This article describes a cheap, lightweight and effective solution to dew for night sky photography.
One of the more annoying aspects of Night Sky Photography is dealing with dew. Particularly in moist climates on a cold night, moisture in the air can condense onto the cold front surface of your lens quite quickly. This can bring a night of winter photography to a rapid close and can rule out long star trail exposures or timelapse sequences.
Astronomers doing serious astrophotography routinely use Dew Heaters with four channel controllers to keep their telescopes free from dew. However, they normally are working with mains power or at least large batteries available, whereas for night sky photography we are often seeking out remote and hard to access locations. The commercial controllers for these dew heaters have three issues as far as night sky photography is concerned:
Lithium-ion batteries offer a great lightweight alternative to lead-acid batteries, but because their supply voltage quickly drops below 12 volts, they cannot be used effectively with the existing commercial dew controllers. In fact with the high resistance of the small dew heaters for wide-angle lenses, they could be run directly from 12 volt supply without any controller, but there is no easy way to plug the standard RCA connector on a dew heater to a DC power supply (RCA was a dumb connector to choose as the standard for dew heaters).
To solve all the above, there's a very simple solution. There are just three parts to what I now use as a lightweight, cheap and very simple dew heater for my own night sky photography:
You can find these on ebay with a search for "12v 6800mah Lithium-ion battery".
There are some that are nicely packaged in a black housing. Others are blue and look closer to the raw product from the factory - some of these are very cheap.
I use the Dew-Not dew heater for 2 inch eyepieces (DN003). These are considerably cheaper than the Kendrick Firefly alternatives. While the Kendrick heaters are high quality, the two available options end up being just a little too large or small - more on that below.
You need an adapter cable with RCA female at one end and male DC 2.5mm plug at the other (to connect male RCA of dew heater at one end into female DC socket on battery at the other end).
You can buy components for this on ebay searching for:
"2 RCA female" ebay example
"2.5mm 5.5mm DC male" ebay example
Then you have to cut and solder the two wires from one female RCA to one DC connector together.
I'm not particularly fast or efficient at making these myself, but if you'd rather buy this adapter cable than make one yourself, I have some you can buy on eBay.
Of course, you could just cut off the RCA end of the dew-heater and solder the DC connector on, but then you lose the ability to use it with the commercial controllers, should you ever want to. If you want to make the dew heater itself, there are instructions here. As referenced on that page, you can also buy very cheap 12 volt dimmers on ebay which reduce the power demand from the heater when you don't need much power to prevent dew.
The Lithium-ion batteries run at around 11.5 volts (anything from 12.6 fully charged down to less than 10 at full discharge). The Dew-Not 2" heater has 38 ohms resistance. This draws current of 0.3A and power output of 3.4W which is plenty for just a lens. On paper you should get 22 hours run-time from the 6800mah battery but I would not assume anything like that. I do comfortably get 8 hours run-time out of them (perhaps less in extreme arctic cold conditions though). If they are flat, the batteries can take a *long* time to recharge so put them on to recharge early in the day if you want to reuse them again the next night.
The 2" Dew-Not heater (~20cm) plus elastic and velcro is long enough to go round all my lenses, although there can be a bit of a gap in the heater material with just the elastic reaching round the last bit of the lens, but in practice this is fine.
The 2" Kendrick heater (~15cm) is a bit shorter leaving a larger gap and in some cases even the velcro does not reach round to attach at all around very big lenses. But the higher resistance of 49 ohms draws less power (0.23A, 2.7W) meaning longer battery life. In many situations this may actually be preferable. The 3" Kendrick heater (~25cm) has a lower resistance of 29 ohms and probably draws more than ideal (0.4A, 4.5W) from the Lithium-ion battery.
When it's all connected together, it looks like this. The dew heater and battery can now all fit in one pocket and it's much easier to carry round and hook up.
The aim is to limit cooling by radiation of heat from the lens to the sky and enhance (where possible) the warming of the lens through conduction/convection with the air around it or by exposure to infrared radiation from trees/rocks close the camera. Some of these ideas are occasionally useful in practice. Often though you will be fighting a losing battle and some form of external heat (dew heaters) is essential.
It is also possible to use some types of chemical heat packs as a crude dew heater. The cheap single-use dry powder type packs available widely now are attractive but I have not had great success with them. They seem to need some continued agitation (as they might get in your pocket for instance) to keep some air moving through the powder and the reaction going. Strapped firmly to the outside of a lens I find they don't stay warm for as long as they should.
If you want to read further, here is an interesting article on what causes dew on objects outside at night: Dew, Frost, Condensation and Radiation. It is written primarily from an interest in camping, but is still a great explanation and exploration of the issue of dew.