SUMMARY
56 allsky pointing measures are analysed and a new 7-parameter telescope model installed.

With the recent inclinometry suggesting a certain stability in the antenna, and new TMU positions determined following a mechanical repair on 990303, it seemed an opportune time to check the overall pointing model. Fifty-six data were obtained in 5 hours and showed raw rms scatters in (daz,del) of (1.8",2.3") :

The most obvious systematic is of del -vs- time (center-right panel above), which we interpret as being -vs- temperature :

The relationship derived :

          del = const - (6.2 +- 0.8) * (T_front - T_back)

is highly significant and implies that the current value of temp_slope in TEL_FULL.ifl should be changed from 5.0 to 11.2 ("/deg).

With this slope removed the rms scatters become (1.8",1.6") :

The most significant systematic now is that of del with elevation (center panel above), indicative perhaps of an erroneous value for the flexure of the secondary support structure. These data are run through the FIT7 program to derive changes to the 7-parameter telescope model. The changes suggested are shown below in the 5th column :

 N           OLD    FIXED=1      +          +            NEW

 1           9.5       0        0.0       -0.5           9.0
 2          17.7       0        0.0        0.4          18.1
 3          68.7       0        0.0      -14.1          54.6
 4         171.0       0        0.0      -15.1         155.9
 5   319 35 51.2       0        0.0      -10.4   319 35 40.9
 6   351 19 14.6       0        0.0       -1.7   351 19 12.9
 7         -12.3       0        0.0        2.7          -9.6

The fairly large changes suggested to parameters ##3,4,5 are in some sense only a set of self-consistent values, and, when optimised individually or in pairs, small (~2") changes are suggested. A solution keeping these three fixed yields a similar improvement to that above, and, following Occam, this solution is adopted :

 1           9.5       0        0.0       -0.5           9.0
 2          17.7       0        0.0        0.5          18.2
 3          68.7       1        0.0        0.0          68.7
 4         171.0       1        0.0        0.0         171.0
 5   319 35 51.2       1        0.0        0.0   319 35 51.2
 6   351 19 14.6       0        0.0       -1.7   351 19 12.9
 7         -12.3       0        0.0        2.7          -9.6

The data are plotted below as if taken with this new model :

The curvature of del with elevation is reduced but not eliminated, and the rms scatters are now (1.7", 1.4"). This is our standard performance level, but we seem to have gone around in a circle somewhat in restablishing this.

The new model has been installed (UT 19990306:13)