The observational study of the dynamics of solar small-scale structures is of high astrophysical interest, and the quality of the measurements has mostly been limited by the instrumentation. This motivated us to complement the focal plane instruments of our telescope with a newly designed filter spectrograph. The key characteristics were chosen to optimize the instrument for the investigation of magnetic elements and sunspot dynamics and magnetism, as well as for observations of chromospheric waves and their relationship to the photosphere. In the case of the sunspots a field of view of more than 60 arcsec is important to include the spot and the surrounding moat in the velocity and magnetic field measurements. This allows to treat e.g. the important question of mass conservation of the penumbral flow. (Westendorp-Plaza et al., 1997, Schlichenmaier et al., 1998, Schmidt, 1998). Chromospheric observations require a field of view that covers several supergranulation cells. In both cases oscillation and waves with periods down to 180 s are involved, chromospheric grains have even shorter characteristic time scales of about 100 s (v. Uexküll & Kneer, 1995). The investigation of these phenomena therefore requires high temporal resolution. On the other hand one needs sufficiently long sequences to achieve the necessary frequency resolution for the study of oscillatory phenomena. Long-term stability of the instrument is essential for this kind of measurements. Furthermore it is very desirable to perform simultaneous or nearly simultaneous measurements in different wavelength regions in order to cover some height range in the solar atmosphere. This was indeed one of the main drivers for the design of a new instrument.
Spectroscopy is the backbone of observational solar physics, it provides the relevant information for a proper understanding of physical processes in the solar atmosphere. Unlike broadband imaging, spectroscopic measurements are always photon-starved, especially when observations near the resolution limit of the telescope are concerned. Classical grating spectrographs enjoy very high spectral resolution, better than 1 picometer, or . On the other hand, their intrinsic field of view is limited by the length and width of the spectrograph slit, the latter being less than about an arcsecond to match the telescope's resolution. Covering a reasonably sized field of view at fixed wavelength requires scanning of the solar images across the slit, a time consuming procedure which also deteriorates the effective resolution due to changing atmospheric seeing.
A 2-dimensional filter spectrograph with a 2D detector array circumvents the problem of spatial scanning and its field of view is rather large: one to two arcminutes, depending on the instrument characteristics. However, the large field of view has to be paid for: now the wavelength information is obtained by scanning through the spectral region of interest. But this scanning procedure takes typically only 10 - 20 steps, and it is rather straightforward to combine the individual narrowband filtergrams to a full 2D-spectrogram.
The main objectives are summarized in the following section. Sect. 3 deals with some important considerations about FPIs. A detailed description of the instrument and its operational aspects is given in Sect. 4. The instrument performance and "first light" results are presented in Sect. 5.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998