Dew Control?

When I was using an SCT, I used a dew cap and a heater around the corrector plate area almost all year around, until it was dry in the winter... and cold. I also had one on the finder scope.  Everything else would get pretty moist without damage.  I never needed to add additional heaters to any of the connecting tubes. I left things attached 100% of the time.  Back in those days I used a Kendrick system. I still do, but I am not sure if they even are made any more! 

~john

I live on an island in the US Northeast, and humidity is my constant companion. But, after 20+ years of using an SCT in this environment, I can offer some experience.

First, I find it useful to distinguish between internal and external condensation. External condensation forms on the corrector plate and on the tube exterior (the two coldest exposed surfaces). I keep a dew heater strap around the rim of the corrector plate year round. During the summer I also add a flexible dew shield. Those two are sufficient to prevent external dew under pretty much any conditions short of actual fog. (BTW, I've used both a 10" and 14" SCT, and had similar experiences with both. And I observe from sunset to sunrise.)

Internal condensation is more insidious and tricky. There are three sealed volumes in my system, and each has its own dew point:

1. The OTA: This volume starts at the back of the corrector plate and extends to the front of my focal reducer lens.
2. The CFW: This volume starts at the back of the focal reducer lens and extends to the front of the camera's glass window.
3. The Sensor: This is the volume internal to the camera that is part of the thermo-electric cooler system.

For each sealed volume, there is a diffusion rate -- the rate at which water molecules pass through holes and seals at the system's boundaries. I've concluded that there's negligible volume-to-volume moisture migration; instead, moisture exchanges between the observatory atmosphere and each of the internal volumes. When you divide the volume of the space by the diffusion rate, you get a characteristic time constant. For the OTA, the time constant is measured in days; for the CFW, the time constant seems to be measured in hours; for the sensor, the time constant seems to be months (in large part because of the dessicant that keeps sucking water molecules out of the air in that volume).

In each sealed volume, there's some partial pressure due to water molecules, and that partial pressure determines the dew point for that volume. Any time an internal surface temperature within that volume drops below the dew point for that volume, you're going to see internal condensation.

For the OTA, the two cold surfaces are the inside of the corrector plate and the inside of the tube. I don't really care about condensation on the inside of the tube, but condensation on the inside of the corrector plate can be a problem. For quite a while, this was a significant problem for me, until I realized that the time constant for the air in the OTA was several days. As a result, problems with water on the inside of the corrector plate tended to correlate with weather patterns, rather than tonight's weather. Once I recognized this, I put a dehumidifier in the observatory and run it continuously. This reduces the observatory humidity, reduces the diffusion of water molecules into the tube, and solved my corrector plate condensation problems. (I've never seen condensation on either mirror; neither is ever as cold as the corrector plate.)

For the sensor, I rely on the dessicant. The coldest surface is the sensor itself, and the first sign of condensation is the formation of frost on the sensor surface. I recharge the dessicant (a few times a year) and that becomes a non-problem.

For the CFW volume, the coldest surface is the front of the camera's glass window. When moisture gets too high in this volume, I see condensation form on the camera window. (It's a distinctly different appearance in images than frost on the sensor itself.) This only happens under one condition for me: a series of rainy days ending with a cold front that brings a drop in the dew point. The rain causes the dehumidifier tanks to fill, which trips off the dehumidifier and the observatory humidity skyrockets (because it's actively raining). Then the rain ends, the sky clears, and I go out that evening and discover the dehumidifier tanks are full, and decide to observe anyway. At that point, the humidity inside the CFW volume can be quite a bit higher than ambient atmospheric humidity. The OTA time constant is long enough that I'll usually see the camera window condensation before I see corrector plate internal condensation. The short time constant on the CFW volume lets the water diffuse out pretty quickly, and within a few hours, the camera window condensation will go away. I can speed up that process by changing the camera cooler setpoint to a warmer setting. Pretty quickly, this will indirectly warm up the camera window a few degrees to get rid of the window condensation. (And this is with a QHY268M with a heated window.)

I've always been envious of people who don't have to stay constantly aware of dew point trends. (I monitor dew point just as closely as I monitor cloud cover.)

- Mark M (MMU)