Cleanliness and calibration stability of UV instruments on SOHO

Optical instruments for solar observations from space have in the past suffered from degradation of responsivity caused by contamination from various materials used in spacecraft and instrument construction. This was particularly detrimental in the ultraviolet range of the spectrum where the solar irradiance is weak, compared to the visible, yet strong enough to cause polymerization of organic contaminants on optical surfaces that are continuously exposed. The Solar and Heliospheric Observatory (SOHO) mission could largely avoid such effects. Material selection and special design features of instruments and spacecraft contributed mostly to this success. The various kinds of remote sensing instruments on SOHO have achieved a stability of their responsivity through special cleanliness requirements. This contribution will highlight those design aspects which are generally useful for future solar missions. Analysis of instrument responsivity data shows that under these circumstances the stability was actually not limited by contamination but by the instabilities inherent to the present detector technology.