How to Use ATCASCHED

ATCASCHED, which runs on XBONES, is used to prepare schedule files. ATCASCHED checks the schedule for completeness and, where possible, checks choices such as observing frequency for hardware compatibility. Drive times, and source azimuths and elevations are computed automatically.

Three steps are involved in using ATCASCHED:

• You fill in a screen of fields that describe a scan
• You add the screen to the schedule, resident in computer memory, and start to describe the next scan
• When all the scans have been defined, you write the entire schedule to file.

A variety of editing facilities are available: the schedule can be modified by the addition, deletion or replacement of other scans kept in separate files. Global changes of some fields are possible. Scans can be copied from one place in the schedule, and pasted at another point in the schedule. Previously used schedules can be recalled and modified. When constructing a schedule, consider:

• Is the sequence of sources sensible, and how much time is expended on the drive times?
• Does the schedule contain all the sources, and at the appropriate frequencies and bandwidths?

ATCASCHED offers several listing options to help answer these questions. You are most strongly advised to study these listings very carefully before observing commences. ATCASCHED considers drive times: all coordinate entries are automatically followed by a calculation of the azimuth and elevation of the source, as well as the drive time from the previous source, using the entered value for LST. When entering your schedule file, you specify how long to integrate on each source (and calibrator source). Note that, except in the case of mosaic schedulers and DWELL scantypes, these integration times include drive times, so the specified time must be somewhat longer than the amount of integration on source required. This is particularly important for secondary calibrators, when the drive time (up to a minute or so) may be comparable to the integration time (about two to three minutes). Alternatively, the sctype field can be set to DWELL to ensure a source or calibrator is tracked for the specified scan duration.

The schedule file submitted to CAOBS (the online Compact Array control task) need have no specific start time. Rather, the schedule file asks CAOBS to start the first scan as soon as possible. The sequence of scans will be exactly as specified in the schedule file. However, when using ATCASCHED, the calculation of azimuths, elevations and drive times is possible only if the starting sidereal time is known (at least approximately). ATCASCHED therefore asks you to provide an indication of the local sidereal time at the start of the schedule. The sidereal time is then incremented automatically by the scan duration each time a scan is added to the schedule.

The online software usually automatically sets attenuators so that the sampled signal falls within an optimum range. If more than one frequency is being used, or if your sources differ in amplitude by more than about 20Jy, you should use different envflg settings in ATCASCHED to instruct CAOBS to reset the attenuator levels for each source. You can record the settings for up to 128 sources per frequency.

To use ATCASCHED from a remote computer, ssh from an ATNF machine, e.g. venice, or nelle, to xbones and login with the username atcaobs. Alternatively, download it from

http://www.narrabri.atnf.csiro.au/observing/sched/index.html

to your machine and run it locally.