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| Paper: |
Using the Radial-Velocity Spectrometer at the Dominion Astrophysical Observatory |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
98 |
| Authors: |
Skuljan, J.; Hearnshaw, J. B.; Cottrell, P. L. |
| Abstract: |
Precise absolute radial velocities have been measured for several hundred late-type stars (GO--M5) using spectra which have been collected at Mt John University Observatory with the 1-m telescope, fibre-fed echelle spectrograph, and Thomson CCD. Six echelle orders in the green (500--560 nm) are used. Many delicate steps have been undertaken in order to maintain exactly the same conditions both in recording and reducing the spectra, over a period of 17 months (Nov 95 - March 97). All images have been filtered for cosmic rays and background scattered light, and a 1-D flat-field correction has been applied to the extracted spectra. The wavelength calibration has been performed by fitting a 2-D polynomial to a set of thorium emission lines in all orders simultaneously. The actual choice of thorium lines turned out to be critical, causing a significant shift in radial velocities. A cross-correlation technique with theoretical spectra computed by R Kurucz had been chosen to determine the absolute radial velocity. Blue sky spectra have been used to monitor systematic zero-point fluctuations from one observing run to another. An additional correlation between the measured velocity and average number of A/D units in the continuum has been discovered and used for fine adjustments, significantly improving the results. A random uncertainty of 13 m/s has been reached for solar spectra over the whole period of time, and about 20 m/s for stellar spectra having intrinsically constant radial velocities. Velocities for about a dozen standard RV stars have been computed and compared with their IAU values. The residual velocities for all spectral types have a standard deviation of about 0.4 km/s, with no significant zero-offset. |
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