Analysis of recent elevation pointing

Recall that, earlier in the year, elevation pointing data revealed that del may be dependent upon the mean leg temperature, 0.5*(Tf+Tb), as well as upon (Tf-Tb), the difference between the temperatures of the front and back legs, particularly after the telescope has been exposed to the sky during the days - the so-called `comet-observing' effect. The coefficient on (Tf-Tb) has remained at 3.5"/deg throughout, and since inspection of the data in this period rarely showed any significant deviant behaviour of elevation pointing attributable to an incorrect value of temp_slope, this analysis focusses exclusively upon temp_mean_slope.

The table below shows the recent data suitable for analysis. The suggested coefficents incorporate, i.e. correct for, the current value of temp_mean_slope. The columns show

1. the dataset, in yymmdd(_hh) format
2. the FE
3. the number of data
4. the raw rms scatter in daz
5. the raw rms scatter in del
6. the suggested value of the coefficient of 0.5*(Tf+Tb) (for my own benefit as much as anything I note that the current value of temp_mean_slope of -1.0 matches a slope in the del -vs- 0.5*(Tf+Tb) plane of +1.0).
7. the rms scatter of del if this value had been used - an asterisk follows if the improvement is significant or, at least brings the elevation data in line with the azimuth data.
8. the range of 0.5*(Tf+Tb) covered by the data
9. comment regarding earlier `comet' observing : time of last observation, if after normal end of 2nd shift. If the directory name is not indicated the FE being used was SCUBA.

      1       2  3     4   5        6         7         8           9

  dataset    FE  N    raw rms    coeff of   new_rms   range   comet obsn ?
                      daz del   0.5*(Tf+Tb)           (deg)   HST last obs

  970710      B 13    1.2 2.3  -1.6 +- 0.2    1.9    2 - 7      no
  970712_18   S 20    1.8 1.6  -0.5 +- 0.2    1.3*   0 - 4    13:00 C08
  970713_17   S 10    0.9 1.5  -1.5 +- 0.3    1.3*   0 - 4    13:15 C08
  970720      B 10    1.9 2.5  -1.0 +- 0.3    2.5    3 - 9    13:10 C08
  970725      B 11    1.3 4.3   0.1 +- 0.4    3.5    3 - 10   10:30 N07

  970726      B  9    1.4 3.1   0.1 +- 0.5    2.5    5 - 10   13:30 C08
  970727      S 16    2.5 2.9  -1.5 +- 0.6    2.9    7 - 11   13:00 C08
  
  970803a     S 15    1.8 3.4  -1.7 +- 0.3    2.2*   4 - 10   13:00 C08
  970804      S 12    1.9 2.9  -2.2 +- 1.5    2.8    3 - 5    14:30
  970805      S 12    1.1 3.3   2.2 +- 0.7    2.0    4 - 6    14:00
  970806      S 31    1.5 2.6  -0.6 +- 0.2    2.5    4 - 10   10:00
  970807      S 27    1.2 3.5   0.0 +- 0.3    2.9    2 - 8    10:00

  970808      S 22    1.2 2.8  -0.5 +- 0.3    2.7    2 - 9     no
  970809      S 20    1.0 1.6  -1.0 +- 0.2    1.6    3 - 8    11:00
  970810      S 17    0.9 1.7  -0.9 +- 0.2    1.7    3 - 9    13:00
  970811      S 16    1.3 2.4  -0.6 +- 0.2    2.1    3 - 10   13:30
  970812      S 15    1.5 1.7  -0.7 +- 0.2    1.6    3 - 9     no

  970814      S 24    1.1 2.4  -0.3 +- 0.2    1.9*   0 - 7    13:30
  970817      S 23    1.9 1.3  -0.5 +- 0.1    0.9*   3 - 8    13:30
  970818      S 35    2.1 2.4  -0.6 +- 0.1    2.1*   3 - 10     no
  970819      S 90    4.2 1.6   0.1 +- 0.1    1.1*   4 - 9      no
  970820      S 14    1.8 2.9   0.1 +- 0.2    1.7*   4 - 10     no

  970821      S 10    1.2 3.9   0.8 +- 0.2    1.2*   5 - 11     no
  970822      S 24    1.6 3.6   1.0 +- 0.2    1.3*   5 - 10     no
  970823      S 19    2.0 3.0   0.7 +- 0.2    1.2*   5 - 10   12:30
  970824      S 20    2.7 2.6   0.0 +- 0.2    1.7*   5 - 11   12:00
  970825      S 27    3.1 3.8   0.5 +- 0.2    2.0*   4 - 11     no

  970905      S 30    1.8 3.8   0.8 +- 0.1    1.2*   4 - 10     no
  970906      S 27    1.6 1.9  -0.2 +- 0.3    1.7    5 - 9    12:45 C08
  970907      S 26    1.8 2.5  -0.3 +- 0.2    2.2    5 - 11   12:45

For those results above deemed `significant', pointing data following daytime (`comet'-) observing suggest values of temp_mean_slope of between -1.7 and 0.7 "/degree, with a mean of -0.5 +- 0.3 (s.e.m.), whereas data not preceded by daytime observing suggests values between -0.6 and 1.0"/deg, with a mean of +0.4 +- 0.2 (s.d.).

Conclusion
No clear trend is obvious from the above results.
Since comet observing as such is becoming less of a daytime activity (see HEM) I shall disable (set to zero) the temp_mean_slope parameter until further notice. It is certainly not clear that any (other) value is any more beneficial to giving the general impression of accuracy in pointing.

I recommend however, that regular pointing be done in any case to counter the lingering effects of any recent treatment of the telescope, especially as regards exposure to the sky during the day.

Return to POINTING REPORTS

Iain Coulson
08 September 1997