SUMMARY
3c279 was tracked for an hour, primarily to obtain beamsize data, but in passing repeats recent tracking experiments, such as that on 991228 that reveal an apparent discontinuity in the elevation pointing across transit. The discontinuity at transit is again seen.

Data was collected with SCUBA, using continuous jiggle mapping, as described here , on a night when tau was too high (> 0.1) to do the scheduled observing.

• from (az,el) = (168,64) at HST 00:12 to (213,60) at 01:38.
• transit occurred at HST 00:33 ata ana elevation of 64.4 degrees.
• (Thanks, Henry).

The resulting pointing residuals are shown below :

• The rms scatters in (daz,del) are (0.6",1.6"), although the subset of data with azimuth > 180 degrees have a scatter in del, and incidently in daz, of 0.4", which is probably a better measure of the intrinsic tracking error. The seeing during this hour was also recorded as 0.4".

• The performance is similar to that seen previously ( 990618 , and particularly 991228 ). Beyond a generic feeling of possible hysteresis in the elevation bearing, we have not identified the cause of this sudden 4" change in elevation pointing.

Beam Map

This map was intended for analysis for the SCUBA beamsize : given the sky opacity this restricts the meaningful results to 850um. It was scheduled so as to provide a large amount of field rotation and hence the smallest possible angular resolution. In this particular hour or so, the parallactic angle (= the field rotation) changed by almost 60 degrees, which, by chance, is the angle between inner ring LW bolometers. With 30 integrations spanning this separation of about 25" the resolution of the final image may then be claimed to be less than 1".

Individual integrations of the LW data were reduced as normal for a jiggle map and the centroid determined. The map was then reconstructed with a pixel size of 1" in the AZ/EL plane with the centroids aligned. The lowest 10 percentile contours are shown in the image below :

The salient features of the reconstructed image are

• The core of the image appears reasonably symmetric. There are small, symmetrically located sidelobes (maybe side-'features' only) lying in the chop direction. They are of amplitude 2%, which may be within the noise in the data judging by the other stuff in the frame, and an explanation in terms of deficiencies in the 120" chop waveform seems confounded by their location 40" from the image centre.

• Analysis of the image using psf gives a mean FWHM of 14.9", an axis ratio of 1.06, the orientation of the major axis is 135 degrees, and the value of Gamma (the radial fall-off) is 2.14 (a gaussian has a gamma of 2).

• For what it's worth, the 450um image gave a mean FWHM of 6.9" (ratio 1.29, orientation 164 degrees, Gamma 1.8).