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TELESCOPE MOUNT Mount description The major factors influencing the design of a telescope mount is its intended use and the materials used for construction. This design is aimed at remote observing and astrophotography and is not intended for visual observation. The availability of woodworking tools led to the choice of birch plywood as the construction material. The approach to the design was to make the most critical components affecting performance as large as possible in order to keep within the required tolerances.
Using wood for construction makes the mount large for a 305 mm aperture telescope. The advantage of a large mount again is the amount of weight that it can carry. The width of the mount is set at 700 mm, a little less than that of a doorway, keeping it portable.
Using a trammel, large precision discs can be made out of wood. This led to the idea of using a friction drive system utilising large wooden discs in place of the conventional worm gear and toothed wheel arrangement. There are a number of advantages in using a friction drive system, most notably the absence of periodic error, backlash and the requirement for a clutch system. The horseshoe design eliminates the need for a counterweight as required for German equatorial mounts, although slippage in the friction drive will occur if the telescope optical tube assembly is unbalanced. Balancing is done by changing the centre of gravity of the optical tube assembly through the arrangement of its components and the addition of small counterweights. This was thought to be the major disadvantage of this mount but testing has proved that balancing is not that critical to its successful operation.
Right ascension and declination drives The right ascension and declination drives are of similar design. The friction drives produce a reduction rate of about 4000:1. This amounts to 0.81 arc-second rotation of the telescope for each half-step of the stepper motors. A lower drive ratio and the micro-stepping of the motors, which would produce faster slewing speeds, were avoided as micro-stepping does not result in consistent accurate positioning of the motors between full steps.
A hardware pulse-width-modulated system limits the current supplied to the stepper motors to about 60% of their maximum rated current. Step pulses are timed by the microcontroller. About 18.5 half-step pulses per second are sent to the right ascension drive to maintain sidereal tracking. The length of the pulses is limited to a maximum of 10 mS in order to keep the power consumption low and to prevent the motors from heating up. The maximum obtained slew speed with this arrangement is approximately 27X the sidereal rate. The right ascension drive:
The declination drive:
Mount calibration and operation After polar alignment the mount is calibrated by slewing to the zenith and setting its right ascension and declination coordinates to the local sidereal time and latitude. The position of the zenith is detected through gravity by means of a pendulum type switch sensor. After calibration sidereal tracking commences and the mount awaits a slew instruction.
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