microprocessor and remotely accessible via a RS232 link.

One of us (J.-L. Prieur) has developed a program in the PC/Window environment to facilitate the remote control of PISCO. All the basic functions are available (Fig. ) with mouse-driven menus. The program controls in real time the atmospheric dispersion correction according to the telescope position, the filter and the atmosphere parameters (§2.2). A log file is also produced at the end of each night with the PISCO setup parameters of all the exposures taken during that night.

PISCO was primarily designed to be used at the Cassegrain focus of TBL, but it has also been made mechanically and optically compatible with the Cassegrain foci of the Canada-France-Hawaii (CFH) and European Southern Observatory (ESO) 3.6-m telescopes.

[*2.2. Atmospheric dispersion correction with Risley prisms*]{}

10000 For an astronomical object observed from the ground at an elevation different from zenith, the atmosphere behaves as a dispersive prism (see f.i., Simon, 1966). Polychromatic images are spread into a small vertical spectrum. For instance, for a 250 nm bandwidth centered at 500 nm, the typical atmospheric dispersion is 1 for an elevation $h = 60^\circ$ and 2  for $h =30^\circ$.

In PISCO, the atmospheric dispersion is 