Telescope Limits

The tracking programs are aware of the fact that the telescope can rotate through 720$^\circ$. The limit of 4$\pi$ is due to the ability of the cables to wrap before pulling out of their connections. This is the range covered by the azimuth readout displayed on screen 1. An algorithm compares the telescope's actual position with the commanded position (labeled REQUEST on screen 1) and selects the zone (i.e 0-360 or 360-720) in which to operate and displays that number as the azimuth position labeled COMMAND. This algorithm only applies when entering a commanded position in terms of RA, DEC, L, or B coordinates. If the user enters an AZ,EL position, then the telescope moves to that actual position in azimuth (i.e. no algorithm is applied). However, if the user enters only an elevation, then the requested azimuth is set to the commanded azimuth just prior to entering the elevation.

In elevation, if the readout falls outside the range 1$^\circ$ and 87$^\circ$, the telescope will halt and remain in the HALT mode as long as the telescope is beyond the limits. To point again, it is necessary to manually move the telescope to an elevation within the limits and then type POINT. In azimuth,, there is a monitor that prevents the azimuth from going into the physical cable-wrap limits as long as the telescope is under computer control. When the readout azimuth is less than 40$^\circ$ or greater than 680$^\circ$, the telescope will halt itself and print an error message on the Tek4023. It will remain in the HALT mode as long as the telescope is outside the azimuth limits and you must manually move the telescope in azimuth until it is back within the limits and then type POINT. The following instructions demonstrate how to manually back out of a hardware limit situation.

1. On the control console, simultaneously press the the LIMIT BACKOUT, POWER ON, and STANDBY switches for the particular telescope drive (AZ or EL) which has reached a limit.
2. When LIMIT BACKOUT light goes off, release the LIMIT BACKOUT switch.
3. Release the POWER ON and STANDBY switches
4. Return drive to computer control.