Dew Heaters.
The impact of Dew on your Astrophotography Lens.
Dew forming on your lens has the effect of clouding the image or stopping guiding and must be avoided.
What is Dew?
Dew is not like rain that is water that falls from the sky. It is water that condenses onto a surface from moisture in the surrounding air.
The moisture in the air is also called humidity. When the relative humidity is at 100%, it means the air is saturated and can not hold any more water at that particular air temperature. The warmer the air, the more water it can hold.
The temperature at which the relative humidity is 100% is the dew point.
Consider that the air is at a certain temperature and is holding the maximum amount of water. The relative humidity is then 100%.
If the air temperature rises, then, because the warmer air can hold more water, the relative humidity percentage drops.
If the air temperature drops, then the colder air can not hold the same amount of water it currently holds, so water leaves the air and is deposited on a surface as droplets of water or dew (or as frost if it condenses on a surface that is too cold).
Dew will form on a surface when the surface temperature drops below the dew point temperature.
An example of dew forming on a surface when it suddenly drops lower is when you take a cold bottle of drink out of a fridge, which has a dry surface when it's inside the fridge, and moisture starts condensing on the outside of the bottle.
Note: The barometric pressure also changes the amount of water the air can hold. An increase in barometric pressure will decrease the ability of air to hold water and increase the dew point. As such, as you go higher in altitude, in general, the barometric pressure will decrease as will the dew point.
Keeping dew off the surface of the lens.
If the surface temperature is kept above the dew point temperature, dew will not form on the surface.
Heated straps can be wrapped around the front of the Telescope near the lens to raise the temperature. The ideal temperature of the surface is just above the dewpoint.
Dew Heater power consumption.
The amount of power used to keep dew off the lens depends on the way the heat is controlled. The power consumed is more important if you are running on batteries. In addition, too much heat can cause unnecessary air turbulence which will impact your imaging quality.
The following are ways of supplying power to heat straps. Each option using less power with the dewpoint measurement using the least power and creating the least air turbulence due to heat. The options also go up in price as they use more sensors with the last option having to constantly calculate the local dewpoint from humidity and temperature measurements.
1) Straps supplied with constant power with the resistance of the strap and the voltage used controlling the heat.
2) Straps supplied with variable power usually averaged from a PWM (Pulse Width Modulated) controller.
3) Straps supplied with PWM power depending on the ambient temperature and the measured temperature of the surface.
4) Straps supplied with PWM power depending on the dewpoint temperature and the measured temperature of the surface.
Dew Controllers
Re option 1. above. I know no commercial product available that would use this simple approach.
Re option 2. above. This seems to be the most common controller. It's characterised by a control knob to vary the power to the heater bands. The issue is that it's a bit of a hit and miss guesstimate. The dial setting selected might or might not be enough power. If it is enough, it would be impractical to not provide too much power with the corresponding excess heat produced and battery power consumed.
Re option 3. above. Measuring the ambient temperature and maintaining the heater straps just above the ambient is a relatively simple feedback method of automatically keeping the heat generated low and using less power. This should be the minimum capability to aim for in a Dew Heater controller.
Re option 4. above. Measuring the ambient temperature and relative humidity and maintaining the heater straps just above the dewpoint is the best feedback method of automatically keeping the heat generated the lowest and using least power. This should be the best capability to aim for in a Dew Heater controller.
Commercial Dew Controller.
I have used for a long time now, the Kendrick DigiFire 12 controller. It is an ambient temperature measurement device. The current model is the DigiFire FX-Pro.
Kendrick has been flagging a dewpoint controller called the Premier Controller for a few years but the product has not yet materialised.
The USB_Dewpoint Controller looks impressive considering its size but is a bit expensive (particularly the shipping to Australia).
DIY Dew Controller
There are many DIY (Do It Yourself) controller and heat straps available.
I prefer the commercially locally available Kendrick heat straps.
My Favourite dewpoint controller is the Arduino Nano Dew Controller Pro (DIY) by Robert Brown. This controller also has a companion windows application. This project needs electronic skills as well as configuring files and uploading programs to an Arduino Nano board.