SUMMARY
These data are a continuation of the effort to reveal the extent and size of the elevation pointing errors induced by the reversal of direction in elevation that becomes so apparent during transit tracking (see the interim summary report of 20000408 , the problems page on the web, the update of 20000417 , or just follow the reports during April 2000 ).

Tracking data of 0923+392 through the north at a transit elevation of 70 degs do not show quite the same pattern of residuals as seen in the south, but both transit regions have the +4" change in common.

Pointing data of 0923+392 during transit were collected on UT20000421 with SCUBA in its conventional POINTING mode during the period HST 18:50 to 20:50. The behaviour of the azimuth and elevation residuals in the azimuth plane are shown below left and in the elevation plane below right:

There is strong trend of the elevation residual with the mean temperature of the antenna running through these data, but the plots ( -vs-az and -vs-el ) show little qualitative change after this thermal trend has been removed.

The seeing during this period was about 0.4". While there is a slight (2") hump in the residuals following transit (remember that time runs right to left through the azimuth diagram), and while the noise in these data is higher, there is no strong similarity to the transit data from the south. Repeats of this experiment during the following two evenings are shown below. Those of UT20000422 used the POINTING method, while those of UT20000423 used map16 :

The data of UT20000422 are rather limited but resemble the corresponding part of the earlier data. The data of UT20000423 are much more extensive. (Short gaps in the data are where the focus was checked). Azimuth residuals of 0.9" rms illustrate the rather poor quality of the night (seeing was recorded as typically 0.5"). Even so, there are similarities in the elevation residuals, with a general rise after transit and some confirmation of the 2" hump at azimuth 350^o. Although without such a clear pattern as is seen in the south, the elevation pointing performance of the telescope during transit in the north also involves an overall +4" change :