By radiating heat to the night sky, the corrector plate of a Schmidt-Cassegrain-Telescope is lowering its own temperature. As soon as the temperatur falls below the dew point, little drops of dew or ice crystals begin to form, starting at little pollutions, f.i. dust particles.
The SCT design with the exponated glass on front is very susceptible for dewing up, much more than a Newtonian.
Wheater makes some extremes possible - some nights the telescope's temperature won't fall below dew point at all and in other nights, even in summer, the corrector plate could be dewed up in less than half an hour. When the sun sets in nice red, much water vapor is in the air.
The radiation of heat can be reduced by using a dew cap. Such a cap limits the amount of sky the telescope sees and can radiate to.
Personally, I use a dew cap from Kendrick. But making such a cap yourself is not too complicated. It is important to use a heat isolating, rough, non reflecting material at the inner side. The outside will be getting wet or icy, make it water resistant.
For long observing in wet or cold weather it is not sufficient to minimize radiation. You must supply enough heat to keep temperature above the dew point. Among other factors, the radiated heat is proportional to the radiating area. If you use the method described here for smaller or bigger apertures than the 11 inches of the C-11, please use the square of the ratio of diameters as a factor for the value of the resistor. The smaller the aperture, the higher the resistance you need !
Some dew remover systems place the heater inside the dew cap in front of the corrector plate. This variant has the advantage of applying the heat directly where it is neccessary, minimizing the power needed. In turn, it is hard to avoid turbulences caused by heat convection, which would deteriorate seeing.
I choosed the variant to heat from the outside. The corrector plate of the C-11 is in good thermal contact with the tubus, allowing a heat current to flow from a warmed up outside to the glass.
I employ resistance (NiCad) wire to generate the necessary heat. At the place of the corrector plate I've put insulating tape on the outside of the OTA. On this tape I applied 3 windings of wire with a resistance of 5 Ohm per meter, covered with thick, waterresistant insulating tape. Since the circumference of the C-11 OTA is one meter, the resistance is 3 x 5 Ohm = 15 Ohm.
The foam inside of my dew cap sits exactly on this heater, diminishing heat loss.
For connecting the heater to my little passive control box the same type of connector as used by the Losmandy control board is employed. It is available with cable already connected. It has to be cut to the right length. I took the ends of the heater wire twice, twisted it, threaded the cabel in, and twisted it around before soldering.
The socket in the middle is connected to the power supply. On the left side the heaters for the Schmidt corrector plate and the finderscope will be plugged in. On the right side there are two sockets for eyepiece heaters or a piggybacked foto lens.
The control box is attached to the OTA using Velcro.
The heater is powered by my 12V, 32 Ah accumulator. Electric power is calculated by dividing the square of the voltage by the resistance: P = U*U/R, yielding 9.6 Watt and a current I = U / R = 0.8 A. Most of the time less power would be sufficient. Maybe I'll add an active control box with an 555 timer and a switching transistor later.
The power of about 10 Watt isn't sufficient to clear an already dewed up plate, because of the additional vaporisation energy needed. It is therefore important to switch on the heating before the dew point is reached to establish a thermal balance. To handle cases where dew removing is needed, I installed an additional switch to shorten one of the three turns of the winding. With only two turns the resistance is 10 Ohm, yielding 14.4 Watt heating. This can remove some dew when dewing up is starting.
Especially important for me are the two power indicating red LEDs, assuring me that the heaters are powered up.
Everything said applies for the objective of the finderscope, too. Of course, since the finderscope is much smaller, the dissipated and to be replaced amount of heat is much smaller. With wire having 170 Ohm/meter I made a winding summing up to 360 Ohm.
To fix the turns of this very thin wire, I've put one layer of isolating tape with the sticky side outsides on the finder. The wire was layed tense on the glue. Cable and connector were soldered to the wire. At last, the wire was covered with a layer of isolating tape.
This heater can be slipped on and of easily. While observing, I use a dew cap made of cardboard above it.
In spite of different causes, the effect remains the same. The eyepieces do not point towards the sky, in fact they are receiving some amount of heat emitted by the ground. But they are cooled down by convection and conduction to other parts of the telescope and moistened by the eyes and the breath.
The eyepiece heater must be flexible in length to be adapted to different eyepieces. It can be made the same way like the finderscope heater. I tested another widespread variant by arranging 27 10 kOhm resistors in parallel between two thin copper cordons. Please be sure to make very tiny spots of soldering plum/tin to keep the cordons elastic. Overall resistance is 370 Ohm, that is maybe a little too much, but I never had problems with the resulting 0.4 Watt and a current of only 32 mA.
The finished circuit was sewed into strips of cloth: thin nylon inside, Jeans cloth at the outside, 4.5 cm wide and 25 cm long. A 10 cm Velcro strip allows usage for different eyepiece diameters, up to the thick 35mm Panoptic, my favorite DeepSky Eyepiece.
I am very impressed by the performance of my simple heating system. After observing 8 hours at -14° centigrade, about 7°F, almost all of the mount and the OTA were covered with ice. Only 2 inches of the OTA and the Schmidt plate were perfectly free and clear. I looked at the Trapezium, the heart of the Orion nebula. At a magnification of about 300 six stars were clearly and steady visible.
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