The display on the HOP page can be in one of three operational contexts. The ``Current'' context is the normal display, and it reflects the actual state of the HUT configuration. The ``Plan'' context is used to change the instrument parameters located in one of the five sequence files stored in the DEP memory, either by starting from scratch, or by using data from a specified sequence load. The ``Preview'' context displays the configuration to be used for the next observation and loads it into the buffer used by the DEP when the SETUP command is given. The instrument configuration to be used can be changed in the ``Preview'' buffer before the SETUP command is given.
The DEP itself can be in one of eight operational states: Reset, Load, Ready, Setup, Locate, Observe, Pause, or Slew. The DEP enters Reset mode upon power up or when given a hardware reset. After completion of the reset sequence, the DEP is ready to accept the initialization load and program data. Loading the DEP requires 7 minutes. After a successful load the DEP enters the Ready state.
The SETUP command to configure HUT for an observation places the DEP in the Setup mode. The observation sequence residing in the preview buffer is then used to configure HUT for the next observation. At any time in Setup mode, the configuration can be changed by sending appropriate commands in the Current context.
Once HUT has reached the requested configuration (this can take a few minutes if several mechanism motions are required), the DEP proceeds from Setup mode to Locate mode. The DEP will then locate the target using the method specified in the sequence load. Table 3-2 summarizes the locate modes, the typical target types they are used for, and the method used. The DEP configures the HUT TV camera appropriately for the chosen locate mode. For Source and Manual locate, the TV camera magnitude is set to the target magnitude. For Guide Star and None locate, the TV camera is adjusted for the mean guide star magnitude.
|Mode||Target Type||Locate Method|
|Source||Visible point source||Use the target itself|
|Manual||Complex field||Use the cursor location|
|Guide Star||Invisible or extended sources||Use guide star positions|
|None||Moving targets, background measurements||None|
After the PS is satisfied that the target is properly centered in the HUT aperture, the BEGIN command places the DEP in Observe mode. HUT can be reconfigured during an observation by issuing appropriate commands while in the Current context, or the next observation can be configured by editing a sequence in the Preview contexts. The DEP exits Observe mode either upon receiving a PAUSE, QUIT, or SETUP command, or if the planned observation time is completed. The observation time buffer is decremented starting from the BEGIN command, and it does not stop for any reason.
When the DEP enters Observe mode, it moves the primary observing slit into place if not already there, sets the TV camera parameters for the mean guide star magnitude, and locks on to the selected guide stars at their locations when the BEGIN command was issued. All subsequent pointing errors are referenced to this initial location of the guide stars. Complex observations can be pre-programmed into an observing sequence. A ``dither" will change the selected telemetry mode, the observing slit, and any mask applied to the spectrum by the SP after a selected time interval. (The SP can be directed to ignore events in certain regions of the detector such as around geocoronal Ly by selecting a particular ``mask".) When the time specified for the Secondary observation interval is completed, the DEP will repeat the Primary observation interval, and so on until the observation is complete.
Up to 3 Offset pointing positions can be specified in the sequence definition, each with a requested observing time. The DEP moves the HUT mirror to achieve the specified pointing offsets. A regular pattern of offset pointings can be accommodated by using a Raster observation. The sequence load specifies the step size Y and Z in the HUT coordinate system, the number of Y and Z steps, and the integration time per dwell point in the pattern.
The PAUSE command can be used to suspend a Dither, Offset, or Raster observation. Data continues to accumulate in the current configuration in the Pause mode. If a Pause is suspended with a PROCEED command, Observe mode resumes. One may also exit Pause mode by commanding a QUIT, BEGIN, or SETUP.
When the planned observation time runs out or a QUIT command is given, the DEP enters SLEW mode. The DEP uses the special sequence 0 stored in memory to configure the instrument for SLEW mode. The maximum attenuating filter is placed in front of the TV camera to protect it against bright objects during a SLEW. If the detector is on, background can be accumulated either as dark count if the slit is closed, or airglow data can be accumulated as the shuttle maneuvers to acquire the next target.