Telescope Dew Prevention

Introduction to telescopes and dew

Latest: Zero cost dew heater for LX200 - from a toaster!

Unless you are fortunate enough to have your back garden situated somewhere like Antarctica, you are, sooner or later, going to be annoyed by dew. The theory is, as soon as an object becomes colder than its surroundings, water vapour in the atmosphere tends to condense onto it. Our telescope mirrors and lens tend to leak out heat during the course of a nights observing. Much of this heat flows out as infra red radiation. Hang on you say - Heat can't travel from a cold body to a hotter body! How does something get colder than the air around it? Its got something to do with outer space being a tad chillier than your back yard. Trust me, it happens.

When it does happen, our telescope optics get covered in a fine mist of water. If you are standing next to your telescope doing something daft like breathing on it, it even worse. The air you breath out is much warmer and wetter than the surrounding air. Breath on a mirror indoors and you'll get the idea. If its really cold, you end up with what we might term "hard water" - icicles hanging off the secondary make for horrible diffraction spikes.
All this misty water tends to have a nasty effect on the performance of our scopes.

Fighting Back...

Short of living somewhere inclemently dry, or taking our telescopes into outer space (which brings on a whole raft of problems which makes dew look simple) we have to work at it keep our optics shiny and clean without resorting to damaging our optics with, say, a mop.

The Dew Shield

I have a Newtonian telescope. A Newtonian comes with its own handy built in dew shield - the OTA itself. Rarely does water collect on my primary mirror. Most of it appears to get stopped from going into the OTA by the secondary mirror. :(
So I have a long tube made from a camping mat which is attached to the front of the OTA. This has a number of effects.
1. Stops moisture "falling" from the skies (like mist) onto the mirrors.
2. Stops the mirrors radiating their heat into outer space
3. Stops the observer's wet outgassings from getting into the OTA. This point is often overlooked with Newtonians, but think where you mount/nose is when looking in the eyepiece...

A camping mat? Yes. Its cheap (about 10ukp) and one roll will do at least 2 OTAs. Its also longer than most commercially available solutions. It doesn't look terribly good. But hey, its dark, who cares?? The downside is it can act like a bloody great sail and make the scope wobble in the lightest of airs.. so don't attach it too firmly to the telescope.

The Hairdryer

This is a brute force approach. Take off the camera. Look at the secondary. Its all wet. Insert hairdryer into focus drawtube. Blast for 10 seconds. Dew gone. Simple as that. Only drawback... unless you have tried it, don't think for one second you know how much noise is created by a hairdryer in the back garden at 3am.

One word of warning... try to keep this hairdryer separate from those used for more mundane purposes. You don't want to be blasting hair into the OTA.

The secondary electric warmer

You recall the bit I said about objects falling below the ambient air temperature? Well, one trick is to supply a tiny amount of heat to the secondary. Not enough to make it actively warm, but just enough to keep the moisture settling on it as seen on the right.

I decided to rig up such a heater this autumn (2004) as I remembered all the nightmares I had with ice last winter. I read about a bit on the Internet. I looked at commercial heaters... I expect they work, but I'd rather not spend that kind of money. I looked at DIY ideas. The general idea is you pass some electricity through a resistor. The resistor heats up, and this heat warms the secondary. However, some of these had very complicated electronic feedback PWM style controllers. Such complexities are not for me.

I had a couple of high wattage 47 ohm resistors. So, I plugged one of them into a 10volt supply and measured the current. 200mAmps. A bit of maths tells me that this gives me 2watts of power. Seems a little more sensible than an 800 watt hairdryer methinks.

Using some wonderful thermal glue from RS I attached one this resistor to the back of my secondary.

Fridge time. I placed the secondary inside the fridge and turned on the power and left it for half an hour. After this time the resistor was just warm to touch, and the mirror itself was just below room temp. I decided this was probably near enough.

Using very thin wire taped to the spider vanes (trying to minimise the impact on the diffraction spikes) I wired it to a spare 10volt supply that was sitting around doing nothing near my electric focus motor. This runs back to a switch in the control room.

And finally, it works! Here is the scope trying to take a spectrum of Uranus' atmosphere after 4 hours observing. The OTA is soaked with moisture, however, inside the OTA, the secondary is dry.