IOT Overview: Optical Multi-Object Spectrographs
We give an introduction to the several instruments that ESO operates and which are able to perform optical multi-object spectroscopy. We point out the standard ways of reducing these spectra, the problems that occur, and the way we deal with them. A short
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What is Multi-Object Spectroscopy? Quite self-explanatory, multi-object spectroscopy is an instrument mode in which spectra are taken of several objects at the same time. In general, this is achieved by placing individual slits on the targets (see Fig. 1 for an illustration). Pre-images are used to define these slits, and depending on the instrument specifics, masks are then manufactured according to these definitions and placed in the instrument. During the acquisition, a best match has to be achieved between the targets and the slitlets. A grating or grism is then used to disperse the light of all targets at the same time.
2 ESO’s Optical MOS Instruments ESO offers six optical instruments that can be operated in MOS mode. On La Silla, these are EFOSC2 at the 3.6 m telescope and EMMI at the NTT; on Paranal these are the two FORSes, VIMOS and FLAMES. In the following we will give a short overview on these instruments and their MOS capabilities. 2.1 EFOSC2 The ESO Faint Object Spectrograph and Camera v.2 (EFOSC2) [4] is a focal reducer instrument, allowing imaging and low resolution spectroscopy. The instrument has multi mode capability including normal/polarimetric imaging/spectroscopy (several submodes in each), multi-object spectroscopy and coronography. It is mounted in the Cassegrain focus of the 3.6 m telescope on La Silla. In general, MOS is executed in visitor mode. The MOS plates are individually punched, the possible slit widths are 1.15, 1.35, and 1.75 arcsec.
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Fig. 1. An illustration of multi–object spectroscopy. Small, individual slits are placed on the objects (left) to get a spectrum (right) for each of them
Up to 5 masks can be loaded at the same time, and an exchange of masks is also possible during the night. However, as this results in a time-loss of about 30 min it is recommended to avoid such changes if possible. EFOSC2 offers several grisms which provide FWHM resolutions between 0.6 and 6 nm and cover the range from 320 up to 1100 nm. 2.2 EMMI The ESO Multi Mode Instrument (EMMI) is a versatile instrument, allowing a wide range of observations like imaging and low, medium, and high resolution spectroscopy while using either the red or the blue arm. EMMI is mounted in one of the Nasmyth foci of the NTT. Again, the MOS observations are normally executed in visitor mode. The MOS plates are individually punched, the possible slit widths are 0.8, 1.02, 1.34, and 1.87 arcsec. Up to four masks can be loaded at the same time, but no exchange of the masks is possible during the night. The MOS mode of EMMI is only offered on the red arm in low resolution, thus constraining the spectral range between 400 and 1000 nm, and the resolution between 0.4 and 4.5 nm. EMMI’s MOS field of view is 8 × 5 arcmin. 2.3 FORS1 The FOcal Reducer Spectrograph, v.1 (FORS1) [1] is designed as an alldioptric optical instrument. It allows imaging and spectroscopy and has the
IOT Overview
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capability for polarimetry and spectro–polarimetry that can be combined with the MOS mode. FORS1 is mounted in
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