The problem of dew: Moisture condensing on the front lens of your telescope (in other words, dew) can cut short an evening's observing or ruin a long exposure deep space photograph if it forms during the exposure. If enough moisture condenses, it also has the potential of getting into the optical tube and causing damage.

A dew shield is a metal or flexible plastic tube, approximately the same diameter as the optical tube of your Schmidt-Cassegrain or Maksutov-Cassegrain telescope. It slips over (or threads into) the front of your telescope to extend the length of the optical tube. This keeps breezes from blowing across the corrector lens, thereby slowing the rate at which the lens cools to the dew point (the temperature at which moisture starts to condense out of the atmosphere). The dew shield also keeps the heat that radiates from the telescope optics close to the corrector lens. This further extends the time it takes before the corrector reaches the dew point and starts dewing up. Dew shields do not eliminate dew, they merely slow its formation.

The problem of the neighbor’s yard lights: In addition to their dew-retarding functions, dew shields also act like the lens shade on a camera lens. By shielding the scope optics from extraneous light, they improve visual and photographic contrast to make faint nebulas and galaxies stand out more clearly against a darker sky background. This is particularly true in city and suburban sites where the observer often has little control over the neighbor’s backyard or security lights.

Heated dew shields and heater strips to completely eliminate the formation of dew (not just slow its formation) are also available. These are listed in this visual accessories section under the headings Dew Shields, Heated and Dew Heaters.

Refractors come with dew shields as standard equipment. The mirrors of reflectors are mounted inside long optical tubes that act as their own dew shields. Schmidt-Cassegrains, Maksutov-Cassegrains, Maksutov-Newtonians, and Schmidt-Newtonians all have corrector lenses located at the front of the optical tube, where they are prone to dewing. The dew shields below are designed for these types of scopes.

There is no difference in performance between flexible plastic and metal or hard plastic dew shields. Metal and hard plastic dew shields either thread into the front of the scope or slide over the optical tube to be held in place by nylon thumbscrews. Flexible plastic dew shields are rolled into tubes, held closed by Velcro strips, and slipped over the telescope optical tube. Foam rubber strips on the inside of the flexible dew shields hold them in place. Flexible plastic dew shields take up less space during transport since they can be unrolled and stored flat when not in use.