A dew shield is a device used to prevent moisture or condensation buildup on the lens of a camera or telescope collector during night observations. There are two general ways that a dew shield can work, which is by either just providing shade for the lens in use or providing a small amount of warm air directly in front of the lens or collector to prevent dew collection. The shields that utilize a heating elements have the objective of trying to keep the desired lens too warm for dew to collect on the surface.
The reason for utilizing a dew shield is that dew can cause a major problem for the astronomer is that unlike rain, dew condenses from the ground. This means that dew forms when the ground water is roughly equal to the air temperature and then coat surfaces. Although this won’t happen to a telescope that has been recently brought outside from a warmer source, after some time of observing, the air and telescope temperatures will equalize. This will mean that if the dew temperature is at the right temperature as the air, the dew can coat the lenses of the telescope and the eyepieces, which cause many problems.
Here is a situation in which a telescope is left unprotected in an observing area susceptible to dewing. When the telescope begins to accumulate dew, observer will begin to notice through the eyepieces that dim stars and galaxies becoming harder to see, which will be followed by noticing that bright stars will develop fuzzy halos. Moreover, when trying to wipe down these surfaces, the observer might not make much of an impact as more water condenses when the wiping stops. There are ways to prevent this from happening, including dew shields and lens caps and is a great example of why one should use a dew shield.
Choosing whether to get a heated or non-heated dew shield is up to the astronomer, which depends on the typical viewing conditions of the areas that one observes in. In the Pacific Northwest, this would typically be during the Fall and winter months. A vendor that was recommended to us by a friend and that we personally use is astrozap, whom we have provided a link to below. Their products work really well, but there are other vendors as well who have have quality products to offer. The key is to make sure that they dew shield fits around your telescope so the dew does not get to the lens surface.
In the Pacific Northwest, when Dew Shields are required, one can often can expect to see the Winter Triangle.