As I described, there are currently several novel ways to control dew. At the same time, there are standard practices adopted over many decades to have a telescope operate as well as possible. Key to the approach is 1) thermal equilibration with the environment, 2) Good dome seeing and 3) Minimal air turbulence in front of the scope.
The use of fans within or in front of the telescope, and the heating of surfaces in front of the telescope, both violate these core principles. In contrast, if you are able to apply heat effectively by direct or indirect means to the corrector plate of an sct, in principle you will avoid dew and not violate any of these core principles.
SCT’s are special since they are closed systems and some warmth within the tube will collect at the top since it is pointed upwards. There is no reason to expect it to be turbulent, and there are decades of good results, both deep sky and planetary, with that approach.
If 3C sounds ok, then what about 5C? Or 10C? If you can heat the dew shield and not have it impact the view, do you really need multiple dew straps at 3C, or can you have one at 10C? Do you suddenly get concerned it will impact the view? For me, and based on professional guidance, I would be concerned about 1C.
If there is some wind possible at times, many people would want to use a much shorter dew shield. That would be no problem if you can tolerate a warm dew shield, so just make it shorter and heat it more.
The core principle involved is simply to have a large solid angle a bit above ambient to compensate for a small solid angle well below ambient. So why wouldn’t that scale to large observatories? Current professional practice for dew avoidance is very simple and efficient and anyone can do it: If dew might form, shut everything down. But they already spend tons of money on environmental control to keep everything equilibrated with the outside. Why wouldn’t they just cover the interior surface of the dome with a slightly warm material, if it doesn’t impact the performance? It would add many nights of observing.
A key challenge with these recent approaches is that it is very hard to tell if the view is impacted at all because you would need diffraction-limited evidence. For example, if a telescope is not well equilibrated the view will be terrible, but turning on internal stirring fans can greatly clean things up. So you can conclude it will help if the telescope isn’t equilibrated, but it doesn’t tell you that it hurts the view when the telescope is equilibrated.
A nice thing about the fan approach is that you don’t need heat, and there is no real need for careful control and feedback based on conditions and measurement. It’s based on the assumption that high power stirring of the air will prevent dew and improve the seeing with the stirring. But I would be concerned about the turbulent region where the stirred air mixes with the atmosphere, which will always be happening.
For deep sky everything is more relaxed and you may not need to worry about a small bloat from something like this. In which case I would lean towards a short dew shield and a high power fan with no heat.
But for me, as I have said multiple times, I don’t want anything above ambient in front of the scope because it violates a core principle of a good setup. But I look forward to results people may come up with in the future that provide convincing evidence the view is not impacted. And any professional adoption of these approaches would be interesting to see.
Frank
