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Subsections

8 Observing strategies

The east-west nature of the Synthesis Telescope requires that an object be tracked for 12 sidereal hours to obtain full hour-angle coverage. We describe this as an observation, although the total duration may sometimes be less than 12 hours. Twelve such observations produce a complete set of visibilities, fully sampling the available baselines; we describe such a set of data as an observation set.

In normal use the telescope is scheduled in a 4-day cycle, during which 7 back-to-back 12-hour observations with interleaved calibrations are completed, leaving approximately 8 hours per cycle for moving antennas and maintenance. The cycle is repeated 12 times to gather data on all possible spacings for the 7 fields. Occasional equipment failure and additional maintenance can add an extra day per field; an observing rate of 46 to 52 fields per year can be achieved in this mode of operation.

8.1 Calibration

Amplitude calibration and phase referencing are achieved by observing a set of strong, compact calibration sources which are known not to be time variable. The derivation of calibration coefficients uses standard antenna-based algorithms, with the complication that, since the Synthesis Telescope has a very wide field-of-view, it is necessary to account for nearby extraneous sources, which would otherwise cause undesirable hour-angle (and possibly time) dependencies. To this end, visibility models of the region surrounding each calibrator have been derived from observations, and are used to remove the effects of the extraneous sources. These models are updated periodically to allow for possible source variability, and are used routinely in deriving the calibration coefficients.

The most commonly used calibrators are listed in Table 5. The flux densities used for these sources are internally consistent and referenced to 3C 286. The flux densities adopted for the calibrators, based on measurements made in 1994, are consistent with the scale of Ott et al. (1994). The same sources are used to determine polarization calibration parameters. The stability of telescope gain and phase is such that it is only necessary to observe a calibrator prior to and following each 12-hour observation. Variations on shorter time-scales are corrected using self-calibration techniques.


   
Table 5: Calibration sources
Source S408 (Jy) S1420 (Jy) p1420 (%) $\theta_{1420}$ ($^\circ$)
3C 48 38.9 15.7 0.6 -
3C 147 48.0 22.0 <0.25 -
3C 286 not used 14.7 9.25 33.5
3C 295 54.0 22.1 <0.25 -
Note: quoted flux densities are based on measurements made in 1994.

Calibration of the spectrometer is difficult because the narrow channel bandwidth leads to high noise levels in individual channels. Calibration is achieved in a two-step process, by using calibration coefficients measured in the continuum bands. At the point where IF signals enter the central telescope building, a broadband noise signal can be injected into all IF paths, at a level sufficient to produce a correlation coefficient of $\sim 0.3$, adequate to give an accurate measurement of gain and phase in each spectrometer channel in 15 minutes. The continuum calibration measures antenna-based gain and phase which vary relatively slowly with frequency, but may vary with time. The IF calibration measures mostly the channel-to-channel amplitude and phase differences, which arise largely in the filters which define the bandpass, just before the signals are digitized; these are quite stable with time. This IF calibration is performed once every 4 days.

In later stages of image processing, amplitude and phase corrections are determined for individual antennas from self-calibration of Stokes Iimages. These corrections are also applied to polarimeter images. Under some circumstances it is beneficial to apply them to Hi images (e.g. when there is a strong continuum source in the field).

   
Table 6: Details of synthesized beams
Weighting Frequency Beamwidth First sidelobe Second sidelobe
    (MHz) (half power) level radius level radius
Untapered 1420 49'' -13% 1' 5.9% 1.65'
Untapered 408 2.8' -13% 3.5' 5.9% 5.85'
Gaussian* 1420 58'' -2% 1' 2.4% 1.65'
Gaussian* 408 3.4' -2% 3.5' 2.4% 5.85'
 
* A Gaussian taper falling to 20% at 144L (617.1 m).

8.2 Antenna tracking and pointing

The antennas are capable of tracking down to an elevation of 12$^\circ$, so the observable sky from the observatory's terrestrial latitude of 49$^\circ$ nominally extends from declination -29$^\circ$ to +89$^\circ$  (the latter imposed by the telescope drive system). However, full hour-angle coverage is possible only for declinations north of +18$^\circ$. Sources north of declination +54$^\circ$ are circumpolar, and there is an overlap in hour-angle coverage at lower culmination, making it possible to track a circumpolar source for 26 sidereal hours without interruption. Non-sidereal tracking rates are available for observing solar-system objects.

Since they are equatorially mounted, the antennas track only in hour angle, although pointing corrections are made in both hour angle and declination. A pointing model for each antenna accounts for zero offset and ellipticity of the position encoders, misalignment of the polar axis, and non-orthogonality of the declination and hour-angle axes. These parameters are empirically determined from "nodding'' observations of selected calibrators at many hour angles and declinations. A "nod'' consists of measurements on-source and at 4 off-source positions situated 1$^\circ$ north, south, east, and west of the source, to which two-dimensional Gaussians are fitted to obtain errors in hour angle and declination. The rms pointing accuracy across the observable sky is 2.4' in RA and 3.1' in Dec.

For a given observation, the pointing corrections at the declination of interest are calculated from the pointing model for every 30 min of hour angle, and interpolated linearly to intermediate hour angles. The tracking accuracy is then determined by a feedback loop in the control system, which measures the deviation of the antenna position from the requested position; at present errors in antenna position exceeding 3'are corrected. Such corrections typically occur on timescales of 30 min.


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