Dew (condensed moisture) is one of the astronomer’s worst enemies, and all telescopes are subject to the dew problem. If no precautions are taken, it can easily destroy a perfect observing night, as it forms on mirrors and lenses, thus it is required for almost every amateur astronomer to have some kind of dew control system for his equipment.
The most effective solution for dew control is the use of a heater to heat the optical surface above the dew point. The do-it-yourself heating system described here consists of two parts – a Heater Strip and a controller. The Heater Strip wraps around your telescope over the point your looking to protect from dew. Inside of the heater strip is a heating element that distributes heat into the strip. The strip is held in place by elastic/velcro attachments. Attached to the heater strip is an RCA output jack that will plug into the Controller.
The strip can be fabricated using a Nichrome wire loop of specific length. Nichrome wire is an alloy of nickel, chromium and iron. It is strong, flexible, can withstand very high temperatures and is available in a whole variety of thickness with different resistances. For making the strip, take required length of duct tape and carefully place the pre-cut nichrome wire on its sticky surface. Next, cut another piece of duct tape to cover the nichrome wire loop with it, connect an RCA plug to the loop using suitable wires, and carefully wrap the whole assembly with duct tape one more time. Finally attach an adhesive velcro stripe to the finished Dew Heater strip.
Note that the wire loop has specific resistance, and will draw specific current from the power source. So, conduct some experiments using a 12VDC supply to make sure that the heater strip won’t overheat. If the heat available is not adequate, then combine multiple Nichrome wire loops in parallel as a single heating element.
Dew Heater Controller is nothing but a simple Pulse Width Modulator (PWM) controller, enables you to adjust the heat level and feed power to more than one dew heater strip if necessary. It is better to power the controller from a 6V/4.5Ah SMF battery (battery charger circuit not included) through the DC input RCA jack (J1).
The PWM is designed around one of the most popular ICs on the planet-the 555 timer chip! The 555 (U1) is configured as a single-channel pulse width modulator which runs at a frequency determined by resistors R1, R2, P1 and capacitor C3. The 555’s output directly drives a logic-level MOSFET (Q1) that switches the current through the heater coil connected at the output RCA jack (J2). The 47K Potentiometer (P1) controls the PWM operation and thus the dew heater strip temperature. The Red LED (D3) is a simple power status indicator.
Since the controller has no feedback (no automatic temperature control) mechanism, extreme care should be taken not to overheat the delicate telescope optics.