Dew Question

[PCH]

Hi,

I have a question which I hope will have a simple answer.

Here goes ...

at any time of year, winter through to summer, is it true to say that if your equipment is at ambient temperature then dew will not form?

What I'm getting at here is to find out if by keeping my gear in the shed outside, whereby it is clearly as close to ambient temp as possible permantently, will it still get dewed up when I get it out to use?

Thanks in advance for your suggestions :thumbsup:

[sheeny]

The short answer is no.

Dew formation depends on the dew point of the air (which is a function of humidity and temperature). When the air, or your scope, reaches the dew point (or cooler) dew will form.

Al.

[PCH]

Thanks Al, - so on any cooler night such as in winter we use dew heaters to keep the gear above the dew point - right ? :thumbsup:

[montewilson]

The reason your equipment is dewing up is because the surfaces are dropping significantly in temp not just one or two deg below.

Look at this calky http://www.decatur.de/javascript/dew/index.html

for a normal scenario, 20C ambient 50%RH the dewpoint is about 9C.

The glass is looking into space that is -270C so it sheds heat real fast. That is why a clear night is so much colder than a cloudy one. This is also why your car bonnet can somehow be at -10C when the air temp only got to zero. One night about an hour after sunset my car roof was already below zero but the ambient was about 10C. Weird science!

[PCH]

Hi Monte,

thanks for that addition. I'm understanding why it happens a little more clearly now.

So, am I right in thinking that I really can't do without a dew heater if I'm to use my gear reliably without dew forming. There isn't another way?

Cheers, :thumbsup:

[astronut]

Paul,
The answer to your question is YES!!
You will need dew heaters if you want an uninterrupted observing session, whether it's summer or winter.
You can either make them yourself, or purchase the system commercially.
I have heaters for both ends of the finder, main eyepiece, laser, secondary mirror and the filter slide..........oh, also leave the fan running the whole time.
Whilst others are having to pack up, or are trying to dry off their optics to continue viewing, my optics are dew free all night.:thumbsup:

[PCH]

John et al,

thanks for these comprehensive replies. It's definitely coming together now.

John, I see you have a 12" LB like me, - do you have any dew heater mechanism installed for the primary mirror ?

And what do you power this lot from also?

Cheers :thumbsup:

[Paddy]

Hi Paul,

You might find that if you can direct the flow from your primary mirror's fan across the face of the mirror, you both prevent dew formation and improve seeing by breaking down the boundary layer at the face of the mirror. Have a look at Scott Tannehill's IIS article

http://www.iceinspace.com.au/63-400-0-0-1-0.html

I did this with my 12" GSO and it made a big difference in both reducing dew and improving seeing. The box in which the primary of my 16" sits also has this effect and the primary never gets dew formation.

[astronut]

Paul,
Do not heat the primary mirror, the fan will be sufficient to keep dew at bay. You don't want to introduce more thermal currents into the tube.
One thing I forgot to mention, make sure you have a shroud for your scope.
I power the lot with a S.L.A 12V 7a/hr battery.
On the longer nights from late Autumn to early Spring, with observing lasting from before sunset to 3am- 4am, I will go through one full battery and 1/3 of another.
I always take two batteries with me year round, just in case.
I hope this helps, if you need more info please ask.:thumbsup:

[tnott]

Secondary heater is the most important one for a big Dob.
Then the finder, then the other bits like eyepieces etc. Most dob users I know don't have any heating though.

If the primary is generally an inch or thicker it will not usually dew up due to it's thermal mass. It can't radiate heat enough to match the descending fall in ambient temperature during the night. Getting rid of excess heat is more of a problem here. It is not the same as a thin SCT corrector plate.

A shroud may stop the optics radiating heat out into space a little, but once they have reached dewpoint the shroud won't make any difference. The shroud itself can collect dew and drop this on your optics.

I have always just used the Astrosystems Dewguard on my dobs' secondaries, kept the telrad dewshield in place, swapped eyepieces regularly, and kept the fan behind the primary running. Never had a problem.

Heating on the finder, eyepieces and filter slide would be handy though if you're happy with the extra cost and fuss in setting up.

For cooling, fans that scrub the boundary layer off the front of a primary only make a difference if you are wanting to do high-powered viewing in the first few minutes while you are waiting for the mirror to drop in temp. One might help stop dew forming on the front face of a thinner primary though, if the fan behind the mirror was not enough.

If you are going to run a fan all the time, it will move air more efficiently if you seal the bottom of the tube/mirror box, or at least make sure it has a baffle around it. Otherwise lots of air coming out of the front of the fan just circulates round and gets sucked in the back over and over again.

[PCH]

Hi John, tnott and Patrick,

many thanks for your replies. I can see there are differing opinions on some of these issues. So, in order to start on this road, the first step as I see it is some sort of multi-output heater controller.

Tnott mentions an Astrosystems unit, - but does anyone recommend another brand/model for some reason?

Many thanks for your time and trouble taken to reply guys, I really appreciate the info. Some of KR's gift money can go on this, so it's my duty to spend wisely. :D Don't suppose any of these items are Aussie made are they? If so I would check those out first.

Cheers

[astronut]

Paul,
I had a friend make the control box to my specs.
And I put together everything else with parts from Jaycar.
It's a project that will keep you busy.:thumbsup:

[Chippy]

Thoasand Oaks is another brand that sell them. There are a few Aus dealers.

[PCH]

Hmmmmmm ..... that may be beyond my abilities John and I wouldn't have a clue where to start. But otherwise a great suggestion :lol:

Chippy, thanks for that. I've had a little look around the usual sites and have sorted out two or three brands that look like they will do the job.

Have a great long w/e guys :thumbsup: