Summary
Recently, large elevation errors were seen as a function of azimuth. They most likely suggest a ~1s error in the clock. This was addressed during the early part of the shift. New software was tested that should correct the elevation pointing problems most easily seen during transit. A sign convention error was detected and corrected. Subsequent allsky pointing suggested that the pointing was good (1.5" rms in elevation), although a confirmatory transit tracking measurement is still needed.

Poor pointing was reported during UT 20010617/8/9. Adding these data together it was clear that the elevation pointing was a very strong function of azimuth : see below left.

TPOINT solutions indicated either a clock error, IH, of +10" (i.e about 2/3 of a second, above right) or a similar error in AW (the main axis tip, E-W : hopefully nothing this drastic could have happened; the corresponding corrected data are similar to those above right).

Data from UT 20010613 might have suffered similarly, while those of 20010527 do not. In between times there is insufficient data to judge when the problem may have started.

UT 20010620
Nick Rees examined the clock and discovered a frequency error. The IRIG-B clock signal is not reaching the antenna servo micro (ASM) and the ASM has probably been free-running for a while. Nick's estimate of the error is about 1 second, which correlates well with the pointing data above. The clock signal looks the same entering the ASM as leaving the clock, so the problem seems to lie in the ASM or its boards. Nick reset the ASM, and it synched OK to the clock, but still now generates/shows a time offset error and a frequency offset error. This will probably allow the ASM to keep time well enough for a night while free-running internally, but needs a longer term solution. Nick fetches the clock board from UKIRT, but upon return the offset errors have gone !

Initial pointings indicate no major impact of the corrected leap-second count. Various SCUBA observing mode tests were done. A transit tracking was done on g343.0 using the new algorithm described on 20010523. The resulting data are shown below :

The large change (~12") in the elevation residual is nearly double that previously seen at this el*evation, implying a probable sign error in the algorithm. The parameter SCURVE.AMP held in (/net/ieie)/jac_sw/itsroot/src/tcs/thi/THI.tide was negated (now -4"). It would have been nice to repeat the transit tracking but no suitable sources were available and the repeat will be requested from future EAC shifts. Meanwhile some allsky pointing was performed :

Given the seeing during most of this was ~1" the rmss of (2.0",2.1") seem quite acceptable.