FURTHER INFORMATION ON THE Dew-Guard HEATERS
The Dew-Guard heaters are a temperature controlled heating system designed to help prevent dew formation on a telescope's secondary mirror. The uniform distribution and controlled heating avoids air currents and optical distortion.
INSTALLATION OF DEW-GUARD HEATERS
The Dew-Guard is attached to the back of the secondary mirror using silicone adhesive (RTV). Apply a small amount (about 1/32" thick) on the surface of each small square heater. Center the Dew-Guard on the back of the diagonal, wire end of the circuit board higher, or toward the open tube end of the diagonal, and allow it to cure overnight. You will need to supply the Dew-Guard with 12 volts DC (9-12V DC is suitable). The preferred place for high capacity (usually heavy) batteries is by the primary mirror or rocker box to have the smallest effect on balance. We recommend gel-cell batteries for capacity, charging characteristics and low temperature performance. A thin wire can be attached along the edge of the spider with super glue producing 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. Attach the pigtail sensor and LED away from the secondary in a convenient location with free air circulation. 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. The unit is supplied with a 9 volt battery snap connector. A 9 volt battery will work for 3-12 hours, depending on the particular environment. Twelve volt operation is best, therefore cutting off the snap connector or using another snap connector (watch 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 at while connected to a power source to operate the temperature sensor.
Note: 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.
The Dew-Guard senses both the ambient temperature and the glass temperature using two sensors: one on the circuit board and one on the pigtail. It compares these and applies power to the heaters when the glass to air-difference is less than or equal to the preset value. A 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 adjustable potentiometer is preset at the factory with heat being applied 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 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 cycles on and off as the secondary mirror is gently heated.
We have found that the factory setting of 1-3 degrees C above air temperature has been sufficient for virtually all applications to date. We 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, so small changes are all that is needed.
||(Small) - turn potentiometer counter clockwise to increase heat
||(Medium) - turn potentiometer counter clockwise to increase heat
||(Large) - turn potentiometer clockwise to increase heat.
|Temperature control Operating range Reverse polarity protected
||+ or - 15 degrees C 12 volts DC nominal, (6-18 volts will work)
|Elliptical size range
||Max. 2 watts
||Max. 3 watts
||Max. 4 watts
|Current (@12 v)
||Max. 0.2 amp
||Max. 0.3 amp
||Max. 0.4 amp
||(4) 1/2 watt
||(6) 1/2 watt
||(8) 1/2 watt
|Actual size (minor axis)