Inclinometry & Pointing     970625 - 970630

• Inclinometry done during poor weather (25th & 29th) and as part of a scheduled pointing shift (970630).
• On 970630 there was a 5.5mag earthquake at 05:50 HST, and a leap second at 14:00.
• Scheduled pointing on 970630_E was preceded by inclinometry also.
• New track models installed on 970629 and 970630.
• Pointing with SCUBA on 970630 confirmed that the leap second was accomodated by the updated software, but indicated non-random azimuth pointing residuals in azimuth.

Inclinometry (CW)

   Date      Start time (HST)    Mean leg teamperature (deg C)
  970625            03:00                   3.4
  970629            17:45                   5.0
  970630            17:45                   5.5

Comparing the data of 970625 with those following the last track weld on 970605 shows changes of as much as 3", not particularly associated with any joint, but with a hint of the 14-segment structure of the track.

Comparing the models of 970629 and 970625 shows only +- 1 arcsecond excursions. Rms scatters in (F1,F2,F3) for the difference between the two track models are (0.50",0.32",0.50"). However, these are not entirely noise spectra, with particularly noticeable spikes at azimuths 95, 120 & 145, and at 276, 300 & 326, where wheels 1 and 3 traverse the joints 2/3, 3/4, 4/5, 9/10, 10/11, and 11/12 in some order. A new track model was created and installed at 20:50 HST 970629.

A strong earthquake centred south of Pu'u O'o rattled the Big Island at 05:50 HST 970630. The opportunity was presented to repeat the inclinometry on 970630. Comparing the models of 970630 and 970629 shows +- 0.5 arcsecond excursions. Rms scatters in (F1,F2,F3) for the difference between the two track models are (0.16", 0.19", 0.27"), and this time they look like mainly random noise. A new track model was created and installed at 20:45 HST 970630.

Pointing was then done with SCUBA. Conditions were fairly decent, cso_tau about 0.09 throughout, seeing 0.2". 25 data were logged in about 2.5 hours, and gave rms scatters in (daz,del) of (1.8",1.4"). The leap second was clearly well accomodated by the software updates. (See the plot ).

This is somewhat disappointing. There is a slight (4-sigma) trend between del and the absolute temperature of the legs, with a coefficient of 2.8"/degree : recall that previous such trends seen particularly after the telescope had been pointed towards the Sun during the day showed average coefficents of about 1"/degree. Correcting for the 2.8"/degree effect reduces the scatter in del to 1.1". The azimuth residuals however behave in a less comprehensible manner, with possible glitches producing the observed distribution; certainly one would have expected better parity with the elevation residuals. These effects may be due to the faulty central bearing - further monitoring of the tracking performance is needed.

No improvement in the residuals is achieved by optimising the 7-parameters of the telescope model, so that model is left unchanged.

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imc 970630