SCUBA clock error report

Summary

On 23 May 2000, it was noticed that the clock on SCUVAX (the SCUBA data acquisition computer) was running 6 minutes and 22 seconds behind the GPS clock connected to MWTTEL. In the current SCUBA design all the times stored in the data headers are determined from the SCUVAX clock rather than the telescope control computer. These times (in particular the local sidereal time for the start of every switch) are critical for the data reduction software since sky rotation and telescope elevation are calculated in software.

In most cases the positional error at the edge of the array will be less than an arcsecond. In a worst case the positional error could be as high as 10 arcseconds. IN ALL CASES TELESCOPE TRACKING WAS CORRECT.

In general only data taken at high elevation requiring accurate positions at the edge of the SCUBA array will be affected.

A fix has been developed and version 1.6 of SURF includes compensation for the clock error.

A document explaining fully the problem and the fix is available here

If you are worried that your data have been affected a program to calculate the error for a particular data file is distributed with SURF. Simply install SURF and run the 'scuclkerr' program.

Please feel free to email us with any additional questions.