Dew Do's [snicker] and Don'ts

Dew (frost when its cold enough) forms when the surface temperature is cooler than the dew point. Having a heater is step one to alleviating this. So long as you can keep your optics at or above the dew point will not have a dew problem. 

I use a dew band on everything from my Guide scope to my 5" refractor  to my 9.25"Edge all the time regardless of weather. 100% humidity, -40c, 1/4" ice on everything except my equipment. The effects are sometimes star bloat. Never had an optical defect from heating. It is possible that you can be too hot but on the lowest setting? unlikely.

Some tips would be passive air from a fan (good if you have a small observatory) 
Refletix - great passive insulation. Keeps dew off, helps equalize temperatures. 

Some people will tell you to use a hair dryer, this is temporary and not ideal. We don't want to overheat our equipment but we don't need it to be cooler than ambient temperature. We are doing long expsosure imaging that will unlikely be affected by our small amount of heat compared to those doing high resolution planetary imaging.

For refractors, put the dew on the body and not the dew shield – you will create air currents that distort the image if heat is placed on the dew shield.

You’ve received some good advice here but I have another two cents to add to the discussion.  There are three main mechanisms of heat exchange that affect your rig—conduction, convection, and radiation.  When you take your equipment out of a warm house to a cold air temperature outside, both convection and radiation cause it to cool down.  Even a light breeze will greatly increase convective cooling which is the exchange of heat with a gas.  The outside surfaces get cold first and heat conducts from the interior parts so after a while, the parts will reach a state of thermal equilibrium with the surrounding environment.  The most important thing to understand is that under a clear sky, the temperature will stabilize at a temperature below the outside air temperature.  That’s because of heat exchange with the sky through radiative transfer.  The radiative temperature (more commonly called the black-body temperature) of the sky depends on a lot of factors but if it’s crystal clear it can be in the range of -20C to -35C.  That means that anything exposed to the sky will get colder than the air temperature.  As others have pointed out, as long as the temperature of any exposed surface is above the dewpoint, dew will not form.  The problem is that when the front element of your lens is looking up, it will quickly become colder than the air temperature—and if the dew point is only a little below the air temperature, dew will form.  You have to worry about dew when the temperature-dewpoint spread is less than about 3C and it’s best to start the heaters when the spread gets down to about 5C.

The first thing that you can do to prevent dew is to limit the view of the sky for the front lens element by using a dew shield.  Just remember that the dew shield will also be exposed to the sky so it gets cold too, which means that although a shield helps (a lot) it isn’t perfect.  A good rule of thumb for the length for a dew-shield is about 2x - 2.5x the diameter of the front element (although there are ways to make it shorter if needed.). Dew shields get more challenging to make for really wide field lenses but for most systems they can be a simple tube.

The second thing you can do is to add heaters.  It’s easy to add a heater, crank up the heat, and you’ll never see dew; however, there is a lot more to it than that.  The first thing to understand is that heat creates turbulence through convective heat exchange with the air.  That’s not a very big deal if you are simply using a short focal length camera lens, but it’s a really big deal if you are running a longer focal length telescope.  The second thing to remember is that if you can simply maintain the scope at the ambient air temperature you won’t get dew—so long as fog doesn’t form!  That’s the “minimum heat” condition for as long as you can see the stars.   So that defines some of the considerations for how to run your heaters.

The first thing you can do is to make your dew-shield more effective by heating it.  As others have pointed out, you don’t want to actually heat it.  You simply want to supply enough power to maintain the temperature of the dew shield within a degree or so of the ambient air.  That does a very important thing.  Keeping the dew shield at the ambient air temperature replaces some of the heat lost to the sky from the first optical surface with heat from the dew shield.  For larger scopes where a long dew shield acts like a sail in the wind, heating the dew shield allows the use of a shorter shield.  Keeping the temperature of the shield within a few degrees of the air temperature will not add significant air turbulence.  The second thing you can do is to heat the OTA tube (or camera lens) itself.  Again the goal is to minimize the amount of heat that you add to the system.  Creating turbulence inside a larger OTA can be as bad as creating it in front of the telescope.  The goal is to only add just enough heat so that it conducts through the front optical elements to keep the front surface at or above the air temperature.  Remember that heating the OTA assembly (or your lens) requires both conduction and convection to get the heat from the heater strip to the front optical surface where radiative cooling has the biggest effect.  More is not better!

There are a few dew controllers out there that allow monitoring the temperature of the air and the heater to control the temperature difference.  My favorite is the Dew Buster but I’m sure that others are as good.

Finally, be very careful about bringing your gear into a heated house after it has been outside in cold humid air (particularly if the humidity in your house is pretty high.)  That’s when you can wind up with really soaked gear—and it won’t necessarily get soaked only on the outside.  Remember that your cold gear may be far colder than the dewpoint inside your house!  It is far better to cover your gear and let it warm up slowly as the sun comes up or to put it into a cold garage for a few hours before bringing it back inside.   Another strategy for small systems is to bag it with a large desiccant pouch (available from Amazon) before raising the temperature.  Be careful to keep it above the local dewpoint at all times!

Good luck with it!

John

I do remember that once I had frost on my DSLR sensor (and everywhere else) although the dew heater running at full blast kept the lens clear. Never ever thought that possible but it was. Not that having the dew heater helped in that case, I had to ditch the affected frames. So beware. Conversely, I never had issue with dew/frost forming on any of the CCDs/CMOS sensor in the cameras I have/had once they run long enough. The issue is usually if you have minor leaks of outside air into the "sealed" chamber of the cooled camera then frost starts to form on the sensor soon enough.

John Hayes:
I didn't want to get too much into the weeds so I didn't get into the issue of frost but since you brought it up, here's a little on that subject.  First, frost is not frozen dew.  Frost does occur when the temperatures gets below freezing but the frost-point is actually a little above the dew point.  Frost forms by sublimation, which is the process of a gas going directly to a solid form without becoming a liquid first.  Your experience with your DSLR is similar to having a primary mirror in a SCT frost (or dew) up even thought the front of the corrector plate remains clear.  That happens when the surrounding structure radiatively cools below the frost-point for the primary mirror--or your sensor.  Heat rises so the parts at the lowest level, below the heaters can get cold enough for dew or frost to form--even with the heaters running.  No question about it:  Things get tricky when the temperature-dewpoint (or frost-point) spread gets really small.  In most cases, it's better to simply shut down when the spread goes below 2C - 3C and that can be hard when the sky is crystal clear!

John

Small correction: 
Sublimation is the transition from solid to gas without becoming liquid in between. 
The reverse path from gaseous directly to solid is called desublimation or deposition.

One thing I do now is call the local airports ATIS. That gives me their temp and dew point.

Here is the most obvious of the images (only 18 minutes here - I know it won't end up in a coffee table book!)