I live in coastal Southern California and I have always struggled with dew prevention on my 10” DIY newt… I’ve tried dewshields, dew heaters, burning sage, shamans, you name it. Dewshields are ungainly and huge. Dew heaters would generally overheat the primary and secondary and cause distortion or tube currents.
I finally had the idea to try a DIY dew heater system using an ESP32C3 Zero microcontroller and temperature sensors to create a closed loop temperature controlled system that maintains a mirror temperature about 1-1.5 degrees C above the ambient temperature at all times. Since the dew point is always at or below the ambient temperature, the theory goes that a mirror that is warmer than ambient (even slightly) will always be above dew point.
I started with the primary mirror. I wanted to avoid a giant heater behind the mirror and I already have a Noctua 120mm fan in a 3D printed housing embedded in a foam plug behind the primary mirror cell. I figured if I could warm the air that the fan is blowing slightly, there would be a slight thermal pressure on the fan to maintain temperature above ambient. I used two 12w adhesive strip heaters and put them on the rear-most plate of the aluminum mirror cell (about 35mm from the back of the mirror). Then I placed a DS18B20 temp sensor in the airflow between the heated portion and the mirror. A second DS18B20 is outside the OTA for ambient measurement, and a third DS18B20 (in TO-92 form factor) is held to the side of the primary mirror with a small square of waterproof (silicone) electrical tape. This gives me ambient temp, fan exhaust temp, and mirror temp.
I programmed the ESP32 with ESPHome (since I use Home Assistant, I figured it would be easier to update wirelessly as I dialed in the settings). I created selectable sliders so that I could adjust the fan speed (set at 25%), desired temperature delta above ambient (1 degree C), max and min duty % for the heater (15% and 3% respectively), and maximum delta between the heated air off the fan and the mirror temp (0.5 degree C).
The ESP32 is powered by a small step down converter so that I can power everything by 12v. The heater circuit is controlled by a small MOSFET soldered to the PCB which routes the 12v to the heater circuit when directed by one of the GPIO pins on the microcontroller. The PID controller takes the ambient temperature, adds the selected temperature delta, and attempts to gently add heat to the air behind the fan by heating the rearmost part of the Primary Mirror Cell. I limit the amount of heating by a selectable amount; e.g., I select that I only want the heated air to be 0.5C above the mirror temperature at the most. So if the mirror starts at the same temperature as ambient, the air heats 0.5C above ambient until the mirror starts to warm, once the mirror warms by 0.1C above ambient, the air can get 0.1C warmer and continues heating the mirror a little more. The process is VERY slow, but is generally assisted by the fact that the temperature at night is generally dropping, and so the process doesn’t so much heat the mirror up as keep it from cooling off as fast as ambient is dropping. Because the heated air and mirror stay within about 1.5-2C above ambient, there isn’t enough heat to cause any noticeable distortion or tube currents; seeing remains the limiting factor.
The process for the secondary was similar. I had an old 40 ohm kendrick split ring dew heater that I had removed from my old secondary and it happened to barely fit. I had stopped using it because my dew controller always overheated the secondary and distorted the image. I 3D printed a shroud to cover the heater and back of the mirror to prevent excessive radiative cooling and routed the wires along the spider. Inside the shroud there is the heater and another TO-92 digital temperature sensor siliconed to the back of the mirror. The secondary only has an ambient temp sensor and mirror temp sensor since the mirror is heated directly.
I placed the telescope outside last night at about 7pm for a test since it was supposed to be cloudy most of the night and I knew it would be quite dewey. You can see the temps dropping immediately as I took the OTA outside. Condensation was already building on the OTA as I was polar aligning the mount. The clouds cleared around 11pm and I was actually able to image until the morning. As you can see, the mirrors were kept around 1C above ambient the whole night and were completely clear of dew throughout the night and into the AM. You can see the amount of condensation on the back of the secondary shroud, the OTA itself is soaked, but the optics are completely dry.
I realize this solution is not for everyone. You need a modicum of know-how with micro-controllers, soldering, and ability to 3D-print parts, but for those of you with the skills, this is a very solvable problem and for a lot less money than an off-the shelf plug-and-(maybe)-play solution. I used about $8 in PETG filament, $10 in micro-controllers, $3 of wire, a couple resistors, $2 of heating strips, $2 of MOSFETs, a couple of 12v barrel plug connectors, and a lot of my time. Most of the parts I already had on hand. The most expensive part was the Kendrick split ring heater, I think they go for about $120 now, but I already had that, and if I didn’t I could have built one with some nichrome wire and silicone sheets.
Anyways, hope this helps someone, let me know if you have any questions.
Clear skies,
Ryan
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