What controllable factors influence how much dew forms on your scope?

[g__day (Matthew)]

I was wondering why there is such a big jump in the amount of Dew that forms when I move my scope from our largely enclosed first floor balcony to the backyard, on concrete but near a swimming pool.

What factors most effect how much dew forms (that one may control)? If you had to rank them in order of importance (their individual contribution to the amount of dew formed) what are the rankings of:

1. Height above ground
2. Whether your site has walls
3. Whether your site has a degree of roof covering
4. Proximity to a large body of water
5. Proximity to a discernible heat source (the house itself for my balcony location)
6. Whether your scope rests on grass, pavers or a wooden deck?
7. How still the air is around your scope?

or are the majority of these factors minor and its all down to temperature differential between the scope and the moisture ladden night air?

What in priority are the best ways to eliminate dew in a fixed (backyard, on pavers, next to shed) location? I assume they would be:

1. Warm end of tube (I already have a dew shield attached to end of scope)
2. Cover tube ends when not in use during the night
3. Build say 6 foot walls around the scope
4. Build a retracting roof

All thoughts greatly appreciated guys!

[Merlin66 (Ken)]

Humidity and temp are the key ingredients. When the ambient temperature drops the surrounding air can't hold the same amount of moisture and it condenses as dew.
Keeping the scope slightly above ambient will reduce and delay the amount of dew, hence the dew heaters etc. I'd assume your balcony is holding some heat around the scope whereas outside the ambient is probably lower. The grass/ pavers can assist.... the pavers will hold some heat during the day and give a a slightly higher temperature for a little while ( down side can be adverse temperature seeing effects!!??)
Dew also falls vertically to earth, hence the marginal improvement using dew shields; extends the front of the tube so the front lens gets more protection and can "contain" a warmer blanket infront of the lens.

[[1ponders] (Paul)]

Ditto to Merlin. Also you may also find that there is a temperature gradient from gound level up to your balcony, ie colder at ground level; dew point reached more quickly, warmer at balcony level (and more air movement which also affects dew forming); less chance of reaching dew point.

The other thing to remember is if you are near vegetation they will be transpiring moisture laden air into the surrounding environment which will increase the relative humidity of the localized microclimate.

Is the pool heated? Covered? Whether the pool will greatly effect the local RH will to a large degree depend on its temperature in relation to the local ambient air temp. If it is significantly above ambient then it will likely be contributing to the RH over the pool, but if non-heated and fluctuating with the ambient it is much less likely. It has more to do with the convection of warm moist air above the pool away from the pool and then cooling down to dew point, rather than moisture coming directly from the pool. If the temperature around the pool is low enough for dew to form then the air is unlikely to absorb moisture directly from the pool, it can only happen if the air above the pool is warmer than ambient and can absorb moisture.

[Lester]

Hi,

Another point in my opinion walls would reduce air flow, but would increase the chance of dew forming. Even a fan blowing air over the telescope can reduce the formation of dew, but in high humidity will not stop it. So a combination of dew shield, heater around optics to only aply minimal heat without disturbing seeing conditions and a fan, this is what I use with good success.

[g__day (Matthew)]

Hmmm, thanks. I notice inside my rather large shed / garage all the metal surfaces (e.g. weight set, tools on my tool rack wall) always appears far less dew covered (i.e. read zero apparent dew) than my scope sitting outside just 5 feet away. The roller door to this garage (dimensions 5 * 4 * 3 metres) is nearly always open so why do two areas only feet away from each other have such vast apparent difference in humidity condensing on metal? Is it the roof and walls? Is it the construction materials of the shed (painted timber and tiled roof)? Is it slightly warmer in my shed than just outside it? What is causing the differential?

From my limited physics I would assume air cools down after sunset slower at ground level due to retained heat from the ground, so you have a layer of moist air 3 - 10 metres above ground waiting for the ground air to cool over an hour or two after sun set. When this occurs does the moisture mostly simply fall like a very fine rain of mist - meaning a shed roof will protect gear under it, or does it mostly condense out of thin air, particularly on any exposed surface with a temperature differential that allows moisture droplets to easily form - meaning a shed will have a large surface area that acts like a lightning rod for moisture - it sucks the moisture out before it reaches the metal gear inside?

It's got me intrigued!

Will enclosing my scope significantly affect how much dew can form on my scope and if so does a large working area around my scope better protect it from dew than just a modest area? In other words would a 2 metre dome be more / less / or no different than a 3 or 5 metre enclosure sheerly from a dew protection standpoint (ignoring operating convienencearound the scope for a moment)?

[[1ponders] (Paul)]

There was a very good thread quite a while ago about different materials and colours and which ones dew up the worst/quickest. Just as an example if I'm using my ED80 and there is a 10/12" GSO dob next to me then the dob will be running with water on the tube while the Orion is only just starting to get damp. Go figure

[g__day (Matthew)]

This is worth some more research, on my part and others. My guess is you can either warm the area around a scope or warm the scope itself,or cool an area near a scope to act as a dew syphon! This last point may be interesting, if one places rough metal tubing with alot of surface area near your scope will this act as a syphon to radically drain moisture out of the air before it ever reaches your scope?

Dew

Dew is moisture that condenses at the ground level. The layer of air within about a metre above the ground experiences rapid changes in temperature from day to night. Therefore the temperature may be cooler on the ground than the air above. Consequently, the air temperature close to the ground may cool to below the level needed for condensation. Moisture therefore is observed on grass and on the surface of some objects such as those with metal surfaces. If the air above also cools below the condensation level, fog will form.

[ballaratdragons (Ken)]

Originally Posted by [1ponders]

There was a very good thread quite a while ago about different materials and colours and which ones dew up the worst/quickest. Just as an example if I'm using my ED80 and there is a 10/12" GSO dob next to me then the dob will be running with water on the tube while the Orion is only just starting to get damp. Go figure

Yep,

At one of our SVAA nights we watched to see which scopes iced up first.
All scopes out in the open on the same viewing field within 20 feet of each other. Short mown grass with the nearest building 40 feet away.

1st was my silver/grey 12" Dob.
2nd was Darrens White 12" Dob.
3rd was the Clubs Silver/grey 8" Dob.
4th was a members Dark Blue 4.25" Reflector
and then they all iced up

BUT . . the last one to ice up was the Black ED80!!!

[[1ponders] (Paul)]

And for those technically minded (Me! .... ) there is this thread by Sheeny Bad Physics/Thermodynamics Where Al, bird, Bert and a few others discuss the technical side of the issue.