Star-tracking

Astronomers using a pair of eight metre diameter telescopes will focus deep into outer space using Heidenhain encoders for precise targeting.

Astronomers using a pair of eight metre diameter telescopes will focus deep into outer space using Heidenhain encoders for precise targeting.

In the year 2000, when astronomers make full use of the two eight metre Gemini telescopes located on top of Mauna Kea, Hawaii, and Cerro Pachon, Chile, they will be relying on the accuracy of Heidenhain LIDA 105C exposed linear encoders for pin-pointing and tracking their targets.

The object of Gemini, which has been funded half by the USA, 25 per cent by the UK and smaller investments from Argentina, Australia, Brazil and Chile, is to provide observatories from which astronomers can gain access to a clear night sky in both the northern and southern hemispheres.

The installation of the Hawaiian telescope is expected to be completed before 1999, with so-called 'first light' in December, while the Chilean telescope, which is currently being erected at the Telas/NFM factory in France, is scheduled for completion by the year 2000.

The Royal Greenwich Observatory (RGO), Cambridge has already carried out a retrofit on the William Herschel 4.2m telescope now located in the Canary Islands using measuring systems operating on the inductive principle.

Says Martin Fisher, RGO's head of electronics: "The Heidenhain optical tape system offers improved resolution and accuracy which is necessary for modern, highly specified telescopes on first-class sites." Heidenhain's optical based Lida 105C exposed linear encoding system was selected for use on the telescope's mount control systems, after extensive comparative testing.

Lida is an optical system based on an incremental linear encoder with a 40 micron grating deposited on an Aurodur steel tape.

The tape is used in conjunction with a non-contact sliding optical head which generates Moire fringe patterns as it passes over the length of the graticule.

An optical sensor then picks up the fringes as a signal and the spaces are electronically counted to determine the precise length of travel.

The performance of the encoder system was tested against the specification required for the essential areas of pointing and tracking.

This ensured the ability of the system to position the telescope at a point in the sky to an accuracy of 1 arcsec rms and to track a target to about 20 arcsec rms over most of the hemisphere.

A special rig was constructed in the RGO laboratory to allow tests to be carried out between the two measuring technologies.

The tapes were mounted side-by-side, with a proprietary linear stage running between them.

Among other items, the stage, which was supplied by Physik Instrument, comprised a DC motor, encoder, gearbox and leadscrew arrangement.

A saddle holding the heads from both measuring systems could be moved up the rig in preset steps.

At each step, the stage was stopped and the encoders logged in order to determine any positional error.

Martin Fisher explains: "Separate tests were carried out under simulated conditions for three main causes of obstruction: dirt, moisture and scratches." "In order to check contamination effects we used toner from a photocopier, mixed with a drop of oil, which was then smeared on the tape," he says.

The azimuth and elevation of the telescope is measured by the encoder system.

For azimuth control, the encoding tape is stretched tightly around the circumference of a 9.5m diameter mounting ring, giving an overall tape length of about 30m.

For elevation, two 8.5m tapes are mounted on segments which are positioned on either side of the principal axis.

The azimuth encoding tape is used in conjunction with four 'read' heads which are fitted in diametrically opposing pairs.

By using this configuration, errors can be averaged down and a degree of redundancy provided to accommodate head failure.

"In the case of the elevation encoders, it was decided to have a mounting surface for the encoding tape on either side of the axis, using a single head on each tape," explains Martin Fisher.

The final decision on the choice of encoding system for the Gemini project rested on whether linearity and resolution could be guaranteed.

This meant that the tape had to be produced without any discontinuities in the scale.

The advantage of the production process used by Heidenhain in manufacturing the scale enables the 30m azimuth to be produced as a single length.

This eliminates the need for multiple joints.

In addition, the simpler trace on the Lida tape makes it inherently easier to match the pattern at any join line to an accuracy of 1 micron, which is essential for the purposes of the telescope.

"Hence, it was decided that the Heidenhain Lida 105C system was the better choice," maintains Martin Fisher.

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