Pointing on the evening of 970409

Summary : Pointing with SCUBA (Mars + blazars only), plus a flatfield on Mars.

About 45mins lost at the start with SCREEN task problems, although the weather was still poor then. Also some trouble with the 'moments' method of determining the image centroid when there is (not-so-)nearby structure on the array.

42 pointing data were logged.

One azimuth residual looks suspiciously aberrant, and the elevation data are affected by a slightly inaccurate value of temp_slope and so show a systematic decrease with time ( = temperature differential between front and back legs). Removing this one point and correcting the elevation data for the temp_slope effect leaves 41 data with rms errors in (daz,del) of (1.6",2.0").

These data were run through the FIT9 routine to correct the 7 parameters of the telescope model. The resulting model (#445) should behave thus and yield rms scatters of (0.9",1.6").

At about 23:00 John did a flatfield on Mars :

In my hour of observing I took a flatfield scan map of Mars using the long-wave array with a sample spacing in x and y of 3 arcsec (observation 66). The bug that caused the data reduction task to reject exposure numbers greater than 67 appears to have been fixed. A few of the early exposures - 9, 13, 17 are flakey but after that the system appeared to operate consistently. Skydips before and after the scan flatfield indicate that tau varied from 0.20 to 0.21 over that period, though the skydip fit needs some work because the `fixed' parameters weren't very fixed.

Thereafter we tested the new model (#445) : obtaining pointing data for 16 sources with pretty decent coverage of az-el space. The data had (raw) rms scatters in (daz,del) of (1.0",1.8") - confirming that the new model is working (quite well, I'd say, without wishing to tempt fate).

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imc 970410