Dew shields, Lens hoods and Dew Prevention Systems
By Chuck Hawks
Dew shield, lens hood, or whatever you call it, most telescopes need them. Dew is a serious problem at night. Nighttime temperature fall and humidity are major factors in moisture formation, so some areas and climates are worse than others. When air begins to cool, it loses its capacity to hold moisture and condensation begins to form. When the humidity is high, the dew point is reached at a higher temperature. If dew forms on the objective, corrector, or mirrors of your telescope it can put an end to your night's observing, so dew prevention should be taken seriously.
Refracting and catadioptric (CAT) telescopes have a propensity to collect dew on their front lenses, which, after all, are usually pointed skyward. A lens hood can reduce, but does not eliminate, this problem for refractors and CAT's.
A truss type reflector must be shrouded to keep dew off the primary mirror. The long tube of a conventional Newtonian reflector generally serves as an effective dew shield and Newtonians are seldom seen with a supplementary hood. Ditto for Cassegrain reflectors and open tube Vixen CAT's, although dew has been known to form on the secondary mirror and, in conditions of severe dew, even on primary mirrors. I have experienced dew condensing on the primary mirror of a 6" Newtonian reflector with a seamless aluminum tube. That main tube was a lot longer than any practical dew shield for refracting and CAT telescopes.
Refractors often come with either a screw-in or pullout lens hood, but CAT's seldom do. If your CAT or refractor did not come with an adequate dew shield, you must buy one. Usually an appropriate dew shield is available from the manufacturer of the telescope, but if not, they are available on the aftermarket. Lens hood/dew shields are typically rigid (metal or hard plastic) or flexible (ABS plastic or something similar). Both types are about equally effective at delaying the formation of dew, but flexible dew shields are much easier to store or transport and are generally preferred for portable telescopes.
Celestron and Vixen sell flexible dew shields made for their CAT's. Orion offers their similar "Flexishields" for a wide range of Orion and other CAT's. These are made from flexible ABS plastic and wrap around the front of the telescope to form a hood; they are secured by a Velcro strip. This is a convenient design that works well. For storage, the dew shield lays flat or it can be wrapped around the scope's main tube. Orion offers their Flexishield for 90mm (3.5"), 102mm (4"), 127mm (5"), 150mm (6"), 203mm (8"), 235mm (9.25"), 254mm (10") and 280mm (11") telescopes. Practically everyone on the Astronomy and Photography Online staff who owns a CAT uses this type of dew shield. They are also effective sunshades for those who use their CAT's for terrestrial viewing or as telephoto camera lenses. Cost is usually moderate, typically about $25-$35 online for an 8" SCT.
Kendrick offers somewhat less elegant flexible dew shields in a variety of sizes to fit telescopes from 3.5 to 14 inches. I used one of these on my Meade ETX-90 and it worked fine. Kendrick also sells a dew remover system, which is a portable electric heating system designed to prevent the formation of dew. It is available in sizes to fit most telescopes, oculars and even the Telrad red dot finder.
Orion (Dew Zapper), Astrozap, Vixen and others also sell portable, battery powered (DC) heating systems to prevent dew formation on front lenses, finder scopes and oculars. These are available in the form of single, Velcro attached, heating bands in lengths designed to fit most telescopes, or multi-channel heating systems that can protect up to four telescopes, finder scopes or oculars simultaneously. Each heating band has its own rheostat to independently control its temperature. Heating strips of various lengths are available separately. Such devices are usually powered by a car battery via cigarette lighter plug or a portable Power Tank type rechargeable 12-volt battery. Vixen's Dew Heater 2, a 665mm long heat strip, comes complete with a handy battery pack that uses eight D-cell batteries for about $135 (Woodland Hills Telescope, 2010).
Another device designed to combat dew formation is the heated dew shield, such as those from Astrozap. These are usually much like the flexible dew shields described above, but add a thin foam rubber strip on the inside of the dew shield. Beneath this strip are heating elements that warm the corrector lens of the telescope to keep it above the dew point. A cord connects the heated dew shield with its controller, which is in turn connected to a 12-volt DC power supply via a cigarette lighter plug. The dew shield keeps the gently warmed air close to your scope’s corrector lens. This is claimed to reduce the battery drain required to keep your scope dew free when compared to using a separate dew heater strip. On the other hand, warm air currents may form inside the dew shield, which does not happen with separate heating bands wrapped externally around the scope's main tube behind a conventional dew shield. Astrozap makes heated dew shields for scopes from 3.5" to 16".
The heated dew shield may well be the most elegant method of combating dew formation. It is, however, probably the most expensive. For example, an Astrozap heated dew shield for an 8" SCT retails online for about $75, the Astrozap two channel, four output heater controller runs about $105 and a Celestron 7 amp hour Power Tank costs about $65 (all in 2010 dollars).
A less expensive, active dew control method is a battery powered (cordless), hot air hair dryer, used as needed in conjunction with an ordinary dew shield. A low wattage model is best for telescope use (unlike for drying hair!). Make sure the model you choose has a low heat setting, since you do not want to create any more hot air currents or heat your corrector more than necessary. I have successfully owned and used these. One way or another, preventing dew formation on optical surfaces is crucial to observers. How sophisticated and expensive a system you need depends on your wallet and your typical observing conditions.
Copyright 2010 by Chuck Hawks. All rights reserved.