The movement of the telescope is controlled by a Perkin-Elmer 3220 computer which is located downstairs in the computer room usually called the CLIP centre. Its system terminal (CST) is the furthest of the two VT220 VDUs to the left of the console in the telescope control room. Once the night's work has started you will rarely if ever want to use this terminal. You will use the VT220 on the telescope console which is where the User Interface (UI) appears. Immediately to the right of this VDU you will see the information display which is a continuously updated report of the status of the whole system. The working of the control program is completely transparent to the user but in outline it works as follows. When a star position is demanded from the User Interface the computer invokes a subroutine called MODEL which allows for the geometrical and empirical errors of the telescope mounting. The errors themselves depend on whether the telescope is East or West of the pier and whether the f/8 or f/15 top end is mounted. The errors are held in the files F15E, F15W and F8E. F8W doesn't exist but could easily be set up in response to user demand. From here on the telescope is under control of the subroutine TNHZ which polls all the encoders ten times a second. If TNHZ detects a large difference between where the telecope is and where it wants to be it tries to eliminate the difference by slewing at the maximum rate allowed by the hardware. Once the difference is less than an arcsecond it drops into tracking mode. If the autoguider is running it injects its errors into TNHZ at this stage. You can follow all this happening by watching the information display which is updated as opportunity offers but about once a second.
The information display is largely self-explanatory but two mechanisms require comment. When the telescope is slewing it pre-computes where the dome should be on arrival at source and keeps the dome turning until it has reached that value. Once the telescope is tracking it ignores the dome until its azimuth is wrong by 3.3 degrees whereupon it corrects it. The dome can turn only at one speed, full, and is either off or on. There is one position near the zenith where the dome has to turn through 180 degrees in about a minute. This results in the dome going into a stop/go mode which can be alarming to a new observer. New observers should also remember that because of the nature of its mounting the telescope is never at the centre of the dome and in general the telescope azimuth and the dome azimuth will be several degrees apart. The drive program knows all about this so don't worry.
It is possible at large hour angles East with the telescope East to point the telescope so that it is partially vignetted by the dome wall and similarly Telescope West. At the moment the drive programme does not allow for this so you should consult the appendix to this manual or the circular graph on the notice-board when in doubt. It is planned to include this problem in the drive program at a later date.
There is a thermistor on the telescope steelwork whose temperature is displayed at the bottom right hand corner of the information display. This temperature is updated only every five minutes and the value displayed during the first five minutes after startup is wrong. When the drive program detects that the focus has changed by half a degree or more it will adjust the focus according to an internal calibration. The program has no absolute calibration; the initial focus is set by the observer using the FOCUS command from the User-Interface or SETFOCUS from the instrumentation computer usually following a focus run. Once the initial focus has been set the program follows the temperature from that point onwards. Although the control 3220 and the instrumentation 3210 are quite distinct computers they can communicate by an Inter-Processor Link (IPL). Thus the Instrumentation computer can set the focus and the telescope control computer can provide the co-ordinates of a source to the Instrumentation computer to enter a data file descriptor.