Dew prevention - advice needed

okiscopey (Mike) · #1 24-02-2007, 11:08 PM

My last observing session (Kulnura) was cut short by dew problems, so I've ordered one of those diddy little 12v hair dryers for the EPs and a dew-zapper from Bintel which fits on the front of my ETX125.

It seems these zappers need a controller, but they're a bit pricey.

As I already have a Jaycar digital thermometer which reads the temperature difference (to 0.1 deg. C) between two K-type thermocouples, so my cheapskate plan is just to keep an eye on the thermometer and simply switch the zapper off when temperature between the corrector and the night air is in the right range.

I'm not imaging, just doing general observing, and all of this is just the learning curve for hopefully bigger and better things.

Can anyone help with these questions:

1. Will this 'manual control' work reasonably well, or will the temperature swings be uncontrollable?

2. What is a workable temperature range? I understand if the corrector is the same or one or two degrees warmer than the air, dew won't form.

3. I only have a 7Ah battery but the zapper draws a couple of amps. Any guess as to the likely percentage of time it'll have to be on in Kulnura-type conditions?

4. How and where is the corrector sensor fixed?

Any links, comments or ideas will be gratefully received!

sheeny (Al) · #2 25-02-2007, 09:35 AM

G'Day Mike,

Quote:

Originally Posted by okiscopey

My last observing session (Kulnura) was cut short by dew problems, so I've ordered one of those diddy little 12v hair dryers for the EPs and a dew-zapper from Bintel which fits on the front of my ETX125.

It seems these zappers need a controller, but they're a bit pricey.

For under $40-$50 you can build your own:
http://www.iceinspace.com.au/index.p...63,285,0,0,1,0

Quote:

Originally Posted by okiscopey

As I already have a Jaycar digital thermometer which reads the temperature difference (to 0.1 deg. C) between two K-type thermocouples, so my cheapskate plan is just to keep an eye on the thermometer and simply switch the zapper off when temperature between the corrector and the night air is in the right range.

I'm not imaging, just doing general observing, and all of this is just the learning curve for hopefully bigger and better things.

Can anyone help with these questions:

1. Will this 'manual control' work reasonably well, or will the temperature swings be uncontrollable?

2. What is a workable temperature range? I understand if the corrector is the same or one or two degrees warmer than the air, dew won't form.

3. I only have a 7Ah battery but the zapper draws a couple of amps. Any guess as to the likely percentage of time it'll have to be on in Kulnura-type conditions?

4. How and where is the corrector sensor fixed?

Any links, comments or ideas will be gratefully received!

I've never tried this sort of manual control, but I imagine it could get pretty frustrating. It requires a lot less power to keep the corrector warm so the dew doesn't form than to evaporate it after it has formed, just because of the latent heat of evaporation of water. Hence the value in the dew heater controller.

In terms of temperature range of a couple of degrees I suspect you could be right. While dew forms it also evaporates at the same time. Whether dew forms or not is just a balancing act between the two processes. If evaporation is dominant, no dew forms. If condensation is dominant, dew forms. Le Chatelier's principle. So you don't need must temperature difference to sway that balance. However, if you are manually controlling, and you let the dew form, it will the take a significant amount of power (or time) to clear the dew.

I would also recommend another battery to drive the dew heater. 7AH won't last you long.

Al.

astronut (John) · #3 25-02-2007, 09:39 AM

Mike,
I can help with a couple of your questions. I use a controller for my dew heaters and it works very well.
Generally a two degree gap is all that's needed and yes it will keep the dew off.
I find that a 7a/hr properly controlled lasts me between 6 to 8 hours depending on the severity of the dew.
With this in mind I carry two batteries with me.
If the size of the battery does not matter, you could use the next step up a 12V 18a/hr.

Quote:

Originally Posted by okiscopey

My last observing session (Kulnura) was cut short by dew problems, so I've ordered one of those diddy little 12v hair dryers for the EPs and a dew-zapper from Bintel which fits on the front of my ETX125.

It seems these zappers need a controller, but they're a bit pricey.

