The telescope is prevented from moving to dangerous positions both by restrictions imposed by the computer control system and by electromechanical switches.
The primary limitation is in zenith distance. The computer will refuse to slew the telescope to zenith distances greater than 66° (the `ZD computer limit'). Under manual control, however, 66 can be reached (the `ZD prelimit') before a speed limit of 270"/s is automatically imposed. On reaching 70° (the `ZD first limit') the telescope is brought to a halt. A further mechanical limit (the `ZD second limit') at 70.5° is there as a backup, but would not be reached unless a fault had occurred.
The region between 66.5° and 70° is called the `ZD prelimit' region, and here only slow motions, including tracking, are permitted. A source can be tracked all the way down to ZD 70° before anything happens to disrupt the observations. Operating problems arise when a source must be acquired at a ZD of more than 66° but less than 70°. It is necessary to slew to an imaginary star near the source but just inside the limit, then switch the telescope drives to manual control and drive the telescope to the source (at reduced speed below ZD=66.5°) before switching back to computer control. Normal computer-controlled tracking then takes over, and the source can be centred and observations started.
There is a similar array of limits in hour angle and declination, as shown in Figure 2.18. In hour angle, the computer limit is at 6h35m; but the telescope can be slewed in declination, 21° beyond the pole so that sources south of -69° declination are always accessible (though the windscreen vignettes the telescope from about 10° below the pole). The computer system knows when to slew the telescope `below the pole' from the source RA and Dec. When a source can be observed in either of two ways (i.e. the hour angle is between 5h25m and 6h35m, and the declination is <-69°; these regions are shaded in Figure 2.18), the computer selects the one for which the source can be tracked longest. The control computer can be told not to slew the telescope below the pole, or to set the below pole limit anywhere between -90° and -69° (note that the south limit overrides the ZD limit). Mechanical limits in hour angle and declination are provided just outside the computer limits as for the ZD case.
Figure 2.18: Hour angle limits for the AAT. Shaded areas mark the region where sources can be observed either above or below the pole.
In the north the limitation on observing is set by the ZD limit, though the telescope can move within a very narrow corridor which leads down from the ZD limit to the prime focus access position.
These restrictions are completely independent of the telescope's optical configuration - there are no additional limitations when using coudé, for example.
Computer-controlled tracking functions throughout the polar region, and observations could probably be safely undertaken as far south as declination -89°58'. Attempts to observe further south are, however, likely to be frustrated by gross field rotation effects and by the limited tracking speed in hour angle (199. 999"/s) available for beamswitching, etc.
The parallactic angle () (the angle between the hour circle and a vertical circle drawn through the source) is shown as a function of hour angle and declination in Figure 2.19.
Figure 2.19: Parallactic angle
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