9 Observing with the WSRT

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9.1 Schedule

The WSRT scheduler is currently R. Smits (smits [at] astron [dot] nl). All primary allocations are intended to be carried out in the semester immediately following submission, i.e. within 2 to 8 months after the deadlines of March 15 and September 15, but scheduling constraints may force occasional slips into the next semester. Projects are kept for at most 14 months without re-evaluation. Backup projects, which will only be scheduled in holes in the block schedule that would otherwise go unused, are never carried over between semesters.

A weekly schedule of the WSRT is available on our the web pages.

User attendance is usually not required, and even PuMa observing computer schedules can now typically be prepared in advance and be automatically executed. The observations are mostly executed by operators, based on project details given in the proposal and settled in consultation with the PI before the start of the run. The sciencesupport [at] astron [dot] nl (WSRT support team) will assist the PI, keep him/her informed on how the observations proceed and inspect the data taken for the projects.

In order to optimally accomodate changing system capabilities and the need for test-time during the remainder of the WSRT upgrade, the WSRT block schedule is adjusted frequently, and observations are sometimes only settled a few days ahead of time. Some proposers will only find out the date of their observations after they have already taken place. This allows guaranteed rescheduling in case of major failures detected by observatory personnel during routine data inspection.

This system is also designed to allow a quite flexible response to target-of-opportunity scientific projects, and, increasingly in the future, variable atmospheric and RFI conditions. However, PIs who require observations to take place at a given time and date, either for scientific reasons, or because they plan to be present during or immediately following the run, can arrange fixed dates with the scheduler upon being notified of the success of their proposal.

9.2 Observing Procedure

At the time of proposing, the desired setup should be specified as accurately as possible, and must certainly be adequate to judge the technical merits of the project. After time is allocated, actual computer setup files should be prepared for the observing run(s), well in advance of execution. The files must currently be prepared by observatory staff, based not only on the proposal, but also on additional information and details, which can be given by the local friend and, soon, by the PI, through a modern administration system, MoM (Management-of-Measuremnts), now under development. It is intended that specification and detailed setup will gradually come under closer direct control of the PI. Typical local synthesis observations will need 15-30 minutes of calibration time before and after the target observation. Current defaults will be to observe strong and unresolved continuum sources (see below). However, PIs are encouraged to design their own optimum calibration strategy or supply own calibrators. This can be done in consultation with WSRT staff.

  9.3 Calibration

WSRT amplitude and phase calibration has traditionally been done via observations of a small set of non-variable calibration sources. 3C 286 is the WSRT primary standard (using Baars et al (1977) fluxes) at ALL frequencies. Excellent secondary standards are 3C 147, 3C 48 and 3C 295 (at 21cm and longer) whose fluxes were tied to 3C 286 and are regularly checked. 3C 48 and 3C 147 may be weakly variable at the 1-2% level at 6 cm and shorter wavelengths. CTD93 has recently been added to the list of well-calibrated flux standards. At the lower frequencies models for the surrounding fields are available.

In general WSRT positions should be good to about 0.5" (assuming a good S/N) using standard calibration. Better accuracy can be achieved using nearby phase calibrators.

Polarization calibration requires the observation of both a polarized and unpolarized calibrator. Once is enough, because of the equatorial mount. 3C286 and 3C 138 are excellent polarized calibrators at all but the longest wavelength; at 92cm it is advised to observed 3C 303 or 3C 345. Good unpolarized calibrators are CTD93 (at all but 92cm) and 3C147 (at all but 3.6 cm).

Calibration sources need to be observed only once before and once after a (long) observation. 12h runs need not be interrupted unless very high spectral dynamic range is required. Requested and allocated time should include time for calibration. Cross-calibration using the phase calibrator is, however, usually not enough. Phase changes at least 20o are often present during the 12h of observation. Self-calibration should be therefore used to improve the final image/cube. Note, that self-calibration needs to be performed not only for continuum observations but also for line observations where strong signal is present.

Accurate flux calibration also requires careful attention to system temperatures. These are monitored continuously via frontend noise source injection, and stored in the Measurement Set. Corrections for system-temperature variations can, and should be, applied off-line Especially at low elevations (<10 degrees) and at L-band (Tsys about 25 K) the variations can be quite large (up to 10 K). The telescope elevation-gain is constant to within one percent and need not be worried about. Generally the largest remaining flux density uncertainty is due to atmospheric extinction variations, especially at the shorter wavelengths under rainy conditions (about 5% of the year). Errors can, in principle, be corrected by using the Tsys variations under rainy conditions. Astronomers aiming for accurate flux monitoring (better than about 2%) may contact Ger de Bruyn (ger [at] astron [dot] nl).

Next: 10 After the Observations Up: wsrtGuide Previous: 8 RFI (Radio Frequency Contents
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