14" SCT Dewing Up.

The first and most important thing is to make a dew shield. You may wonder why a dew shield is needed when you have heaters. The answer is because heaters are intended to replace the heat energy being lost to the night sky through radiation cooling. Not having a dew shield is like trying to heat your home with the doors wide open, you lose more energy than you can put in. Adding a dew shield is like closing the doors so the house can warm up and the thermostat (DewBuster) can start cycling the heat to maintain the proper temperature.

Get a roll of Reflectix insulation (Home Depot in insulation section, looks like silvered bubble wrap). It is extremely lightweight, cheap, and works better than anything else I've come across. Try to make it extend past the corrector plate by at least 15" (the tube diameter). The longer you make the dew shield the better, but observatory clearances or wind problems will limit the practical size. It's best to make it overlap the heater strip and tube as much as possible since that helps prevent heat loss and gets more of the heat into your telescope where you need it. Rather than gluing felt to the inside, which would make it heavy, spray the inside with 3M 77 spray adhesive (Home Depot) and let it dry. This acts as a primer and you can then paint it with Krylon Ultra-Flat Black spray paint. This keeps it lightweight but gives a good flat black interior that wont flake off. You may find my directions for making a dew shield helpful.

The next thing is to make sure your battery or power supply can deliver sufficient current. 14" SCT's are big scopes and you can't get away with some of the things smaller scopes can. The heat generated increases by the square of the voltage, so a fully charged battery or 13.8V power supply will generate 30% more heat than a 12V. If the power supply or battery can't supply enough current the voltage will drop each time the big heater turns on. With a big scope like you have, I would suggest a 10 amp 13.8V power supply like the Pyramid PS15.

Efficiency is very important so be sure to place the heater strip around the aluminum tube just behind the corrector plate casting and use a dew shield.

The voltage at the heater is very important since heater wattage is a product of the voltage squared (see chart below). When the big heater turns on, it will cause the battery voltage to drop, and the greater the battery discharge, the lower the voltage will drop. It also helps to have low resistance connections to the battery and use heavy gauge wiring. For this reason I recommend the ring terminal option on 14" and larger telescopes because this eliminates the cigarette plug and provides heavy duty connections, 16 gauge power cord, and low resistance solid state fuse.

A fully charged battery will produce about a third more heat than a discharged battery. Voltage losses occur in fuses, wiring, switches, and connectors so the voltage at the heater will always be less than the battery voltage. Also keep in mind that battery voltage drops when the heater is on, so battery voltage should be measured while the heater is turned on.

If the DewBuster™ controller's AUTO LED remains on constantly this tells you the heater is not applying enough heat to maintain the temperature. Wrapping the heater with insulating material can improve efficiency and solve the problem. If desired, a second heater strip can be used and plugged into the second AUTO output. Since the DewBuster™ controller monitors temperature, it will automatically compensate so that the pair of heaters uses no more power than a single heater, yet it will have the ability to produce extra heat when needed.

Efficiency is very important so be sure to place the heater strip properly and use a dew shield. Also consider wrapping the heater with insulation so that more heat enters the telescope and less is lost.