The SMU topend was removed from the telescope on Tuesday November 18 because too many RMS limit errors were occurring. An inspection, which was made on Monday, made it appear that the problem was mechanical. After the SMU was setup on the carousel floor it was determined that much of the "difference" in how the two axes were feeling was due to one dust boot on the voice coil vibrator on the EW axis being missing and the boot on the other vibrator on the same axis being in pieces. After considerable electronic characterization, the chopper was disassembled and the mechanical parts were cleaned and inspected. When the newly cleaned chopper was reassembled, it was determined that the dynamic characteristics of each axis had changed considerably. Some mechanical pieces were swapped between axes to make the characteristics of each axis similar. The chopper could then be run at full rate and throw without RMS limit errors. The topend was replaced on the telescope on Friday. Over the weekend tests on the sky showed that the beam width in the NS axis was 20 arc-seconds while the beamwidth on the EW axis was the nominal 15.5 arc-seconds (14 arc-seconds actual beam plus 1.5 arc-seconds of "smearing"). This extra smearing along the NS axis was due to an overshoot in that axis, which Per measured to be at least 10 arc-seconds. It was then determined that, if the Crown amplifier on the NS axis is turned up to 10 (maximum), much of this overshoot goes away. Tests will be performed to see if this fix is sufficient for all JCMT operations. Until further notice the Crown amplifier setting for the NS axis should be turned up full and that for the EW axis should be left at 9.

Craig Walther