Keeping your Dob' warm.

[Fraunhoffer]

As Autumn and winter approach, I wondered if anyone had some tried and trusted tips to keep the dew away (apart from copious dew heater patches).
Ive read about wrapping the tube in foil or bubble wrap.

Does it help to put a blanket on the ground ?

[Paz]

A dew shield extending from the front of the ota till help a little, plus any material you might be able to add to the back of the secondary to insulate it from losing heat to the sky, and capping the top of the ota if you have any gaps/rests bewltween observing things will help.

I think insulating the tube should help in theory but that's not something I've tried.

[bosun21]

Insulating the tube really only benefits closed design telescopes like SCT's and Maksutovs as it helps mitigate the thermal currents in their tubes. I just use a dew shield and place a cover around the primary mirror and. I don't mind the tube itself icing or dewing up 

[Carbon Brush]

Excuse my basic unproven physics here. I was taught physics in an earlier century so have forgotten a lot.

All objects radiate infra red (heat) and receive it from the environment.
For example after sunset a house outside wall can be much warmer than the air. Useful to lean against to stay warm😁.
When you see infra red images showing hot people or objects you get a good visual representation.

If you point a (posh expensive) infra red thermometer at objects you get more accurate readings.
If you point the thermometer to the clear sky (remember these?) it may show 30C lower than your surroundings.
Your scope glass, if the tube is insulated, can easily drop below surrounding air temperature, encouraging dew.
The insulated tube does not echange with the surroundings and the open end looks at the cold sky.

If my thinking is anything like correct, then a reflector that is open at the rear, will take longer to dew than one with a closed rear cell.
This is because air will convect down through the tube if the mirror chills below ambient. But it is a slow process.
If the fan is running, it blows ambient temperature air through the tube, so the mirror never chills (much) below ambient.

If you blow warm air into the tube, obviously there is no dew. But you do get thermal currents disturbing the view.
In extreme, uneven mirror heating means you can't get a good focus.

I'm sure someone more knowledgeable will be along soon with a better explanation.

 

[Guest]

So this is where my 10 inch dob sits when i use it.  Notice.... there is no grass under it.    When i got done observing it looked like i had been out in the rain.  No dew shield ever made was going to stop that.  The only thing that saved the night were my heaters.  If i can find the picture again i will add it to this post.  Of course your environmental conditions probably arent the same as mine and you may never see as heavy a dew as i get here 

Edited August 21, 2023 by Mike Q

[Fraunhoffer]

9 hours ago, Mike Q said:

So this is where my 10 inch dob sits when i use it.  Notice.... there is no grass under it.    When i got done observing it looked like i had been out in the rain.  No dew shield ever made was going to stop that.  The only thing that saved the night were my heaters.  If i can find the picture again i will add it to this post.  Of course your environmental conditions probably arent the same as mine and you may never see as heavy a dew as i get here 

Ah i see where the heater is.

What are the rectangular blocks in the second picture?

 

[Mr Spock]

Dew forms when the temperature of a surface cools down to a temperature that is cooler than the dew point of the air next to it. When this happens water vapour will condense into droplets depending on the temperature. The temperature at which droplets form is called the dew point.

An object becomes cooler than its surroundings through heat exchange, much like how a fridge works. Heating a tube, though initially removing dew, will create more heat exchange and consequently more dew. The trick is to get the tube to ambient temperature rather than too warm or too cool. The only way to do this is by ambient temperature air flow over the tube surface. Dew forms on grass because the ground is constantly supplying heat to the surface.

The other way is to lift the scope out of the ground/air heat exchange region. This is usually around 4'. So if you can have your scope on a 4' platform...

[Guest]

1 hour ago, Fraunhoffer said:

Ah i see where the heater is.

What are the rectangular blocks in the second picture?

 

Magnets.   I can move them around to improve the balance.  These are ok but i need to get something better.

I actually have 3 heaters on this scope.  One on the OTA, one the finder and one for the eyepiece.  I also have heater on the laser.  The key is getting just enough heat to keep the stuff dry.  On a normal night I would run my heaters around 25 percent on this particular night I started at about 50 percent and backed off to about 40 percent power.  It kept everything clear.  

Edited August 22, 2023 by Mike Q

[Louis D]

I have a sonotube OTA on my Dob, and I've never had dewing issues like those shown in @Mike Q's image.  Does painted "cardboard" dew more slowly than metal?

[Guest]

7 hours ago, Louis D said:

I have a sonotube OTA on my Dob, and I've never had dewing issues like those shown in @Mike Q's image.  Does painted "cardboard" dew more slowly than metal?

I would suspect the metal tube would cool off faster then cardboad and probably show it more then cardboard.  Just shows the difference between environmental conditions 

Edited August 24, 2023 by Mike Q

[bosun21]

9 hours ago, Louis D said:

I have a sonotube OTA on my Dob, and I've never had dewing issues like those shown in @Mike Q's image.  Does painted "cardboard" dew more slowly than metal?

It will do as the heat transfer of metal is much faster than the sonotube therefore the dew will form much quicker.

[Louis D]

So, wouldn't insulating the metal tube outside and inside (with flocking) slow down dewing?

[Guest]

I would think that would depend on where you are at and the environmentals.  I have yet to see a dob around here wearing a coat.  

[Guest]

Personally i have not ever been convinced that insulating a dob is all that effective.  It seems to me that there comes a point where you just have so much open area that air is going to get in there anyway.  I can see it working up to a point, but once you get past say 8 inch, thats a pretty big open area ya got there.