Hi All,
I thought I would share my recent adventures in dew prevention, as it might benefit someone else.
On the 10" I could get by with just a large 120mm fan mounted on the side of the upper cage, blowing directly onto the surface of the secondary mirror.
However, the new secondary on the 12.5" scope is much larger and thicker (it's 4" on the short axis) so the fan would likely not be enough. With the 10" I had already experienced some dew occasionally on the most humid nights during winter, so I knew I had to find a better solution for the larger scope.
For this purpose I purchased a DewBuster controller and a bunch of resistors to build my own secondary heater. The reason for not using a commercial heater was that I couldn't fint any that would fit the shape available around the stalk mounted secondary. I wanted something custom which would cover the entire area evenly and fit the exact shape.
So I build a heater out of some 330 Ohm 0.5W resistors mounted in parallel, and fixed it with RTV silicone directly onto the back of the secondary. To minimise air currents and maximise the efficiency of the heater I needed to cover the entire backside of the secondary with an insulating material. I chose some nice self-adhesive sound-absorbing/insulating foam that I found at JayCar. This turned out to be perfect for this purpose as it easily molded to fit over every resistor and also stuck extremely well to the back of the mirror. The entire heater is now completely encapsulated in this material which was more than I could hope for.
The heater is temperature controlled so I needed to feed 4 wires to the secondary holder, two for power and the other two for the temperature probes (there is a probe for ambient and another for the mirror itself). Now of course I thought I had a brilliant idea in just using the guitar strings as the wires. I had already tested on a piece of string, ensuring that the current would not cause it to heat up (Stainless steel actually has 7 times the resistance of copper, so that's something to beware of). I concluded that the current required to run the heater would not warm the string to any noticeable degree, it didn't get warm to the touch or anything, so all looked good.
I now hooked everything up to run through the strings to the secondary. The DewBuster controller acts by allowing pulses of current to the heater with a frequency determined by the temperature probes, in order to keep the mirror at the specified setting (usually a couple of degrees above ambient).
I tested visually on a bright star to see if there was any trace of air currents resulting from the warm heater that was now sitting in the middle of the optical path. Such currents had really been my main concern. I defocused the bright star until I could clearly see the large doughnut shape with the secondary in the center. To my dismay I could see significant warm air currents flowing, not from the heater but from the
strings, as soon as I turned on the heater. Apparently it doesn't take much temperature difference at all to create these currents.
I now focused the star to see if the current had any impact on the seeing. To my surprise the entire view in the eyepiece was shifting rapidly back and forth, in perfect sync with the pulses from the DewBuster controller! The reason was that as the strings got heated they expanded and contracted accordingly and this was apparently enough to move the secondary. It of course made the solution unusable, but it was very interesting to observe how perfectly the controller was able to control the positioning of the secondary, and I suspect this could actually be exploited as a form of adaptive optics if the current could be applied in sync with guide star movements...
But after all, instead of running the current directly through the guitar strings I had to run it separately to the secondary holder through four thin 0.25mm copper wires. I twisted these together and bent them in a half circle so as to not introduce an extra diffraction spike in my images. This has worked perfectly on every night since, with no trace of dew forming on the secondary. The wires has had no detectable impact on image sharpness or contrast either. And the DewBuster is a great controller, it really is a set and forget solution.
Attached are some images of the installation, showing the secondary holder with the heater on (it's under the black insulating foam) and the ambient temperature probe sticking out (the other probe is tucked under the foam, touching the back of the mirror).
You can also see the wiring and the controller box.
This was a great little project, and if I find for any reason the primary should need heating I know I can easily create another heater for it.
Larger images are available in
my gallery.
Regards,
Rolf
