This Kendrick Dew-Guard heater is a self-regulating temperature-controlled heating system designed to help prevent dew formation on any reflector telescope's secondary mirror having a 1.5" to 2.5" minor axis. Its uniform heat distribution and controlled heating cycle avoids air currents and optical distortion.
The Dew-Guard heater consists of an oval phenolic circuit board with a 1.25” minor axis. The board has four small 1/2 watt heater elements and a temperature sensor on its face which contact the back of the secondary mirror. A separate ambient air temperature sensor and a small LED indicator light are connected to the circuit board by short wire pigtails.
The Dew-Guard senses both the ambient temperature and the secondary mirror glass temperature using the sensors on the circuit board and pigtail. It compares these temperatures and applies power to the heaters when the glass-to-air difference is less than or equal to a preset value. The small LED indicates power consumption and is at "half" brightness when there is a 2 degree or smaller difference between the air and glass. The indicator LED will only light when the temperature conditions activate the Dew-Guard. The LED will not light when power is applied if the secondary mirror is above the air temperature. An adjustable potentiometer is preset at the factory to turn on the heating elements when the secondary mirror is still 1-3 degrees C above air temperature. A small screwdriver can be used if adjustment is necessary.
Using a small temperature difference preset will cause the unit to turn on sooner and may be used for the most severe dewing conditions. A large temperature difference preset will allow the mirror to nearly reach air temperature before turning on, the preferred setting for typical dew or frost conditions. This will give the longest battery life and have a negligible effect on the optical figure of the secondary mirror while avoiding tube currents. The unit will remain off until the secondary mirror reaches the preset air/glass setting, then will cycle on and off as needed to keep the secondary mirror gently heated. Kendrick has found that the factory setting of 1-3 degrees C above air temperature has been sufficient for virtually all applications to date. They suggest that you first try the Dew-Guard at the factory setting before adjustments are made. If adjustment is necessary, the potentiometer is a 3/4 turn unit, so small changes are all that is needed.
The Dew-Guard is attached to the back of the secondary mirror with user-supplied silicone adhesive (RTV). Apply a small amount (about 1/32” thick) on the surface of each small square heater element. Center the Dew-Guard on the back of the diagonal (with the wire end of the circuit board higher, or toward the open tube end of the diagonal) and allow it to cure overnight. Attach the pigtail sensor and LED away from the secondary in a convenient location with free air circulation at the back (sky side) of the secondary mirror holder. Do not stuff the sensor into the packing that is supplied with most secondary holders. It is however, OK to stuff the polyfill wadding into the holder and against the Dew-Guard.
You will need to supply the Dew-Guard with 12 volts DC (9-12V DC is suitable). The unit comes with a 9 volt transistor radio battery snap connector. A 9 volt battery will work for 3 to 12 hours, depending on the particular dewing conditions. Twelve volt operation is best, however, so cutting off the snap connector or using another snap connector (watch the polarity) will be necessary to rewire the Dew-Guard. If you plan to leave a battery permanently attached to a Dew-Guard heater, it is advisable to put a switch and a fuse in line with the heater. The Dew-Guard consumes about 0.02 watt while connected to a power source to operate the temperature sensor.
The preferred place for high capacity (usually heavy) batteries is by the primary mirror or rocker box to have the smallest effect on balance. Kendrick recommends gel-cell batteries for capacity, charging characteristics, and low temperature performance. A thin wire running to the battery can be attached along the edge of the spider with super glue to produce the smallest effect on diffraction. The spider itself can be grounded to the Dew-Guard and act as the ground wire, eliminating one wire. Red is positive.
The maximum heating power of the Dew-Guard is 2 watts. The maximum current draw at 12 VDC is 0.2 amp.