The sun goes down in the west. No clouds in sight. Transparency is high. You're all set up for a night of observing or imaging at your favorite dark sky site or maybe in your backyard. After a couple of hours of observing, you begin to notice that the eyepiece is fogged over. If you're imaging, you suddenly notice a distinct decline in the quality of your raw frames. What you are noticing is the effects of dew formation on your optics. Dew is water vapor in the air condensing on your equipment.
A telescope will begin losing heat to the atmosphere almost as soon as the sun sets. As the air cools, the optical glass begins to cool as well but faster than the drop in air temperature. Once the temperature of the optics reaches the dew point (the temperature at which the relative humidity reaches 100%), water condenses on the glass surfaces. Unless you're equipped to fight dew formation, your observing or imaging run is over.
Dew can affect all types of telescopes, but refractors, Schmidt-Cassegrains and Maksutov-Cassegrains are particularly susceptible to dew formation owing to their design, which places glass at the front end of the optical tube. Open-tube (or "truss”) Newtonians are also prone to dewing on the primary mirror even though it sits at the back of the optical assembly. The design of truss tube Newtonians exposes the primary mirror to the cooling effect of night air. Combining this effect with the need to allow the telescope to cool in order to reach ambient air temperature prior to observing or imaging leads to shortened use time and frustration. In order to inhibit dew formation and increase the amount of precious telescope time, Astrozap offers two main types of products to combat dew formation. These products fall into two main groups: passive and active.
The mainstay of the passive group is the dew shield. The "classic” dew shield consists of a cylinder of material, usually made of metal or plastic, placed on the front of a refractor, Schmidt-Cassegrain, Schmidt-Newtonian, Maksutov-Cassegrain, or Maksutov-Newtonian telescope. The dew shield creates a zone of air that does not cool as rapidly as the air surrounding the telescope. The objective glass stays slightly warmer for a longer time than it would without a dew shield, thereby extending telescope time. Truss-tube Newtonian owners benefit greatly from the use of light shrouds cloth curtain wrapped around the trusses—that accomplish the same effect. Dew shields serve the second function of reducing or eliminating stray, off-axis light from entering the optical tube, thereby increasing the contrast of the object's appearance in the eyepiece or the image the camera acquires.
However, a dew shield's passive effect alone is temporary, because the time will come when the air temperature inside the dew shield reaches the dew point, and water condenses on the telescope's objective. Most refractors have a built-in dew shield, but it may not be sufficiently long enough to create the proper effect. Also, a dew shield applied to the objective does not protect finders or eyepieces, both of which are also prone to dewing. An adequate dew fighting strategy requires a more dynamic approach, taking finders and eyepieces into account.
The active group of dew fighting equipment consists of heater straps, which uses an electrical current to create and apply heat to the objectives and eyepieces. Using heater straps keeps the glass's temperature above the dew point, thereby preventing dew formation in all but the most extreme of conditions. Employing active dew protection equipment requires a 12 volt DC power supply rated at a sufficient amp-hour number to provide enough current to the dew heater straps to last throughout the observing or imaging run. Dew heater straps are not intended to heat the optics any more than is needed to keep the glass above the dew point.
Astrozap offers both passive and active dew fighting equipment, including metal and plastic dew shields, heater straps and controllers, and an innovative hybrid dew shield with a built in heater strap.