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Astron. Astrophys. 337, 757-771 (1998)

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3. Observations and data reduction

RE 1816+541 was observed with the 4.2m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos on the island of La Palma (Canary Islands) between 25-27 June 1996, using the Utrecht Echelle Spectrograph (UES, Unger & Pettini 1993). UES was used in conjunction with a 31.6 lines mm-1 grating and 1024[FORMULA]1024 pixels TEK 24 µm CCD. A 1.1 arcsec wide slit was used, which corresponds to two detector pixels and a resolving power [FORMULA]/[FORMULA] of 55000 (FWHM). The spectra cover the wavelength region 5350-9231Å which was recorded in 45 orders, allowing simultaneous observation of the H[FORMULA] and Ca II IR3 emission lines. Because of the limited size of the detector it was not possible to fit the entire echellogramme. The resulting gaps between consecutive orders were of [FORMULA]8% in the blue up to [FORMULA]59% in the red end of the range. The integration time was set at 200 s. Time overhead between subsequent exposures was [FORMULA]80 s, yielding a time resolution of [FORMULA]5 min. A log of the observations can be found in Table 1.


Table 1. Log of the spectroscopic observations of RE 1816+541 taken with the WHT/UES on 25-27 June 1996. The start and end times of continuous sequences of spectra are recorded. Gaps of 15-20 min were due to the observations of B stars and another active late-type star which was studied as part of a different project. Phases ([FORMULA]) have been calculated according to the ephemeris given in Robb et al. (1995), HJD=2449927.752 + 0.4589E

The three nights of observations were clear. Meteorological conditions were generally stable. Spectra obtained on the second night were of somewhat poorer S/N due to the high winds. Best seeing conditions were achieved on the last night, 27 June, with an average seeing disk, 1.1 arcsec. For the first and second nights, the average seeing was 1.4 and 1.3, respectively.

An adequate number of bias frames, flat frames and dark cound exposures were taken at the beginning of each night. Th-Ar calibration lamp spectra were taken throughout the night to account for possible shifts of the wavelength calibration. Spectra were processed semi-automatically using standard packages within IRAF (Tody et al. 1986).

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998