The Astronomical Unit


This page last modified 2000 July 27

Dealing with Dew

During the colder nights in particular, dew can be the bane of those of us who observe outdoors as opposed to in an observatory. Yet there are solutions, varying in their efficacy and complexity, to this irritating act of nature.

Dew Formation

In order to know how to combat dew, it is important to have some understanding of why it forms. Water vapour 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 (breath!) that comes into contact with them.

Radiative Cooling

Outside our biosphere is space; far enough out and it is space at a temperature of 2.7K. 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.

Dew Prevention

An observatory will shield the telescope from a great deal of the sky, thereby reducing this radiative cooling. For those of us without observatories, our simplest solution is to reduce the amount of sky which the optical components can "see". Newtonian primaries, particularly those in solid tubes (as opposed to trusses), are usually well shielded. Refractor (including binocular) objectives, Schmidt corrector plates and eyepieces are among the most dew-prone, with Telrad® finders heading the list by a long way.

Objectives and Correctors

Dew caps provide the simplest way of shielding object glasses and corrector plates from the cold sky, but very few telescopes are provided with sufficiently long ones — their length needs to be at least 4 times the aperture that they are shielding. Foam sleeping mats, available from camping suppliers, are easy to cut to size to make a dew cap. The butted ends can be stick with duct- or gaffer-tape, and the inside can be blackened (Krylon® Ultra Matt Black spray is good). As long as the end of the dew cap is not visible in the lowest-power (greatest field) eyepiece, it is not too long. The resulting removable dew cap is remarkably lightweight and makes a world of difference. For those wanting a higher-tech solution, the existing dew-shield can be actively warmed — this is the principle of the Kendrick Dew Zapper®. Although I have not tried this, I know of people who have made perfectly serviceable heated dew caps using resistance wire or strings of resistors taped to the inside of the dew cap. These need not impinge on the light path. Those readers with electronic capabilities will, no doubt, be able to see more sophisticated solutions.


Eyepieces 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 eyecup, which will trap any moist air. On particularly cold nights, remove the eyecup or fold it down if possible. There are two obvious ways of warming eyepieces: an inside pocket or some form of electrical heating. I have never tried the latter, but I routinely swap eyepieces when I am observing in winter.


Diagonals in open-tube Newtonians can suffer from dewing. This can be reduced by a shroud at the top end, but the diagonal can sometimes still "see" a lot of sky when the telescope is used for observing very low altitude objects. In the field, one needs to be creative about warming a dewed diagonal: One solution is to cup one's hands around, but not touching, it for several minutes. It looks worse than before when the hands are removed, because the moist air from the hands has added to the condensate on the mirror, but it will normally miraculously clear after a minute or so, provided it has been sufficiently warmed.


Although I have seen some older binoculars with sliding dew-shields, these are entirely absent on "normal" binoculars made nowadays. I have made dew-caps out of cardboard poster tubes (50mm objectives) and toilet-roll inners reinforced with gaffer tape (30mm objectives) and, although they seem to work, the added length is awkward and they tend to get knocked when the binocular is hanging from my neck. This may be a better solution for mounted 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 using 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. On dewy nights fold back the eyecups if you can, to allow whatever moving air there is to waft away the moist warm air from your eyes.


A specific solution is to use a piece of one of those "plastic and elastic" A4 folders to make a hemispherical-ish shield that covers the entire-length-and-possibly-more of the Telrad. You can secure it with bog standard parcel- or gaffer-tape or, once you're satisfied, you might like to permanently fix it to appropriate plastic bits with small self- tapping screws.