3. Observations and data reduction
High-resolution spectroscopic observations of ER Vulpeculae were obtained during a 4-night observing run in July/August 1993 carried out with the Utrecht Échelle Spectrograph (UES) located on the William Herschel 4.2-meter Telescope (WHT) on La Palma. Descriptions of the telescope can be found in Bingham (1984) and Unger et al. (1988). The UES instrument (Walker & Diego 1985; Unger & Pettini 1993) is located at one of the Nasmyth foci of the telescope. The dispersed light was recorded on an EEV6 CCD detector with a pixel size of 22.5µm and dimensions of 1280 1180. The grating used gave a resolving power (R = / ) of 52,000. This corresponds to a velocity resolution of 5.77 km s-1 or wavelength resolutions of 0.058 Å, 0.115 Å and 0.192 Å at 3000 Å, 6000 Å and 10,000 Å respectively. Observations were timed to encompass the primary eclipse of the target. In total 22 spectra were obtained with 3 minute integration times over the range 4890-8680Å. Fig. 1 shows a schematic representation of the geometry of the ER Vul system for the duration of the spectroscopic observations. Due to the nature of the data reduction and analysis a small set of standard stars were also observed; these were 26 Dra (HD 160269), HR 7504 (HD 186427) and Per (HD 22928). The estimated seeing varied from 0.9 to 1.7 arc-seconds throughout the observations.
CCD reduction and spectrum extraction and calibration was performed with IRAF 1. After bias subtraction, image trimming and flat-fielding, 51 spectral apertures were defined by reference to bright star images and object spectra extracted. Th-Ar calibration frames were extracted and lines identified across all orders (typically 200 lines were defined). It was found that wavelength calibrations did not drift by more than 0.02Å and reduced spectra typically had signal-to-noise ratios in excess of 50 per resolution element. Normalisation of the extracted spectra was performed by fitting a spline function across pre-defined continuum points. Some spectral orders were corrected for the presence of atmospheric lines by means of bright B-star comparison spectra.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998