In order to know how to combat dew, it is important to have some understanding of why it forms. Water vapor condenses out of the air onto any surface and simultaneously evaporates from that surface. The potential rate of evaporation is lower at lower temperatures. Below a specific temperature, the dew point, the rate of evaporation is lower than the rate of condensation and dew forms. The principles of dew reduction are then simple: reduce the amount of cooling of the optical surfaces and reduce the amount of warm moist air (especially breath!) that comes into contact with them.
Under a clear sky, objects, including optical surfaces, lose heat by radiative cooling. Outside our biosphere is space at a temperature of 2.7 K. Although the effective temperature of the sky is perhaps 100K or so warmer than that, it is still a great deal colder than the surface of the earth. Hence, on clear nights (i.e., those good for astronomy) there will be a net loss of heat by radiation from the surface of the earth and things on it, like telescopes. As they cool, they become prone to dew (and frost) formation.
Our simplest way of reducing dew formation is to reduce the amount of sky the optical components can "see." Binocular objectives and reflex finders are among the most dew prone of all astronomical surfaces. Dew shields provide the simplest way of shielding binocular objectives from the cold sky but, of the binoculars that do have slide-out dew shields, very few are sufficiently long. To be fully effective, a dew shield should be about two and a half times as long as the aperture it is shielding. An extension of this length is unwieldy on small and medium binoculars, for which there are simpler methods for dew prevention and removal. Simple dew shields for larger binoculars can be made from stiff plastic (that from plastic folders is usually adequate) or 3-mm (.125-inch) thick polyethylene foam. These can be stored flat and can have their edges secured in use with hook-and eye (Velcro) strips.
For those who want a higher-tech solution, there are proprietary dew heaters, such as the Kendrick Dew Zapper, that are available commercially. These provide a low-level heat to the surrounding of the aperture. A do-it-yourself alternative, if you have the requisite skills, is to make a similar device using resistance wire or strings of resistors taped to the surround of the aperture. These need not impinge on the light path. Those readers with electronic capabilities will, no doubt, be able to see more sophisticated solutions.
For small and medium binoculars, the solution I use nowadays is to hang the binocular inside my jacket as soon as there is any sign of dewing and, on cold nights, when I am not actually looking through them. If you do this, you will find that they immediately dew up even worse from the warm moist air under the jacket, but they soon clear and are ready for use again. Because hand-held binoculars are usually not held to the eyes for very long periods, their objectives tend to cool less quickly, and they are not as prone to dewing as are mounted binoculars.
Eyepieces on larger binoculars offer a different problem. For obvious reasons, a long dew cap is not an option (and eye cups even make the matter worse!). The obvious thing is to avoid breathing on them, but there is another source of warm moist air: our eyes. It makes sense to dry a moist eye before putting it to an eyepiece, particularly if that eyepiece has an eye cup that will trap any moist air. On particularly cold nights, fold down or retract the eye cup. There are two obvious ways of warming eyepieces: an inside pocket or some form of electrical heating. I have never tried the latter (but there are commercially available eyepiece heaters), but I routinely swap eyepieces when I am observing in winter with my 100-mm binoculars.
Reflex finders are particularly dew-prone. A solution specific to these is to use a piece of plastic folder to make a hemicylindrical shield that covers the entire length and possibly more of the finder. This should not be of a radius that it precludes the use of both eyes with the finder. You can temporarily secure it with parcel or duct tape or tape or, once you are satisfied with it, you might use hook-and eye strips.
The practical alternative to dew prevention is dew removal. Several astronomical suppliers provide "dew guns." These are merely 12-volt portable hair driers that plug into the cigarette lighter socket of a car or battery pack (Figure 7.4). These are usually significantly less expensive if they are obtained from a camping store as "traveling" hair dryers.
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