So Your Telescope gets Wet!
Mine Did, But Not Anymore!

I didn't invent this idea, I have pulled information from several sources.

Credits go to the Following:Bill Arnett's Homepage and Sky & Telescope's Homepage

A common problem with Astronomy equipment! You are out at night with a warm piece of metal and glass, both of these items will release heat at a very fast rate. The trick is to keep the glass above Ambient Temperature. All that is required to keep your optics dry is to keep the temperature of the Optics, 1-degree Fahrenheit above Ambient Temperature. That's it!!!! You need to add a very small amount of heat to the glass, by doing this, you can keep you optics dry all night!


You can purchase a commercially manufactured dew removal system from Kendrick for BIG BUCKS or You can build one yourself for less than $10.00 (not including a battery) and 1 Hour of your time. Your Choice!!!!! If you are like me, You're Cheap!, You hate to Open your Wallet unless it's for a PanOptics Eyepiece

Well Here Goes!!! First you need to figure out how much heat you want to apply to your optics SKY and TELESCOPE indicates for a 8" Schmidt-Cassegrain Corrector Plate 3 watts of Heat is plenty. Bill Arnett constructed a very sophisticated multi-stage system with 4 banks of Heat for a total of 30 watts (each switch controlled), so he can turn each stage on/off as desired. For our purpose, I will give the theory and let you decide if you want to fry an egg or not!

I have a 16" Dobsonian with a 4" diagonal, I have 1.8 watts of heat (80 Ohms) on the diagonal, 0.435 watts of heat (330 Ohms) on my TelRad and 35mm PanOptic with 136 Ohms (which will raise it 5 degrees)
and have not incurred a Dew problem at any time on either. Below is the formula for the calculations for electrical resistance heat expressed in ohms. Assuming 12 volts as Input Voltage, OHMS = VOLTS2/WATTS or OHMS = 144 / WATTS

Wattage vs. Resistance (OHMS)

WATT

OHMS

WATT

OHMS

WATT

OHMS

WATT

OHMS

WATT

OHMS

--

--

1.00

144

2.00

72

3.00

48

4.00

36

0.25

576

1.25

115.2

2.25

64

3.25

44.3

4.25

33.8

0.50

288.0

1.50

96

2.50

60

3.50

41.1

4.50

32.0

0.75

192

1.75

82

2.75

52.3

3.75

38.4

4.75

30.3

--

--

--

--

--

--

--

--

5.00

28.8

The last 4.0 and 4.5 wattage can also be used to cook an egg!!!

Since all resistors don't come in the size you want you can put 2 resistor in parallel and the resistance drops in half for Example 2-100 ohm resistors in parallel are the same as a single 50 ohm resistor.

So go measure you diagonal or collector plate and get to soldering resistors together!!!! Remember, No bare wiring touching metal use shrink tube or electrical tape. Don't Burn UP your telescope!!!!!!

Next Star Party, I don't want to hear My Scope had Dew on the secondary mirror or collector plate.

Credits
Bill Arnett
®SKY & TELESCOPE