable supervision, can be made available to anyone who is interested in understanding how our universe works.

Certainly, there are not many communities where a scientific workshop is available on the doorstep.

On a final note, if McKay ever wanted to improve the accuracy of the telescope positioning he could look to the new Sick-Stegmann 18bit encoder.

Due for launch in January 2007, this encoder will provide 262,188 steps enabling accurate positioning to nearly 0.001 degrees.

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