Revisiting the impact of atmospheric dispersion and differential refraction on widefield multiobject spectroscopic observations: From VLT/VIMOS to next generation instruments

From National Research Council Canada

DOI	Resolve DOI: https://doi.org/10.1051/0004-6361/201423459
Author	Search for: Sánchez-Janssen, R.¹; Search for: Mieske, S.; Search for: Selman, F.; Search for: Bristow, P.; Search for: Hammersley, P.; Search for: Hilker, M.; Search for: Rejkuba, M.; Search for: Wolff, B.
Affiliation	National Research Council of Canada. National Science Infrastructure
Format	Text, Article
Subject	Atmospheric movements; Refraction; Scheduling; Surveys; Atmospheric dispersion; Atmospheric effects; Instrumentation: spectrographs; Methods:observational; Overall efficiency; Parallactic angle; Spectral distortions; Techniques: spectroscopic; Instruments
Abstract	Context. Atmospheric dispersion and field differential refraction impose severe constraints on widefield, multiobject spectroscopic (MOS) observations, where the two joint effects cannot be continuously corrected. Flux reduction and spectral distortions must then be minimised by a careful planning of the observations, which is especially true for instruments that use slits instead of fibres. This is the case of VIMOS at the VLT, where MOS observations have been restricted, since the start of operations, to a narrow two-hour range from the meridian to minimise slit losses, the so-called two-hour angle rule. Aims. We revisit in detail the impact of atmospheric effects on the quality of VIMOS-MOS spectra with the aim of enhancing the instrument's overall efficiency, and improving the scheduling of observations. Methods. We model slit losses across the entire VIMOS field of view as a function of target declination. We explore two different slit orientations at the meridian: along the parallactic angle (north-south), and perpendicular to it (east-west). Results. We show that, for fields culminating at zenith distances larger than 20 degrees, slit losses are minimised with slits oriented along the parallactic angle at the meridian. The two-hour angle rule holds for these observations using north-south orientations. Conversely, for fields with zenith angles smaller than 20 degress at culmination, losses are minimised with slits oriented perpendicular to the parallactic angle at the meridian; MOS observations can be effectively extended to plus/minus three hours from the meridian in these cases. In general, night-long observations of a single field will benefit from using the east-west orientation. All-sky or service mode observations, however, require a more elaborate planning that depends on the target declination, and the hour angle of the observations. Conclusions. We establish general rules for the alignment of slits in MOS observations that will increase target observability, enhance the efficiency of operations, and speed up the completion of programmes, a particularly relevant aspect for the forthcoming spectroscopic public surveys with VIMOS. Additionally, we briefly address the non-negligible impact of field differential refraction on future widefield MOS surveys. © ESO, 2014.
Publication date	2014
In	Astronomy and Astrophysics 566, A2 (2014).
Language	English
Peer reviewed	Yes
NRC publication	This is a non-NRC publication "Non-NRC publications" are publications authored by NRC employees prior to their employment by NRC.
NPARC number	21272317
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Record identifier	645a79a9-33d8-4359-91f3-7cbc55bb6f7c
Record created	2014-07-28
Record modified	2020-04-22

Date modified:: 2024-04-19