As I already have a Jaycar digital thermometer which reads the temperature difference (to 0.1 deg. C) between two K-type thermocouples, so my cheapskate plan is just to keep an eye on the thermometer and simply switch the zapper off when temperature between the corrector and the night air is in the right range.
I'm not imaging, just doing general observing, and all of this is just the learning curve for hopefully bigger and better things.

Can anyone help with these questions:

1. Will this 'manual control' work reasonably well, or will the temperature swings be uncontrollable?

2. What is a workable temperature range? I understand if the corrector is the same or one or two degrees warmer than the air, dew won't form.

3. I only have a 7Ah battery but the zapper draws a couple of amps. Any guess as to the likely percentage of time it'll have to be on in Kulnura-type conditions?

4. How and where is the corrector sensor fixed?

Any links, comments or ideas will be gratefully received!

okiscopey (Mike) · #4 26-02-2007, 05:38 PM

Thanks Al and John, advice received and understood!

Have read the build-it-yourself article and all of Kendrick site for more info, Jaycar's catalogue has a 12V Motor Speed Controller (KC-5225) for $23.95.

I'll leave it to that Shakespeare bloke to have the last word:

"Out, damned dew, out, I say!" Adapted from Macbeth (V, i, 38)

netwolf · #5 26-02-2007, 07:39 PM

Mike,

I found this alternative with temperature sensing feedback that you could build. Its was developed by Don Clement (also makes the Clement focuser)
http://www.atmsite.org/contrib/Clement/dewheater/

AndrewJ also seems to have some good tips on this subject seem my new scope LX90 thread.

Other homemade ones
http://www.backyard-astro.com/equipm...dewheater.html

I like this idea of using a coper tube to hold the resistors as a dew heater. Nice comfy fit. But i would not want to have an accident so close to my corrector.
http://www.beevo.com/Dew_Heater.htm

Off the shelf solutions:
I also called up Astro-optical today and they custom make controllers for you. Depending on how many heaters you want to use. I was quoted about 150$ for this, but you can get a Thousand Oaks controller new for the similar price at myastroshop. They sell the Astrozap heater straps and the Astrozap flexi dewshields. Also the Kendrick Mark IV dew heater is in the same price bracket as the thousand oaks one.

I have read many good things about the Dew-not heaters combined with the dewbuster controller. The Dewbuster unlike the thousand oaks and Kendrick Mark IV has temperature feedback. This is about double the cost.

Then there are the Digital Kendrick line of controllers. And another on from the UK called Revelation Pro.

Regards
Fahim

sheeny (Al) · #6 26-02-2007, 08:26 PM

Quote:

Originally Posted by okiscopey

I'll leave it to that Shakespeare bloke to have the last word:

"Out, damned dew, out, I say!" Adapted from Macbeth (V, i, 38)

Very good!

Al.

okiscopey (Mike) · #7 26-02-2007, 09:56 PM

Many thanks Fahim, that was a good bit of googling - a lot more than I found. All bookmarked for future reference.

Because I've already got that super-duper temperature-difference device, I'll stick with finger-powered on-off-ing for the moment with the Bintel heater I've ordered.

If you don't hear from me again on this subject, it's because manual switching was a dismal failure and I'm hard at work with the soldering iron!

netwolf · #8 26-02-2007, 10:41 PM

Okiscope, at this point I would pay someone to build me a solution. Though i understand they theory quiet well. I was never any good with my hands, so soldering is out. I could do it all on a bread board I suppose, but its hard to put a breadboard around my telescope.

Regards

okiscopey (Mike) · #9 26-02-2007, 10:47 PM

Quote:

Originally Posted by netwolf

I could do it all on a bread board I suppose, but its hard to put a breadboard around my telescope.

You'd have to make sure the crumbs don't fall onto the mirror.

Best of luck Fahim, in the end we all need to find our own 'best way'. I'm a bit reticent myself with circuits of this level or above ... they're all right if they work first time, but if they don't you need to be an electronics guru to work out what's gone wrong.