SCUBA Flux Calibration Through the Night

It is well-known that the surface of the telescope is not as good in the early evening and morning as during the night. Observers see SCUBA flux calibration factor (FCF) values that decrease after sunset. It is not easy to see exactly in which period the dish is good because a lot of work is needed to obtain a consistent data set. As part of an ongoing investigation into the SCUBA calibration, I have looked at SCUBA 850 micron data during 2003 when the surface had a relatively constant rms value during the whole year. This note shows some preliminary results of this investigation and is intended to make observers aware of different effects in the calibration. Later as more information is available and possible specific test observations are made, we will be able to study (and publish) this in more detail.

For this study I have used only calibration measurements of Uranus, collected by V.Barnard for the period May to September 2003 when the FCFs seem to have been fairly constant according to the SCUBA Gain Table. However the coverage of the data in Hawaiian Standard Time (HST) is not good - in this period Uranus was not visible at the beginning of the night. To remedy this I have collected some more data from the archive, omitting the period 10 February - 30 March 2003 when the FCF values were particularly bad (see the above web page). I used only days for which B.Weferling had derived csofits at 225 GHz (i.e. in 2003 before 11 July) which were used for the atmospheric calibration. First I took all jiggle maps of primary and secondary calibrators (except IRC+10216) with chopping angle larger than 90". This still did not result in a good enough data set. I added maps with 60" chopping angle of CRL618 and also pointing observations of this source. It appeared that FCF values in V.Barnards list and from the pointing and 60" Jiggle maps were about a factor 1.1 lower than for the other calibrator sources, so I used this correction factor in order to make the data points agree. This ad-hoc factor was not accurately determined and its cause still has to be investigated, and it should not be used for other purposes. The results are shown below (adding 24 hours for clarity for HST less than 12h.

The pointing observations of CRL618 (red symbols) show a steep increase in FCF at the end of the night, consistent with the other sources. Also at the beginning of the night the FCFs are higher, but with somewhat more scatter (for CRL618). Probably it would be better to plot these data versus time of sunset or sunrise rather than HST, which may be done in the near future.

Between 20h HST and 7h HST the FCF values are approximately constant. For this period I have plotted below the FCF as a function of elevation. There might be a slight increase, but it is within the noise. To confirm this it might be good to analyse data from IRC+10216 which reaches 83 degrees elevation, if there is an easy way to correct for its variability.

A previous analysis based on much less data from 2000 does not show the changes of the FCF with time. The reason is unclear at this time.

Jan Wouterloot

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