The orientation of spectral traces on the STIS detectors has been found to rotate slowly over time. Additionally, there is a small random offset in the orientation each time the MSM is positioned. The time-dependent component of the orientation has been calibrated for the most commonly used grating-wavelength combinations in SPTRCTAB reference files that were delivered in August and October 2006. The purpose of the new "mktrace" task is to generate an accurate SPTRCTAB for a specific science image, thus correcting for both the random offset and the temporal dependence in the trace orientation for any wavelength setting of any L or M first-order grating.
The new task computes the trace of the science target using the default aperture location or accepting an input location. A linear fit is made to this trace. The user can choose to exclude portions of the trace from the fit because of velocity structure, low signal-to-noise, etc. A calibration trace is interpolated from the traces at the nearest rows in the SPTRCTAB, and a linear fit is made using the same ranges of columns selected for the fitting of the science trace. Image files of the target and calibration traces and of the linear fits are generated to assist the user in selecting the portions of the spectrum to include in the fits. The difference in the angle of the two linear fits is the correction that must be applied to the SPTRCTAB traces. A new SPTRCTAB is generated, applying this correction to all of the traces for that grating-cenwave combination, thus covering the entire detector.
This task is included in STSDAS release v3.6. Details on the use of the new "mktrace" task are provided as part of the STSDAS release in the form of a help file.