Way to the stars with optical tracking

The ability to position a telescope at a point in the sky with extreme accuracy is important to astronomers. Using exposed linear encoders is said to make it easy

Next year astronomers will rely on the accuracy of linear encoders for pinpointing and tracking heavenly bodies. Heidenhain LIDA 105C exposed linear encoders are being supplied to two 8m Gemini telescopes located on top of Mauna Kea, Hawaii, and Cerro Pachon, Chile, to provide the telescope with the ability to position itself at a point in the sky with extreme accuracy.

The object of Gemini – in which the UK has a 25% stake – is to provide observatories in both the northern and southern hemispheres from where astronomers can have access to a clear night sky. The installation of the Hawaiian telescope was completed late last year, 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 system, which is considered a first as far as optical systems are concerned, uses a steel tape with a grating of 40mm as the measuring standard and can be used for measuring lengths up to 30m.

Tape encoders are the traditional method for applications such as telescopes, as they can be manufactured to any length. In addition, major advantages in place of the more conventional gear-driven incremental encoders are the elimination of any gear error and the fact they form a part of the structure.

For this application the optical system was selected, after extensive comparative testing, over the Inductosyn system, which uses an inductive head to scan an AC signal carried by a printed circuit track laid down as a type of continuous `hairpin’ waveform. The LIDA optical system is based on an incremental linear encoder with a 40mm 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 passed 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.

Before the trials Inductosyn was deemed to be superior by virtue of its resistance to contamination from dust and grease. In the meantime Heidenhain had developed the head optics to provide a multi-facet, or single field, window. This effectively allowed the head to scan four points on the track at one time and as the system is able to average out several fringe patterns, this minimises any problem of contamination.

An additional advantage for the optical system was that the production process enabled the 30m azimuth to be produced in a single length, the alternative system,which is a step and repeat process, produced two 15m lengths, necessitating two joints. The simpler trace on the LIDA tape, it is claimed, makes it easier to match the pattern at any join line to an accuracy of 1mm, which is essential for the purposes of the telescope.

The Gemini telescope located at Cerro Pachon, Chile, is being built in France at the TELAS/NFM factory. It is scheduled for completion by the year 2000. The optical based LIDA 105C exposed linear encoding system was selected for use on the telescope’s mount control systems