An extension to the tube of a refracting telescope to prevent dew from forming on the objective lens, or to the tube of a catadioptric telescope to prevent dew forming on the corrector plate. It works by reducing cooling of the objective by shielding it from cold air currents. The dew cap should be between two and three times longer than the telescope's aperture, with a diameter which does not restrict the field of view at low powers. If it is too long, however, it may impair the seeing because of air currents circulating within it.
Subjects: Astronomy and Astrophysics.