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Multiple snapshots versus extended snapshots

     

Snapshots require phase stability only for the duration of the individual snapshot. There is therefore some debate over whether (for example) an observation requiring 15 minutes of integration time tex2html_wrap_inline2261 is better made as one continuous 15-minute observation or by combining the data from three separate 5-minute snapshots. A single 15-minute observation may give better dynamic range, because ionospheric or tropospheric phase gradients in the form of ``wedges'' may calibrate out of a single short observation, leaving only a position shift. In contrast, three shorter observations that are more dispersed in time and calibrated on a long calibrator cycle may encounter different wedges and so combine to give an image with poorer final dynamic range, unless self-calibration (Lecture 9) can be used. There are therefore some circumstances in which a single observation is preferable, as well as being easier to schedule.

The advantages of combining data from several shorter snapshots are (a) greater protection against total loss of the data for a given source through equipment failures or short-term bad weather, and (b) more even sampling of the u-v plane than in a single extended snapshot. Multiple snapshots are particularly useful when observing at wavelengths of 18 cm and longer in the C and D configurations, as they allow better imaging of confusing sources that may otherwise limit the achieved dynamic range (see Confusion ). The single extended snapshot may be better for observations that must be made at low elevations, where phase ``wedges'' are more likely to arise, and if self-calibration cannot be used. This may be particularly true for observations of weak or complex low-declination sources for which the total hour-angle coverage is anyway limited by the short time that a given source is above the horizon.


next up previous contents index external
Next: Constraints on Spectral Line Observing Up: ``Snapshot'' Mode Previous: Limitations of ``snapshot'' mode

abridle@nrao.edu
Thu Jul 11 16:26:53 EDT 1996