The Astronomical Scheduler Daemon: astrod

The Astronomical Scheduler Daemon (astrod) schedules observations, system startup and system shutdown. It consists of a modified queue scheduler that decides in real-time which field is the best field to image next. Gamma-ray Burst alerts from alertd are automatically put in the front of the queue for immediate processing. In addition, GRB alerts are logged and further observations of the target coordinates (``follow-up'' observations) are scheduled to be performed at logarithmically increasing time intervals. See Chapter 4 for information on how to set up the scheduler.

There are four primary types of schedule items that can be configured, listed here in increasing priority:

• ``Sky Patrols:'' The entire sky is gridded to a pattern, whose spacing depends on the telescope field of view. A configurable region of this grid can be cropped based on right ascension, declination, and galactic latitude. These regularly spaced fields form the ``sky patrol,'' that is the default mode of operation for wide-field surveys. For more details, see Section 4.3.3.

• Targeted Observations: Specific fields can be named as targets in the configuration file. These are considered slightly more important than sky patrol fields.

• Late Burst Follow-up Observations: After a burst alert has been received, the location is logged in a file described in Section 4.5. At late times, the burst location is repeatedly imaged at increasing time intervals.

• Prompt Burst Observations: Upon receipt of a burst alert, if the weather is fine, the sun is down, and the burst is above the horizon, the scheduler will immediately schedule a prompt burst observation sequence.

The parameters for the schedule items are defined in astrod.conf, described in Chapter 4. Each schedule item has a corresponding field or set of fields, and a set of configuration options. These include vetoes based on field elevation, lunar illumination, solar elevation, and camera temperature. In addition, if a field is to be imaged more than once, a cadence can be specified.

Each schedule item also has a corresponding imaging sequence defined in the configuration file. For sky patrols, this is typically set to two consecutive long (60 s) exposures. For prompt burst responses, this is currently set to 10 short exposures, 10 medium exposures, and 50 long exposures. New imaging sequences can be defined by the operator, as described in Section 4.4.1.

The observing schedule is built in real-time. When the previous imaging sequence has finished, the scheduler scans through the list of schedule items. First, each field referenced in all the schedule items is checked for quick vetoes. If a field passes the veto cut then the scheduler calculates a score based on current airmass, whether or not it is a specific target, and whether or not the cadence demands a return to the field. The relative weights given to each of these criteria is configurable as described in Section 4.3.5. The scheduler then has the telescope slew to the highest scoring field which is then imaged with the corresponding imaging sequence. The typical calculation time to determine the best field from a sky patrol list is ${\sim 0.2}$ seconds. Prompt burst alerts are automatically put at the front of the queue with higher priority, yielding faster processing times. If no schedule item passes the veto cuts then the telescope returns to ``standby'' mode, pointed at the zenith.

When astrod receives a SIG_ROTSE from alertd, the current schedule is interrupted and the alert information is copied from shared memory. The burst location is then logged for follow-up, and a burst alert schedule item is put in the front of the schedule queue. If the trigger location is above the horizon and the system is running in good weather, the mount is told immediately to slew to the trigger location, and system begins the preconfigured imaging sequence. If any of these conditions are not met, the schedule item remains at the front of the queue until it can be imaged, or its lifetime expires. If the burst field is not viewable, the scheduler will continue with its normal operations.

Now I will describe an example of a typical observing sequence, when a sky patrol and various targets have been configured. Under normal operation, the telescope will point near the maximum elevation of the sky patrol fields, and fields will drift into this area. As specific target fields become visible, they are imaged, and then the telescope returns to the regular patrol. When a field has a cadence specified, the scheduler tries to keep to the cadence to the best of its ability.

The astrod scheduler is also responsible for telling the clamshell to open at sunset, and close at sunrise. The configuration file is re-read at the end of each night's observing after the clamshell is closed.

When the system is put in manual mode, astrod quietly loops until it regains scheduling control in auto mode.