Skotworld: Dealing with UK weather, the dew challenge

Observing from the UK presents its fair set of challenges, between finding a clear sky, good seeing and dealing with dew.

In this post, I'll cover what I've ended up doing on my battle with dew.

The problem

The dew problem is very common on SCT scopes due to their large Schmidt plate in the front, facing the sky.

Understanding the physics of dewing or condensation helps fighting it. In short, condensation takes place when the humidity in the air is high and a surface colder than the air is present. In this case, the droplets of water in the air go rest on that surface, creating dew and waisting your observations!

Now why is the scope colder than the air? After all it is only sitting there and should be at the same temperature, isn't it? Well things can get colder or warmer through 3 mechanisms:

conduction: this is when 2 parts are in contact and exchange heat
convection: this is when a liquid/air helps transfer heat between 2 parts (your blow dryer is a good example of using convection to transfer heat from the blow dryer to your hair)
radiation: this is due to electromagnetic waves. it happens in the vacuum of space. A good example of radiation is the heat you feel on your face standing a few meters from a very hot wood fire.

Radiative cooling is the problem for us observing the sky. The deep space radiation is very cold (3 kelvin). The atmosphere (water vapor in the air) radiation is still pretty cold but warmer. The ground radiation is warm in comparison. All 3 contributes to the temperature of our optics. On a clear night, where the air has a low water vapor content (as compared to on a cloudy night), you get less "warm" water vapor radiation. And this is sufficient to cool anything facing the sky. This is why on a clear dry night it can feel very very cold whilst a cloudy night with the same air temperature will feel warmer. It's also why you get frost on your car after a clear night eventhough air temperature didn't drop below 0 (but exposed surface did). To learn more about this phenomenom, this is a good read.

Perventing dew

So to fight dew, all we have to do is to keep the optics a few degrees above dew point (the point at which water droplets condensate on surfaces). In practice, think you need to keep your optics above the ambiant air temperature by just a few degrees.

1-prevent cooling by reducing your exposure to radiative cooling. This is the effect of the dew shield. These long tubes in front of the scope simply create a new radiation source at air temperature (the shield itself). They also by their nature reduce the amount of water that can reach the optics as most would condensate on the top part of the shield rather than the bottom where the optics are. The material doesn't matter much but a sturdy plastic or yoga mattress certainly works like a charm.

2-warm your optics. this is what dew heaters are for. they are quite simply resistive bands that you wrap around the scope. put current and they get warm, warming the optics and therefore keeping them a few degrees above the dew point.

FAQ: do dew heaters create turbulence? Well since you are heating only a few degrees above air temperature, you're not going to create much turbulence at all. So you'll be fine.

What I've done

I started with a dew shield as shown below.

It works wonders, but on those humid nights, observing objects high up in the sky, it will reach its limits and in my case, the scope can start to fog up after a few hours. I used to use a blow dryer and remove the dew which works fine, but I wanted a more permanent solution

So I bought from Modern Astronomy at the Astrofest a 10" dew heater strip and a Hitecastro single channel dew controller.

Now getting the dew shield on top of the strip on the scope was a bit of a pb as the strip is too thick for the dew shield to close, so I decided to combine them. I opened up the dew strip on one side, removed the foam from the plastified resistor band since the dew shield already has one and stiched everything back together.

Here is the finished result. As you can see, I kept the velcro band from the dew strip to neatly store the wire.