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Installed at the Royal Observatory Greenwich

For the 28 inch refractor we supplied a new control system based on this product. The telescope was fitted with encoders and we fitted our Quad Decoder Box to modify the signals to a form suitable for our Intelligent Handset. This interfaces to a computer running a planetarium programme (with an ASCOM driver fitted). A large BARS BOX display was developed and fitted to provide something visible from the other side of the dome. The telescope stays calibrated so enabling the finding of planets during daytime.

Heritage telescopes are usually equipped with clutches and maybe a motor driving very slowly on each axis. There will also be hand controls to centre the object. To computerise this is quite difficult and to make it slew under motor control is practically impossible without changing much of the mechanical drive components. This is generally not desirable so an alternative solution was developed.

The essential part is the fitting of encoders. This can be discreet. A 5:1 mechanical ratio with 5000 lines per rev encoder will provide sufficient resolution combined with electronics. 5000 x 5 x an electrical increase of 4 is a total of 100,000 counts per rev of the telescope (12.2 arcsec resolution). An alternative solution fits the encoder directly on the prime axes but needs a computer card or box to drive it - see the HHI Encoder solution adopted for some of the telescopes at Herstmonceux Space Science Centre. We have used lipped discs connected with a timing belt as a cheap method of getting a very low backlash 5:1 ratio. A pair of friction discs is ideal but needs an extra mechanical arragement to spring the two discs together. Separating the discs and coupling with a belt, also sprung to keep the belt taught will have no backlash and can cope with lost centres on one of the axes.

The encoder signals (channels A and B in quadrature) are fed to the Quad Decoder box which provides unambiguous pulse and direction signals. The Intelligent Handset reads this format and computes the coordinates. A version of this software provides an output to drive the BARS BOX.

During GOTO the Bars Box displays the distance to the object as a large solid bar of light. This decreases to zero when the object is reached. Readers will be familiar with this as the NGC-MAX system.

Here are some pictures of mechanical conponents that we have produced.

Sprung belt and 5:1 increase Sprung Encoder mounting


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