The internet is a wonderful place to share ideas at the speed of light but sometimes AN idea becomes THE idea when repeated enough and it may help to examine its premise. Maroubra_boy made an excellent post about preventing SCT dew formation without heat.
Sundown at Kitt Peak means the mountaintop peace and solitude is broken by the roar of fans blowing air across glass not just to prevent dew but more importantly to improve seeing.
“When the optical face of the mirror is even 1-2 ºC (2-3 ºF) or more above the ambient air temperature, observable image degradation will exist. The wavefront will be deformed as it passes through temperature gradients near the face of the primary mirror.” Bryan Greer, September 2000 Sky & Telescope describes how this was quantified using the rainbow schlieren test method. The same applies to corrector plates.
To see the effect of the boundary layer, http://autocostrutto...ng-mirrors.html
Greer continues, “Fans have three benefits. Most importantly, they increase the rate of heat loss from the primary mirror. This results in the mirror being able to track the falling nighttime temperature more closely, as well as reducing the initial cool-down time when the scope is first taken outdoors. Another benefit is possible when fans are directed at the front face of the mirror where they can scramble the optical structure of the boundary layer. The boundary layer is a thin layer of warm air hugging the mirror's entire outer surface. All of the convective heat transfer from the mirror to its surroundings takes place within this layer. The portion of the boundary layer on the front face of the mirror is where the image degradation occurs. Chopping up the structure of the boundary layer has the effect of restoring high resolution to the image. While fans do reduce the duration of the initial cool-down period, this is not their main purpose. The night air temperature is constantly falling, and the mirror cannot track along with the ambient closely enough without forcing airflow over the mirror. Thus, the fans should be left on throughout the observing session.” Again, the same thing applies to corrector plates.
Why apply heat to a corrector when it works against reaching and maintaining ambient temp, is completely unnecessary for dew prevention, and does nothing to reduce boundary layer optical issues?
Dew and frost happen when a surface is below the dew point temp. The dew point temp is always below ambient and you’ll know when they get close to each other as you’ll be standing in a fog. Dew is simply excess water vapor condensing on a surface that is above freezing while frost is the same excess water vapor condensing on a surface that is below freezing. But, in all cases, the dew point temp is below ambient.
With a fan, you are blowing ambient air at the precise temp you want the corrector to be. The forced air flow disrupts the boundary layer, increasing seeing while preventing dew. And, it’s simpler, cheaper, and consumes less power.
Maybe dew strips prevent dew at the expense of all other factors while fans tick all the boxes.
Lack of fan vibration, even with dirt cheap fans is discussed in other threads here on CN. I work at long focal lengths and still worry about vibration so I like magnetic levitation fans. Here’s one solution: standard Celestron C11 dew shield, 40mm 12V mag lev fan $25, plug adapter glued to the side of it $2, and it runs on 5V USB power. If you want the full airflow at 12V, use a USB step-up cable to convert 5V to 12V, $8. Of course an AC converter also works. Since the fan is disrupting the boundary layer, keeping the corrector at ambient, and improving seeing, it runs all the time, even when dew is not a threat.
Edited by speedster, 19 February 2021 - 01:03 AM.