The three-dimensional relationships between quantities like pressure, temperature, velocity and magnetic field, describing the state of the gas in different solar atmospheric structures, can only be investigated by narrow-band bidimensional spectroscopy.
For this reason, a new instrument, the Italian Panoramic Monochromator, has been built in Arcetri in collaboration with the Physics Department of "Tor Vergata" University in Rome. It essentially consists of a Fabry-Perot interferometer (FPI), used in telecentric mounting, in series with a Universal Birefringent Filter (UBF) (Beckers 1971, 1972), used as an order sorter.
This instrument represents an evolution of a previous one for mono-dimensional spectroscopy, the Spectro-Interferometer, which, for many years, was operative at the Solar Tower in Arcetri (Cavallini et al. 1987).
After a first description of the IPM, which can be found in Bonaccini et al. (1989), hereinafter referred to as Paper I, some different versions of this instrument have been built and used in Arcetri for full disk observations, at low spatial resolution (Cavallini et al. 1992). We learnt from this experience that the instrument encounters some severe difficulties in its construction, essentially concerning the correct shape of the overall instrumental profile, its wavelength stability and the spectral homogeneity of the image plane.
As it will be discussed later, these problems have been solved, and when Italy agreed to the THEMIS project in 1992, an improved version of this instrument, for high spatial resolution, has been designed and built. The IPM is operative now and as from April 1997 it is at the disposal of all observers.