Tracking errors

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Tracking errors

The telescope encoders return values in the azimuth-elevation system, so the TRACKING_AZ values from the IMBFITS tables must be multiplied by in order to have a reliable idea of the telescope behavior on the sky. The Figure shows3.8 three real examples for three different scans. While the tracking errors seem comparable in both directions at low elevation, the dispersion in azimuth is completely flatenned near the pole: the same jittering of the telescope along the azimuth axis results in different jittering on sky, depending on the elevation ( ). Question: no wind effect near zenith??

Figure: Antenna position (blue) and tracking errors (red) of three drift subscans from three different on-the-fly scans, with two zoom factors for each. The tracking errors are displayed from a unique central position in order to visualize the dispersion in both directions, and a magnification factor is applied for better visibility. All plots show the azimuth-elevation sphere under an arbitrary orientation and the associated grid. Top: On-the-fly scan from the MRTCAL test data, at about elevation. Center: On-the-fly scan near the zenith (); subscan drifts along right-ascension, nearly equivalent to elevation scanning for the source and date-time of observation. Bottom: Same, but scanning along declination, nearly equivalent to azimuth scanning.