Dew heaters and hair driers

[Guest]

Dew heaters and hair driers are often recommended to combat condensation on various parts of the scope but we are usually told to allow time for the temperature of the scope to settle down before doing any serious observing. But is that a contradiction? If we start applying heater straps and blowing hair driers aren't we going to make the temperature more uneven? 

 

[Carbon Brush]

Yes it can be a problem!

Consider a reflector scope with a thick mirror, stored in the warm house.

You take it outside and the main mirror starts to cool. The mirror cools in an uneven manner. Edges and the thin centre lose heat more rapidly than the other regions.
This causes the mirror to distort a little. It is no longer a well figured parabola. This prevents you from getting pin sharp stars.

Next problem the warm tube and warm mirror cause rising air currents in the tube.
The difference in refractive index between warm and cold air means you get image distortion that varies with time preventing good viewing.

When you do focus, the warm (steel or ali) tube is longer than a cold tube.
As the tube cools the focus has to be adjusted.

Once the entire scope has cooled to near enough outside temperature, everything becomes stable and you get good performance.

Next problem. The clear sky is cold.
A weird statement. If you look around with an infra red thermometer, you will find the sky is colder than the nearby walls, hedges, ground, etc.
This means that as time passes, the scope mirror continues to cool towards sky temperature.

Ever wondered why sometimes in the morning your car windscreen is icy but not the side windows?
Side windows, being near vertical, are at surrounding temperature.
The sloping windscreen points to the sky, so cools towards sky temperature.

This is the point where your scope glass gets dewed up, but not other things.
Though there are nights when everything suffers condensation.

If you can heat the important glass bits, just a little, you prevent condensation.
Just a little is the important bit. Too much heat and you are back to the problems seen when you first take the scope outside.
A red light torch on glass will help you to see condensation before you notice poor viewing.
That is the point when you nudge the heater power up a little.

If you use a dew heater on the glass and put only enough heat into it to prevent dewing, there is minmial heating.
The glass does not distort and the tube does not suffer significant wrm air currents.

A blast from a hair drier over the front of a scope is enough to lose the image.
The hair dryer is really a last resort. Or for drying everything off before packing away.

Hope this helps,

David.

[joe aguiar]

well not really cause dew will start to become an issue after the 2 to 3 hr mark with powered dew control. then you are just putting enough hot air norm on maks/ scts for like less then 60 secs to get rid of the dew. you are not heating the entire ota or thicj mirror

for me 3 hrs is enough time with just a regular dewcap that by the time dew form IF it does iam almost done for the night. alot times if u have the dewcap thats enough to solve that problem BUT depnds too where you live. If u r in a humid hot place dew will settle faster and thats when those people use the hair dryier.

also some scopes cool down fast like 30 min or so (depending on sioze and what kind it is) so if u can get 3 hrs before the dew theres lots time to obseve b4 that happens and call it a night or have to use a hair dryier.

joejaguar

[Guest]

Thank you both for your comprehensive replies.  Dew is a new problem for me so I'm just getting to grips with it.

As a matter of interest, what's the best surface finish on a metal newtonian tube to reduce the problem of dew. I think it's possible to reduce the emissivity with the right kind of finish. I saw on other forums that some people line the tubes with some material but I thought it would be better on the outside.

Cheers

Steve

 

[Carbon Brush]

The tube inside is almost black paint as standard. This reduces internal reflections, improving image contrast. Just how 'almost black' being in part determined by the price tag.
Unfortunately some scope insides seem to be smooth black, rather than matt black.
Many folks use black flocking material to overcome these issues.
The tube inside is therefore the best heat radiator. A white outside being poor for radiation.
But as the tube is thin, there is good heat transfer from outside to inside so don't get worked up about colour.

Tube colour is as much as anything to get us to buy the latest model.
Are Skywatcher green dovetail bars mechanically better than the black ones?
Does a William Optics Redcat refractor work better because of the colour?
I'm sure my LS60 solar scope gathers more detail because of the red trim.😕

Before worrying too much, give your scope a few runs to get the measure of the problem.
If you observe downwind of the local duck pond, you will be in moist air and dewing will be more of an issue than on a hill top.
The same goes for being downwind of a central heating boiler flue.

As you get a feel for the magnitude of the problem, you can take whatever steps are necessary to avoid the dew.
If you don't stay out for hours on end, it may not be a problem.

Keeping eyepieces in warm pockets avoids condensation.
I made a 12V heated box for eyepieces some years back and posted pics on this site.
For some reason it provoked zero response. Perhaps because I made it from scratch, rather than adapting a standard product.

Advice on dew heaters does of course depend on your scope type.

HTH, David.

[markse68]

16 hours ago, Carbon Brush said:

Ever wondered why sometimes in the morning your car windscreen is icy but not the side windows?
Side windows, being near vertical, are at surrounding temperature.
The sloping windscreen points to the sky, so cools towards sky temperature.

Hi Carbon, I don’t understand the physics of this and would have put that phenomenon down to the moisture in air condensing in cold temp and falling like fine rain into the car where it settles on the angled glass but less so on the vertical glass? I’m guessing you are meaning it has something to do with black body radiation which I don’t really understand but isn’t it to do with temperature differentials which shouldn’t be much different from sides of car to top of car?

[NGC 1502]

Having your optics at ambient is best, although that’s not so much an issue for low powers and faint fuzzies. 
But dewed optics is far worse because you’ll see nowt......

So it’s a necessary compromise to apply only just enough heat to keep the dew away, not always easy to get it just right.

For “heat” read gentle “warmth”.....

Ed.

[Carbon Brush]

@markse68

The long explanation. Courtesy of NASA.

https://mynasadata.larc.nasa.gov/sites/default/files/2018-12/Measuring the Temperature of the Sky and Clouds.pdf

Bottom line. Clear sky is colder than surrounding objects, builds, or clouds.

HTH, David.

[markse68]

Very interesting- this is making me think a lot about heat and heat transfer and I think I’m starting to understand better- will give that a read- thanks

On a separate note I was thinking of making a small cordless unheated  blower for de-misting dewed up optics after Friday night when my primary got completely fogged. But I’m thinking that though it should do the job, the absence of added heat would lead to dropping the mirror temperature by latent heat of evaporation so it would likely dew up even quicker straight after 🤔 Probably not such a great idea

[jetstream]

Interesting discussion.

I have never had a mirror in a truss dob go below ambient outside.

My seacan, painted white is a very good stable environment for my scopes. I have extensively measured the internals temps with a Fluke IR gun and found that the top third gets colder than the bottom 2/3 when air temps are dropping. The flat grey interior is radiating heat out through the top.Unless very extreme temp changes the mirror is within 3c-4c warmer of the air temp.

I had one night that the mirror was 8degc cooler than outside and bam., instant fogging. The temp spread overnight to the next afternoon was +10c down to -12c, with the fogging temp of -2c. My mirror was -10c at the time.

I can see how it is possible for an aluminum tube dob to cause the mirror to go below ambient, from the temp gradient between the top of the tube and bottom. I have not however had a primary mirror in an Al tube dob fog up outside. Certain conditions and it could happen IMHO.

The biggest problem is bringing the scope back inside after cold weather use, I don't do this anymore but when I did the scope was in an insulated case.

Edited December 1, 2019 by jetstream