Pointing and Tracking 970404,05

3c273 was tracked over an azimuth range that included two azimuths where the central bearing effect is large :

The residuals suggest that the current TEL_EMPIRICAL correction is no longer adequate, with significant structure appearing in the daz-vs-az diagram at azimuths 110 and 126 - see the current version of TEL_EMPIRICAL

The azimuth coverage is not complete between these, but the peak-to-peak amplitude of the effect is approximately 2.5" at (az,el)=(110,50), and 2" at (126,62), and is compatible with cos(elevation) as before. The data in the figure have not been corrected for the time-lag (and therefore azimuth-lag) of the logging procedure compared with the mid-point of the pointing measurement, but there is no clear evidence of a change in phase or period of the current correction.

Some have questioned the veracity of these conclusions in the light of the poorish azimuth coverage. In any case, I have asked for an immediate repeat of this experiment with TEL_EMPIRICAL disabled in order to re-establish the correction.

Pointing was done in the south on the evening of 04 April and in the north that morning.

The 58 evening data (many of Mars and 3c273) showed a strong systematic of del with azimuth, and curvature in the daz -vs- elevation digaram, indicative of an error in parameter #3 - the orthogonality of the major axes. The data was processed through FIT9 and a new model (#443) created, despite the unavailablity at that time of the southern data due to problems with the pointing.log file structure. The residuals before and after the fit were, in (daz,del) : (2.2",2.4") and (1.5",1.4").

The morning data was dominated by the increase in del with time through dawn - giving another strong correlation between del and (Tf-Tb), the temperature differential between the front and back antenna legs. After removal of this effect the 29 morning data gave rms residuals in (daz,del) of (2.1",2.0") with no obvious systematics - indicating perhaps generally poor seeing.

The 87 data together, corrected for thermal trends give rms's of (2.2",2.8") - the trend of daz with elevation persists, but the del -vs- azimuth systematic is not continued into the 4th & 5th quadrants in any coherent fashion. As a result it is not well removed this time using FIT9 - residuals after this fitting are (1.9",2.2").

The 7-parameter model suggested by all 87 data is not so different from that (#443) produced from the southerly data alone - so I shall leave #443 in place for now. Its performance since its inception has been confused by very poor seeing and by trends of del with time (temperatures) that persist despite the changes to the temp_slope parameter made last month.

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