The Information Display

  The Control System's TNHZ task regularly updates (roughly once a second) the Beckmann Displays at the top of the Control Desk and the Information Display, which can be routed by the Telescope Operator to a selection of up to three monitors in the Control Room; the monitor on the System Control Panel (Control Desk, Bay 2) always carries the Information Display. A sample output is shown in Figure ; some additional comments are given here:

  
Figure: Sample Output on the Information Display

• The top line gives the Date, the Universal Time and the geocentric Julian Date.
• The block labelled INPUT echoes details of the current source as supplied to the User Interface:
  1. The RA and Dec will be either Mean coordinates, in which case the associated Equinox is also displayed, or Geocentric Apparent, as defined when entering the position data. This display will not change when the telescope is tracking or guiding.
  2. The proper motion values shown are as given to the User Interface.
  3. Offsets in RA and Dec are updated when one of the buttons OFFSET or NOM on the Control Desk is pressed (and the offset increments used by the Telescope Control Software are not equal zero). On source change the offset values are reset.
  4. The differential tracking rate display reflects the values passed on with the DIF command. The displayed values are updated (and the telescope acts upon those values) whenever the differential track button DIFFRATE on the console is pressed. Note that the value is reset to its normal value (usually zero) on change of source.
  5. The aperture display shows which aperture position is being used. The RA and Dec position values are calculated from the X,Y rotator coordinates to indicate where on the sky the current aperture is positioned. Note that the offset is executed by pushing the BEAMSWITCH buttons on the System Control Panel. The NOMINAL position usually has non-zero X and Y because the reference point used for the SNAFU procedure is not on the rotator axis.
• The TELESCOPE block of the Information Display gives details of the current telescope parameters:
  1. For the INT is indicated whether the telescope is OVER (above) or below the pole (or, equivalently, for the JKT, whether the telescope is East or West of the mounting).
  2. The status of the telescope drive is indicated with the display of FAST SLEWING, SLOW SLEWING, TRACKING or STOPPED, according to the velocity demand. Note that TRACKING only indicates that the slow-motion drive is turned on, not that it is tracking within any particular error. If the Telescope is in fact tracking but the Dome is still out of position, then the Information Display reads SLOW SLEWING.
  3. The two positions displayed are, firstly, on top, the telescope position (mean or apparent) as entered via the User Interface, and secondly, below, the Geocentric Apparent coordinates.

     For the purists among the readers it is noted that the latter position is strictly only accurate if the telescope is tracking with a negligible position error. This is because the apparent position is calculated by adding the demanded position to the known position error; the position error, however, is determined in the encoder coordinate system, which is skewed with respect to the telescope mean or geocentric apparent systems. Therefore, this calculation is not accurate for large position errors.

  4. The position error represents the telescope's (momentary) distance from the the correct tracking position. These values should be zero (or at most 2 in the least significant digit) under normal operations.
  5. The Local Sidereal Time and Hour Angle displays are accompanied by details of the current source's visibility:
     1. If the source is visible its Local Sidereal Time of Rising and remaining time above the horizon are given. The number of hours left for observation is calculated as the time to lapse before the source goes below one of the telescope (software) limits. This will be either the Hour Angle, Zenith Distance or, for the INT, the Southern Declination limit.
     2. If the source has not yet risen the rise time and possible observing time are given.
     3. If the requested source either never rises or is circumpolar then appropriate messages are displayed.
  6. The three azimuth position angles displayed, refer to the telescope, the demanded dome position, and the actual dome position. (The demanded dome position will be different from the telescope azimuth for an offset-mounted mounted telescope, such as the JKT or the finder.) The actual dome azimuth value is read directly from the encoder, and its alignment should be checked daily because the encoder gearing occasionally slips.
  7. The Zenith Distance (software) limit is set at 70. The airmasses are simply relative to zenith, but no allowance is made for the altitude above sea-level.
  8. For the INT the zenith distances of the edges of the wind shields are also given. These are the actual zenith distances, not when the telescope begins to be obscured! With the lower shutter down the telescope starts to vignette at a zenith distance of 58; with the lower shutter fully up, the telescope is vignetted at a zenith distance of 49and lower. Further, when the shutter is fully open the lower edge of the shutter is beyond the zenith, its zenith distance is without meaning and a row of asterisks (``*'') is displayed.
  9. The Parallactic Angle and Instrument Rotator positions are very useful information when doing spectroscopy at large zenith distances. The Telescope Operator can rotate the A&G box to align the spectrograph slit vertically so that atmospheric dispersion spreads starlight along the slit and not across it.
  10. The control software includes a model of the Telescope Focus as a function of the ambient temperature. Once the focus has been determined at the beginning of a night, the telescope will remain in focus, even when the temperature changes. The displayed values allow on-line inspection of this little miracle.
• The last line displays the selected NEXT object to observe. The telescope will move to this object either by typing GO/NEXT on the User Interface or pressing the SRCE button on the Control Desk. Typing GO is sufficient to go to the NEXT source.