To Dew Or Not To Dew?

RAJAH235 · #1 08-11-2005, 01:58 AM

Hi guys. A question regarding dew & colours @ night!!!!!!!
Has anyone noticed the difference that colours make, to whether or not, things dew up?
A couple of weeks ago, (when we had good weather), I was set-up next to another member's home made, t'scope, (10" DOB). His is full gloss dark blue & my Meade is the original white. (both Sonotubes).
We were no further than 6 ft. apart, but as the night progressed, his started dewing up & the water was almost running off his tube later on. Mine remained relatively dry, & I do mean dry.
Conditions were exactly the same for both. Had to be........distance involved.
Now everybody knows that it doesn't matter what the colour of your tube is @ night, BUT, it made a difference here. We were both dumbfounded.

This is not the first time I've noticed this. It happens with Meade blue SCT's, Orion black DOB's, unbranded silver DOB,(tin tho!),etc etc.. Gotta be really bad to dew up mine.
Only talking tube here, not E/Pcs or accessories.
Has anyone an explaination?????????

L.

bird (Anthony Wesley) · #2 08-11-2005, 12:17 PM

Okay, I'll bite on this, cause it's something I did a lot of research on a year or so back...

I had heard from a friend that aluminium made a good material for scopes cause it resisted dewing, and I was getting a new tube made from aluminium so I wanted to know what that was all. Google to the rescue...

You can find lots of info on this via google, so here's the short version :-)

Dew forms when a surface is colder than the dewpoint of the surrounding air. Dew forms on horizontal surfaces under a clear sky because that surface has cooled below air temperature by radiative loss up into space. Most materials radiate heat away at wavelengths that are transparent to the air, so they can cool by losing heat without warming up the air next to them. This is called "supercooling" and it's a major cause of tube currents in amateur scopes when the tube is 3 or 4 degrees colder than ambient air and you get a "waterfall" of cold air running down the inside of the tube and washing across the mirror.

If the tubes are painted then the emissivity of the paints will determine what happens. If you have a paint that emits IR at wavelengths not absorbed by air then any surface coated with that paint will supercool under the open night sky.

In the case mentioned here, one paint emits IR at wavelengths absorbed by air, preventing supercooling, and the other one does not.

It so happens that aluminium is a good material for scopes because it emits IR at wavelengths that *are* absorbed by air, so it can't supercool and in fact it will follow the nearby air temp quite closely.

I can't remember who it was, but someone at the starcamp in ballarat commented that my tube was still dry when the table and other stuff around me was heavily dewed, so I went through this explanation.

So the best material for a tube as far as I can see is unpainted aluminium.

regards, Bird

janoskiss (Steve H) · #3 08-11-2005, 12:24 PM

Does that mean that if I wrap my scope in Al foil it will keep dew away? Al eyepiece rack/case, Al dew shield? It would be great to be able to passively conquer dew (without heaters, batteries, electronics).

bird (Anthony Wesley) · #4 08-11-2005, 12:27 PM

Sounds like something to try. Dew will form eventually, but not as fast as with other materials. Even my aluminium scope gets dewed when the temp drops low enough.

A heated dew shield is nice for keeping the dew away, but I don't think it's good for imaging to introduce a heat source near the top of the tube.

Bird

janoskiss (Steve H) · #5 08-11-2005, 12:30 PM

But you have a fridge at the back of your scope, Bird.

Robert_T · #6 08-11-2005, 12:40 PM

dew on the tube is one thing, but will the formation of dew on outside of the tube have any impact on whether or how quickly the actually optics will acquire dew?

cheers,

bird (Anthony Wesley) · #7 08-11-2005, 12:45 PM

Robert, the air that gets into the tube will likly have already dropped its moisture on the way if you have a tube prone to dewing. I try and keep my tubed closed when I'm not actually using the scope just to cut down on the amount of moisture that can get inside.

Steve, gotta have somewhere to keep the XXXX :-)

Bird

RAJAH235 · #8 08-11-2005, 10:10 PM

Many thanks, Bird.

So it's all down to the colour of the tube, provided they are made of similar material? Good old IR eh! Never thought of that aspect.
Will have to relay message to our atm's re; materials & colours, for their tubes.
Regards, L.

rowena · #9 08-11-2005, 10:24 PM

Laurie you mentioned the scope was a glossy blue color yet yours is in the white.
Is the paint on your scope a gloss? i don't think it is from what i recall. you may find it is all to do with the paint itself. The two different types of paint may act differently with moisture in the atmosphere!

Call in the Mythbusters i say!!!!

*edit* Doh shoulda read Birds response

RAJAH235 · #10 09-11-2005, 12:15 AM

Sorry Rowena, but it's gloss as well.
Thanks again, Bird.

fringe_dweller · #11 09-11-2005, 12:46 AM

Guy's I am going off thread a little but, its to do with tube currents. Myself and a friend have noticed when we put white light solar filters over the end of our dobmounted newtonian reflectors - the seeing and lack of tube currents even when the outer tube is being heated by the sun, compared to most night time problems, there is a significant average initial difference in steadiness for day time use. We assumed that it is because its sealed from airflow that this is the case maybe - so no movement inside to out and vice verca. We wondered just out of curiousity wether getting the same sort of clear plastic they use for these astronomical use white light/mylar without the coating on it and using it for night time use - it would have to be of amazing light transmission of course tho? Also see it sometimes when you stop down the aperture with a mask a little bit too.

[1ponders] (Paul) · #12 09-11-2005, 12:50 AM

Mak Newt Kearn?

fringe_dweller · #13 09-11-2005, 12:56 AM

hehe yep

Mak Newt Dob

bird (Anthony Wesley) · #14 09-11-2005, 07:53 AM

G'day Kearn, You can get to the same result if you carefully equalize the temperature of your mirror so that it's equal to the surrounding air, and use something like unpainted aluminium for your tube. This combination will stop tube currents.

I cool my primary with an active refrigerating-thingy made from a couple of peltier coolers and some old heatsinks and fans, and it does the job nicely. The cooling unit also closes off the mirror-end of my tube, so that makes it even harder for air to circuate into the tube from outside.

I've written a few short articles about this cooling on my website, http://www.acquerra.com.au/astro/

regards. Bird

fringe_dweller · #15 09-11-2005, 05:18 PM

Hi Bird, mate i have read all of your posts and webpages with great interest from day one

the proof of your sytem is also very evident in your imaging results - its an awesome system you have there, there is no question that it is a seriously effective set up. the only drawback I can see with the refrigeration device in a dob set up would be the extra weight hanging off the back maybe? But i love the concept of the unpainted aluminium tube, i had never heard that before - a very original idea!
I have always covered the smallish holes in the back of the mirror cell support after initial half hour - hour cooldown - on my 8" I use a shower cap - it is noticeable improvement of course and mandatory. It is a shame that these wonderful scopes have this problem - renderring them compromised quite often. your refrig. ideas are the best way i have seen so far of addressing this serious flaw in an otherwise near perfect design.
Cheers