Heating the tube to keep dew of a Newtonian mirror

[Paul Haese]

Big guy, I don't think this will work. Any heat placed into the tube will induce some tube currents. Even minor heat will do this. Which piece of equipment are you thinking of using this on?

I have been looking at this same issue with my RC. It gets dew on the secondary and also the primary. I have extended the length of the tub by nearly double and this is still an issue. As soon as the mirrors reach ambient I find dew starts forming at double time.

[AlexN]

I was hoping you'd come in and comment Paul, As I was saying earlier in this thread, I remember suffering many frustrating nights with my C11 wondering why I couldn't get a sharp planetary image.. You pointed out the heater I was using, and upon removal of that, everything became very very clear.. I don't like heaters much anymore.. on refractors they are a necessity.. at longer focal lengths (>800~900mm) they do start to distort the images...

[hikerbob (Bob)]

Adam I put up some detail on the PWM Dew heater thread last year on work I've done on sensing the dew point and on measuring the temp under heater straps. See the thread http://www.iceinspace.com.au/forum/s...ead.php?t=8514 for that.

I'm in the midst of a rework of the controller and have obtained but not yet tested a cheaper temp/humidity sensor than the one I used originally.

I've not tried heating the primary with it so can't comment on that but for management of dew on the finderscope, eyepieces etc I've been very pleased with the results. I started an article on it some time ago but have not finished that yet. I hope to get to a point of having a design and part's list that relatively easily done on veroboard and not to hard to source the parts. I'm also having a try with a printed circuit board version but I'm still learning my way around that space.

Bob

[strongmanmike (Michael)]

Thanks for all the replies guys! much appreciated

Interesting scenario huh? Mixed feelings

Mike

[TrevorW]

Just a thought in the car we turn the air con on to demist the windows ie: colder air

[adman (Adam)]

Originally Posted by TrevorW

Just a thought in the car we turn the air con on to demist the windows ie: colder air

The aircon compresses gas which cools it. This causes condensation at the point (somewhere under the bonnet) where the heat is exchanged with the air you are blowing through your car. You get nice cool, dry air inside, and a puddle of water outside. You can't cool air below its dew point without water condensing. Same with your home airconditioner - thats why they have those drains that run the condensation outside.

By airconditioning your car, you are lowering the temperature - but also sucking all the moisture out of the air inside, which lowers the relative humidity. Thats why it works as a demister - not because its cool.

If you remember the days before aircon in cars - you had no choice but to crank the heat right up to try to evaporate the moisture on the windows - but that just made the inside really steamy. So you ended up with the heat way up and the window open!

Adam

[DavidU (Dave)]

Mike, I can't see any heating of a Newt is going to give good images.
I'm working on something that will work.

[TrevorW]

Dew forms when a surface cools (through loss of infrared radiation) to a temperature which is colder than the dewpoint of the air next to the surface.

My point was if the dewpoint of the air above the mirror is in fact colder than the mrror would dew form

A good article on dew control can be found at

http://home.comcast.net/~astrophoto/Articles/Dew.htm

[gregbradley]

A simple solution may be an insulating blanket on the outside of the tube where the mirror is.

I bought and used a dew blanket for my refractor. It is really just one of those windscreen sun insulators with a bit of velcro to wrap it around on itself.

You simply wrap it around the APO tube where the objective is at the start of the night. The idea is it slows the temp drop of the objective so it stays above the dewpoint. It worked really well.

My dark site though tends not to be too bad for dew although the OTA has at times been dripping.

So one of those windscreen sun protectors, a bit of velcro strip from Spotlight and wrap it around the tube where the primary mirror is and perhaps that may work. It may not work as well on an open tubed scope as the heat can leave through the tube but it may.

I don't recall ever having dew on my RCOS 12.5 inch with a carbon fibre tube and it always had the 3 fans going pulling air down into the tube so perhaps that does work.

Greg.

[adman (Adam)]

Originally Posted by TrevorW

My point was if the dewpoint of the air above the mirror is in fact colder than the mrror would dew form

OK - I see where you are coming from now. But I don't think that it would work. The best way (not to mention the easiest...) is always going to be to raise the temp of the mirror to above the dewpoint, rather than cooling the surrounding air below the temp of the mirror. Because as you cool the air, it will be able to hold less water vapour, making dew formation more likely

Adam

[peter_4059 (Peter)]

Mike,

My Kendrick Newtonian heaters have finally arrived. Here is how I'm planning to keep the mirrors slightly above ambient so dew isn't a problem this year at astrofest.

The secondary heater has built in adjustable temperature control with ambient and mirror temperature sensors to keep the secondary 1-3 deg above ambient.

I'm planning to install a temperature sensor on the primary mirror so I can adjust the PWM dew controller to just raise the primary a bit above ambient.

Peter

[Insomniac]

Now does anyone know of anyone who sells a heating unit for use on a 20" primary?

Here north of Auckland I've had my primary dew up quite frequently. I've got one small fan on the back of my dob mirror, and I don't even run that all night!! Maybe the laws of physics don't apply around my observing site... It's always been a puzzle to me after reading so many folk say how uncommon it is, etc.etc...

Chris

[stringscope (Ian)]

G'day Mike,

heating the tube sounds like a recipie for tube currents to me.

Cheers

[pmrid (Peter)]

I'm modifying a 400mm dia water-heater scope and am planning on an open truss design. I'm wondering how well/poorly the various condensation-busting suggestions canvassed in this thread might apply to such a design.

Or, are there other ideas/solutions that are more specific to the truss design?

Peter