The purpose of this paper is to point out that
1) proper treatment of polarization by a telescope is not only important for polarimetry, but also for other observations requiring high signal-to-noise ratio, and
2) this does put a constraint on telescope design, which, however, is not unduly restrictive.
Polarimetry is the observational technique which can detect anisotropics in point sources, their environment, or the medium between them and us. Modern optical polarimetry can be linear or circular, and is making progress towards spectropolarimetry and imaging polarimetry using panoramic detectors (e.g. McLean, 1984). Given sufficient photons, the precision obtained is as high as 1 part in 50 000 (see Odell, 1981 for a project requiring this precision), but more often a precision between 1 part in 1 000 to 10 000 is sufficient. (Spectro)polarimetry has been applied to planets, the Sun and other stars, stellar systems and galaxy nuclei; for a few modern investigations, see Baur, 1981; Jones et al, 1981; Schmidt and Miller, 1980. Experience in radio-astronomy has shown that when facilities for polarimetry are offered, many applications emerge from the astronomical community, yielding data that cannot be obtained by other techniques. It is essential that at least some of the large optical telescopes are capable of ob-serving polarization cleanly.