A Do-It-Yourself

Dew Heater

for an SCT

Description

This is an efficient dew preventer specifically designed for a Schmidt-Cassegrain telescope such as the Meade LX series. The heat is generated by many small resistors placed near the corrector plate. This design uses 4 separate heaters each independently switched; this provides 2⁴ (more commonly known as "16") combinations of power settings from 2 to 30 watts which should be adequate for a wide variety of circumstances while using only the minimum necessary amount of power. And, most important, it all tucks away neatly so that there aren't a bazillion wires running all over the place and can be left permanently attached to the scope.

Unlike the Kendrick system this plan has no provision for preventing dew on your finder(s) or eyepieces. But it would be trivial to add such a capability if you find it necessary.

Below are detailed plans for constructing it. Please read all the instructions (and especially the conclusion) before proceeding. Despite appearances, its really quite easy. If you can't build this, you don't belong in "do-it-yourself" stores. It does require a lot of soldering but if you've never soldered before don't despair, this is about as easy as it gets (just remember to hold the cool end of the soldering iron). It does take a while, though, as there are 152 resistors to solder up.

Quantities and measurements here are given for a 12" LX200; only trivial changes would be needed for other SCTs.

Tools and Materials

Theory

Construction

Operation

The corrector plate is always pointing at the dark sky whose effective temperature is only a few degrees Kelvin (ie Really Cold). It therefore radiates away its internal heat quickly and begins to collect dew as soon as it goes below the dew point. Fortunately, the dew point is always less than or equal to the ambient air temperature (usually much less). The purpose of a dew heater is not to make the corrector plate hot or even warm, it is only to raise it slightly above the dew point. (This is A Good Thing since a corrector plate much above the ambient air temperature would introduce some optical distortion.)

Mathematically sophisticated and observant readers will have noticed that the nominal power levels (2, 4, 8, 16) are not arbitrary but in fact are powers of two. (We won't worry too much about the exact values since they vary considerably with battery condition.) Thus the switch positions can be taken as a binary number representing the total power level. Just read off the binary number represented by the switches and multiply by 2 to get the (approximate) power level.

In real life (something computer nerds may find challenging) you just start out low and bump up the power if any dew begins to appear.

Its best to prevent dew in the first place, though its hard to figure how much power will be needed on any particular night. Experiment and record your experience.

If you expect to experience heaving dewing situations, you're going to need a pretty big battery. This heater running at full power draws nearly 3 amps. That's only 10 hours of a 30 amp-hour battery (and you probably want to run your scope, too). On the other hand, if you expect only light dewing you may get away with very little power.

Using a dewshield will greatly decrease the power level necessary to prevent dewing. But a dewshield can be heavy and greatly affect the balance of your scope. On nights with light dew you may want to forget the dewshield and just use a little power. But on heavy dew nights using both will save your battery.

If your battery voltage drops too much it may damage your LX200. Plan ahead.

Note that the switch that turns off the pilot lights does NOT turn off the heaters; it is not a master switch, its just to turn the lights down/off as a courtesy at star parties.

What I would do differently next time

It would be a lot easier if all the resistors were replaced by some nichrome wire but I couldn't find any at the time. Later I discovered a source: SSI.
It might be easier if one made a cardboard mask to put the resistors thru while wiring them up instead of holding them in the foam. Maybe even several layer of cardboard one for each of the 4 circuits. The foam might not even be necessary.
It would also be nice to put some labels on the controller box.
This was fun to construct and it kept me busy during the full moon week. But really, its too much work to justify the cost savings. If I had to do it again, I would buy a Kendrick system!
Dealing with Dew by Alan MacRobert at Sky Publishing's site is useful background
Nick Zivanovic has another set of plans that may be easier to build.
Yet another version by Richard Snow
The Dry Eye, another commercial product similar to the Kendrick system
A very simple dew heater by Steen Weichel
DewBuster, a more sophisticated approach

Tools and Materials . . . Theory . . . Construction

Bill Arnett; last updated: 2003 Jun 3 ; suggestions or corrections appreciated