Dew shields

[Dom1961]

Quick question, how do dew shields work? They seem to just be light blockers?

[bitnick]

Good question! This is my understanding:

All warm things (i.e. things above absolute zero) emit black body radiation (infrared and longer at normal atmospheric temperatures). So the objective will emit part of its heat in all directions, which will eventually bring it down below the dew point.

Space is cold, so it will not give any radiation (heat) back. If you place a dew shield on your 'scope, your objective still emits its black body radiation, but the dew shield also emits black body radiation, so gives heat back to the objective. Only the part of the objective's black body radiation that escapes the dew shield is "lost" to space. This slows the cooling down enough to stop or delay the dew forming on the objective.

This is the same mechanism that causes frost on roads at night even though the atmospheric temperature is just above freezing.

BTW, pointing the telescope towards the ground should be enough to prevent dew from forming. Not useful during observation, but good during cool down and breaks!

[Stu]

Following from your last point, if you are pointing at the zenith then a dewshield is useless so you may need some other form of dew prevention like dew heaters or a hairdryer!!

[Physopto]

Black body radiation affects every thing. If you take a plastic bucket and put in 1/2" of water and then sit it onto a sheet of insulation, you can measure the reduction in temperature when it is left pointing up at the night sky. The water can freeze even in temperatures well above freezing, so long as the air above the bucket is not disturbed causing air currents in the bucket and there is no external energy  conduction into the water. If the sides of the bucket were very deep and insulated, it would work even faster.

My idea of the dew shield is much the same, but the undisturbed air inside the shield acts as a blanket to reduce the speed of dewing. The air has lost some of its moisture forming a dryer blanket and is colder than the air outside the shield, so it is more dense and stays in place. Fitting the dew heaters negates this somewhat but keeps the lens dry.

I could be wrong. It's just my idea of what is happening.

Derek

[baggywrinkle]

You are correct. The shield effectively holds a volume of air that stays slightly warmer than that outside. Eventually this will radiate away but it does slow the effect.

The difference between dew and no dew is a fraction of a degree.

The problem is that all objects radiate away heat and at night this is more marked and the OTA & lens/mirror assembly can drop below the dew point of the air. To further exacerbate the problem warm air holds more moisture than cold so those balmy summer evenings (when we eventually get them...) can be the worst of all.

The tape heater and dew shield combo can raise the lens above the dew point of the air and prevent dew formation on a longer term basis.

[jonathan]

It should be noted that a dew heater only generates a very slight temperature increase around the objective, it might still feel cold to the touch but it's usually enough to keep the dew from forming.

Too much heat, however, and there is a risk of excessive heat currents coming off the objective which will result in disturbed / muddy viewing.  I've never experienced this while using a dew heater tape controller though, even when turned up to full.

[damnut]

All good explanations above. I had a 10" lx 200, I always used a dewshield, no dewshield = dew on the meniscus. It only seriously failed once, at a darksite on a hill in Derbyshire where it was so cold I ended up with a layer of ice over everything!.

[Physopto]

As Jonathan says about the viewing, I've always been careful with the temperature. I use a Kendrick controller. It ensures that the output is set just above the dewpoint. Only time it has not worked was at Kielder when the voltage to the controller dropped too low and it failed to compensate. Unfortunately it has no audible alarm for that. So I now keep a close eye on the voltages. At sometime I will sort out an alarm.  It was caused by a short in a plug in the 12 volt line. Water had gotten in, I hadn't tightened it up enough. A good reason never to use 240 volt at the rig.

Derek

[baggywrinkle]

As part of my work I operate chilled mirror hygrometers which are used to calibarte Rh and dew point sensors as well as test dew point & moistire content in various gases.

I have watched  dew form in very short order with minor temperature differences and I do mean minor, <0.5C or less depending on the gas and mirror. Jonathan's comments is very valid, you do no have to overdrive the heaters.