Spectroscopic observations and data reduction

2.4.1 X-shooter on the Very Large Telescope

X-shooter (Vernet et al., 2011) is the first of the 2nd _{generation instruments at the Very}

Large Telescope array of the European Southern Observatory. The instrument covers a

wide wavelength range of 3000 to 25000 ˚A with three echelle spectrographs: the ultraviolet-

Figure 2.8: Examples of 2D spectra of a ThAr calibration exposure in the UVB (top left), VIS (top right) and NIR (left) arm of X-shooter, illustrating the echelle nature of the three spectrographs (Vernet et al., 2011).

resolution of R = λ/∆λ = 5000 – 9000. The detectors are CCDs, cooled with liquid

nitrogen to temperatures of 153 K, 135 K and 105 K respectively, to minimise dark current. The good resolution and the fact that the instrument is mounted on an 8.0 m VLT telescope, make it possible to obtain phase-resolved spectroscopy of short-period white

dwarf binaries, because exposure times as short as ∼ 5 minutes can be reached while

keeping a good signal-to-noise level (Bours et al., 2014a, chapter 3). The wide wavelength coverage in particular makes it an ideal instrument for studying radial velocity variations in white dwarf + M-dwarf binaries, because the two stars dominate the flux in different parts of the spectrum (Copperwheat et al., 2012; Parsons et al., 2012a,b).

X-Shooter data reduction with ESO Reflex

To reduce X-Shooter data I use the X-Shooter pipeline in the ESO Reflex workflow man-

agement tool2. The reduction includes several steps. In the first step the master calibration

files are created, which includes the master bias, the master dark frame and the master flatfield, from individual bias, dark and flatfield frames. Note that in the standard reduc- tion the UVB and VIS arm data are debiased using the overscan regions, rather than the master bias frame, although the user can specify this according to his/her preferences. In the second step an initial guess is made at the wavelength calibration and the position of the echelle orders on the CCD. This is done by using a physical model of the instrument combined with information on the atmospheric pressure and temperature, and the corre- sponding instrument settings as saved in a data fits file of a calibration arc frame. Then a flat field frame is used to determine the central positions of the individual orders. During the third step a detailed wavelength calibration is performed and spatial distortions are determined to update the rough model made in the previous step. If standard star observa-

tions have been taken within±3 days of the science observations, the instrument response

curve will be determined in a fourth step. This curve will later be used to flux-calibrate

the science data. In the last step the science data are extracted, the orders are connected and the sky level is subtracted, after which one has obtained the 1D science spectra.

2.4.2 STIS on the Hubble Space Telescope

The Space Telescope Imaging Spectrograph (STIS; Woodgate et al., 1998) is a 2nd gener-

ation instrument and a general purpose spectrograph aboard the Hubble Space Telescope (HST) since February 1997. It has an imaging and a spectroscopic mode, both with sev- eral options to set the resolution and wavelength range. The total wavelength range covers four bands in the far-ultraviolet (FUV), the near-ultraviolet (NUV), the visible and near-

infrared and ranges from 1150 ˚A to 10000 ˚A.

The visible / near-infrared detector is a conventional CCD, cooled to 190 K, while the FUV and NUV detectors are multi-anode multichannel array (MAMA) detectors. These are photon-counting photocathodes, which can operate uncooled and reject visible photons to avoid contamination from this regime where sources are often brighter than in the ultraviolet. The MAMAs were preferred over CCDs because the latter’s quantum efficiency tends to degrade quickly at ultraviolet wavelengths and because even before degradation

the combined quantum efficiency of a CCD and the throughput of available filters3is lower

than that of the MAMA.

All STIS data are automatically reduced by a pipeline at the Space Telescope Science Institute (STScI) where the HST headquarters are located.

2.4.3 COS on the Hubble Space Telescope

The Cosmic Origins Spectrograph (COS; Green et al., 2012) is a 4thgeneration instrument

that has been on board the HST since May 2009. The instrument was developed to com- plement STIS and to operate simultaneously with STIS, thereby increasing the wavelength

range of observations. COS has a far-ultraviolet (1150 – 2000 ˚A) and near-ultraviolet

(1700 – 3200 ˚A) optical channel, with a microchannel plate detector in the FUV arm to

maximise quantum efficiency (see Vallerga et al., 2001, for details), and a redundant STIS MAMA detector in the NUV arm. Both arms have a minimum of optical elements to decrease losses through inefficient reflectivity, which can be significant at ultraviolet wave- lengths. Various gratings with different spectral resolution and central wavelengths can be chosen, to optimise the observation according to the science goal. The instrument also has an imaging mode, which is primarily used for target acquisition.

COS was originally designed to operate for 3 years, but it has spectacularly outper- formed these expectations. However, the FUV detector sensitivity has been degrading since the start, and has become significant in the last few years. This complicates the relative flux- and wavelength-calibrations of the detector (private communication J. Debes). This is a general problem of UV-astronomy, and from cycle 21 in 2014 onwards this prompted an HST UV-initiative in which proposals relying on UV observations were favoured in order to make full use of HST’s UV capabilities before the relevant instruments become unusable.

All COS data are reduced by the automatic pipeline at the STScI.