Pointing 970630 - 970706

The table below shows recent SCUBA pointing data :

      1       2     3    4          5           6           7

  dataset     N    rms  rms       del-vs-     new_rms    comments
                   daz  del     0.5*(Tf+Tb)

   970630    25    1.8  1.4     2.8 +- 0.7     1.1   pointing run
   970701    25    1.4  2.5     0.8 +- 0.2     2.0   comet obs. till 13:30
   970702    27    1.3  2.6     2.0 +- 0.4     1.3   2 deviant in daz
   970703    19    0.9  2.8     1.1 +- 0.2     1.6   no daytime observing
   970704    20    1.9  2.9     1.0 +- 0.2     1.8   RxB3 obs. till 13:30

The erractic behaviour in azimuth in the 970630 data (see the report ), prompted speculation that either the behaviour in the central bearing had changed, or that the empirical software correction to the azimuth pointing was ineffective. Only 3 of these pointing data were taken within 1 degree of the azimuths affected by the fault, and none of them are particularly deviant in daz. Rather, the behaviour of daz is erratic when considered as a function of time, indicating an historical effect - i.e. dependent upon previous antenna position. For instance ?   Do the daz 'glitches' correspond to passages over central bearing problem azimuths ?   Well : the 3 largest azimuth changes are as follows :

     change in daz    HST   change in azimuth    change in elevation
         +3.3"       21:50      378 to 366          71 to 30
         -2.9        21:56      366 to 385          30 to 46
         -3.0        22:56      135 to 143          76 to 41

Notes to other datasets :
970701 - There is a strong trend of daz with time (temperature ?) throughout first shift, while daz is randomly distributed in time during the second shift.

970702 - Two data have clearly discrepant azimuth residuals, and both of these data are at azimuths affected by the central bearing. Without these two (N=25) the azimuth residual rms reduces to 0.9". However, there are 4 other data at suspicious azimuths that are not deviant.
There is also a strong trend of del with time (temperature). The doors and roof were open during the afternoon although no observing was done.

I am tempted to conclude :

• The telescope 7-parameter telescope model is good at the 1.0 - 1.5" rms level.
• Elevation residuals worsen with anomalous refraction.
• There are thermal effects upon elevation, and possibly azimuth, that are not not being accounted for. Exposing the telescope during the day to the Sun, or even perhaps the sky, is enough to induce strong systematic change in del through the night. The slope of the del -vs - 0.5*(Tf+Tb) relation at these times is still 1.0, as seen in a previous analysis .
• Data at azimuths most likely to be affected by the central bearing problem are not always discrepant, indicating that
  • the behaviour of the bearing is variable, or
  • the empirical model is no longer valid, or
  • the empirical model never was perfectly valid

• the tracking performance over the critical azimuths.
• the TEL parameter tel_slope_mean .

To this end, the tel_mean_slope parameter in the TEL task was changed to (-)1.0"/deg on 970705 , since the following few days had scheduled day-time comet observing. (The TEL task requires the negative of the slope shown in the above table). Results were encouraging and seemingly pre-empted the expected variation in elevation pointing :

      1       2     3    4          5           6           7

  dataset     N    rms  rms       del-vs-     new_rms    comments
                   daz  del     0.5*(Tf+Tb)

   970705    17    1.4  2.3    -0.1 +- 0.6     1.9   (see notes)
   970706    24    1.3  2.9    -0.4 +- 0.2     2.6   seeing awful

The nights were not ideal, with a decrease in del on 970705 only around dawn, and in a manner correlated more with (Tf-TB) than with 0.5*(Tf+Tb), and with appalling seeing (>5") after 10pm on 970706 . Nonetheless, they support the good azimuth performance, and show that the elevation performance can be maintained independent of antenna temperature with judicious use of the tel_mean_slope parameter.

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imc 970708