4m Cassegrain Instrument Rotator

• 4m Cassegrain instruments can be rotated through a full 360° range under remote control of the Telescope Control System.

• The readout for this is in the lower right corner of the Telescope Status Display Screen; it is the entry called "ROT".

• The allowed range is normally 0°-360°.

• The default rotator position is 90°, which places the spectrograph slits in an E-W orientation. The angle corresponds to normal astronomical "position angle" nomenclature: 0° is towards the north, increasing through 45°= NE and 90° = E, etc.

• Any of the instruments can be rotated at any telescope position (they have been counterbalanced as necessary). But it is worth telling observer support personnel if you plan to do this, so that they can check that the cabling won't get hung up.

• ATMOSPHERIC DISPERSION. The Telescope Control System includes the command "D" (Atmospheric Dispersion) in its Rotator/Image menu. This calculates the correct rotator angle to place the spectrograph slit in the same direction as the atmospheric dispersion for the midpoint of an exposure of any specified length. It calculates it for the present coordinates of the telescope. The normal sequence is:
  • slew to the object coordinate.
  • have the night assistant use the D command.
  • rotate to the correct angle.
  • find a guide star.
  • start observing.