This 0.05” thick elliptical dew heater pad has a minor axis of 3.75”. It will keep dew from forming on a Newtonian diagonal mirror with a minor axis of 4” and larger. The pad is made of two sheets of rubberized fabric cemented together. Embedded between the sheets is a heater element that it warms the entire surface of the heater pad. The heater pad is placed against the back of the diagonal mirror and is held in place by the pressure of the batting found in the diagonal holder. You can also apply a thin skin of silicone adhesive to the heater pad to attach it permanently to the mirror. Since the pad is in direct contact with the back of the mirror, the warmth of the heater element is transmitted very effectively into the glass. The result is that you need less energy to keep your mirror dew-free.
The pad has a pair of thin permanently-attached 17” wires terminated by a male RCA plug. The wires are secured along one of the diagonal’s spider by a method of the buyer’s choice (silicone adhesive, electrician’s tape, etc.) The RCA plug is fed out though a hole drilled through the side of a solid tube or run up to the top of the tube. A separate 70” male-to-male RCA cable, connected to the 17” wires’ RCA jack via a supplied female-to-female adapter, completes the connection to the heater control unit.
For those who want diffraction-limited performance (and who doesn’t) and who know how to solder, a length of very thin copper foil is included with the heater pad. This self-adhesive foil can be cut in two lengthwise and applied along the length of one of the spider vanes. The 17” wires from the heater pad can be cut and the wires leading from the heater pad soldered to the copper foil where the spider van joins the diagonal holder. The remaining portion of the 17” wires, with the RCA jack attached, are soldered to the opposites end of the copper foil leads, at the tube wall. The soldered connections can lift off the surface of the spider vane in time, so the solder points should be epoxied in place. The copper foil leads can be painted black, if desired.
The heater is particularly effective with truss-tube type Dobsonian reflectors, whose open framework often allows the formation of dew. The heater pad draws 0.7 amps at full power (power consumption at normal operating levels will generally be 40% to 60% less than this, as the normal standard controller setting is medium to low).