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Astron. Astrophys. 340, 569-578 (1998)

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2. Objectives

Only a few spectrometers based on two or more Fabry-Perot etalons have been built and successfully used in astrophysical applications. Very early approaches were made by Gehrke and Von Baeyer (1906) and Houston (1927). In the sixties several systems have been built, e.g. the PEPSIOS-type spectrometers (Mack et al. (1963), McNutt (1965), Stoner (1966), Ramsay et al. (1970)) and the triple FPI system of the Culgoora Observatory (Loughhead et al. 1978). Darvann and Owner-Petersen (1994) carried out a thorough study of a possible FPI-filter for the LEST telescope. Very recently, the two-dimensional spectrometer of the Göttingen group (Bendlin et al. 1992) at the VTT on Tenerife has been upgraded to a double FPI system from a combination FPI - universal birefringent filter. In addition to the scientific objectives mentioned in the introduction there were a few more technical requirements that entered into the instrument design. In order to keep setup time to a minimum, reasonable effort was spent to simplify necessary setup work and to automate it wherever possible. In order to minimize environmental influence TESOS was designed as a very compact instrument in a closed housing. All movable components are motorized to reduce disturbances by opening the instrument box. The basic requirements for TESOS were:

  • Wavelength range: 450-750 nm

  • Field of view (FOV): [FORMULA] 100 arc-sec

  • Spatial pixel resolution: 0.15 arc-sec/pixel

  • Spectral resolution: [FORMULA] 250.000 (@500 nm)

  • Frame rate: 3-5 frames/sec

  • Simultaneous measurement of filtergrams and continuum frames

  • Optional Stokes-V measurements

  • Fast prefilter change

  • Wavelength stability: [FORMULA] 0.5 pm/h

  • Thermal stability

  • Convenient user interface

  • Dual etalon system, with upgrade possibility to three etalons

Some of the above numbers are not independent of each other, and trade-offs had to be made between e.g. field of view and spectral resolution.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998