Astron. Astrophys. 318, 198-203 (1997)

2. Spectroscopic observations

Optical spectropolarimetric data were obtained with the 3.9 meter Anglo-Australian telescope on 9 and 11 January 1995. The weather was adequate for spectroscopy on January 9. Useful spectropolarimetric data could be taken on January 11.

The instrumental set-up included a half-wave plate rotator set to various angles to obtain the Stokes parameters and a calcite block to separate the light leaving the retarder into perpendicularly polarized light waves. Two holes in the dekker allow one to simultaneously observe the sky background. Four spectra are recorded, namely the O and E rays of the target object and of the sky respectively. One complete polarization observation consists of a series of four consecutive exposures at different rotator positions.

A 1024 1024 pixel TEK-CCD detector was used, which combined with the 250B grating resulted in a wavelength coverage from 3786 - 7510 Å, at a projected pixelsize of 3.6 Å. Medium resolution observations with the 1200V grating yielded a coverage from 6335 - 6882 Å, in steps of 0.54 Å. Wavelength calibration was performed by observing a copper-argon lamp before each exposure. Spectropolarimetric standards were observed throughout the night. The instrumental polarization was found to be negligible.

One series of 4 spectra of HD 76534 was taken in the low resolution setting on 9 January 1995. The exposure time was 50 s for each wave plate position. On 11 January, 2 4 sets of data were taken in the low resolution setting, again with exposure times of 50 s. Two hours later, HD 76534 was observed at medium resolution. Two series of spectra with integration times of 300 s were taken. A log of the observations is provided in Table 1. In the remainder of this paper, we will denote the polarization spectrum taken on 9 January 'Run 1', those taken on 11 January Runs 2A and 2B for the two series of low resolution spectra, and Runs 3A and 3B for the high resolution spectra.

Table 1. Log of the observations

The initial processing of the data was performed in the IRAF package. After bias-subtraction and flatfielding, the individual spectra were extracted by tracing the pixel rows containing the data. The wavelength calibration was performed by fitting a order polynomial through more than 20 identified lines in the arc spectra, and resulted in dispersion fits with a of 0.5 and 0.07 Å for the low and medium resolution spectra respectively. The resulting spectral resolution as measured from the arc spectra is 8 Å and 1 Å respectively.

The E and O arrays were then extracted into 2-dimensional frames and imported into the Time Series/Polarimetry Package (TSP) incorporated in the FIGARO software package maintained by STARLINK. The Stokes parameters were determined and subsequently extracted.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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