"Dew Heater" question

[raybiker73]

Apparently, every time I go outside with a telescope, that is a signal for Mother Nature to drop us below the dew point, so I've been having a lot of issues with fogging. I've seen what are called "dew heaters," which seems to be just a heated electric element that wraps around the scope, presumably at the lens. Do these work, and if so, how and when does one use them?

[Refractordude]

Hello Raybiker73:
What type of scope do you have? I use a non electrical Astrozap for my 120mm f/8 refractor.


For my binoculars I use a 12v hair dryer. It also works for telescope lenses also. Just give the lenses a 30 seconds warm up every half hour or so.
https://www.ebay.com/sch/i.html?_from=R ... r&_sacat=0

I really like the Zap-on, but have no need for them on the telescope. May get two for my 20x80 binoculars in the future.
http://www.tech2000astronomy.com/zap/

[Lady Fraktor]

Yes they work very well.
I use the Kendrick Premium Dewheaters and have had my telescope out at -35°C with no issues.
I was colder and had more frost on me than the telescope did.

[JayTee]

This thread might be of interest to you. viewtopic.php?f=24&t=3444

Cheers,
JT

[pakarinen]

https://www.skyandtelescope.com/astrono ... ew-heater/

12-volt single channel controllers (if you need one) can be had for about $10 on Amazon.

You can also find motorcycle glove 12v heaters on Amazon. Haven't looked carefully, but IIRC they're a plate that are curved or can be bent a bit.

Usual proviso - Don't do anything with voltages above 12 volts unless you're a trained electrician. You could die or melt something on your scope, etc.

Personally, I just wrap a chemical handwarmer around my dew shield with a rubberband. Cheap, easy, no power needed.

Edit: Found 'em:

https://www.amazon.com/Bro-Universal-Mo ... 37&sr=8-10

If I go electric, I will most likely try these.

[KathyNS]

The thing to understand about dew is that it is not the air temperature that goes below the dewpoint. It is the lens/corrector that drops below the dewpoint. (If the air temperature were below the dewpoint, there would be fog.) The front element of the scope radiates heat to the sky, which is close to absolute zero, so it drops below the air temperature.

Dew heaters add enough heat to prevent the temperature from dropping below the dewpoint. Dew shields cut down on the outgoing radiation, slowing down the rate of heat loss.

[TCampbell]

Think of what happens when you pull a glass out of the refrigerator or freezer and it fogs up... same thing happens to the optics on the scope.

A "dew shield" helps delay this by trapping some of the warm air and reducing the circulation ... in the hope that it will take longer before the dew starts forming.

You are correct about the dew heater bands. There are some caveats.

If you apply too much heat, you wont have a problem with dew, but you'll warm up the optical surface enough that it will create heat currents. This can distort your views. So the idea is to heat the optics "just enough" to where dew wont form... but no warmer.

The bands are typically installed just behind any important optical surfaces.

Dew heaters typically run on 12v power. The amperage is mostly based on the length of the dew strap (larger diameter scopes need more amperage). The strap will typically have the RCA "tulip" style plugs. This typically connects to a "dew controller". The purpose of the controller is to let you set the power and this works by pulsing the 12v power on and off. E.g. if you set, say... 30% power, then it means it will spend 30% of the time in the "on" state and 70% of the time in the "off" state. Sometimes (but not always) the cycles repeat each second. So that would be .3 seconds "on" and .7 seconds "off".



There are some slightly more advance controllers that have temperature probes. The probe can measure the scope's temperature *and* usually measures the ambient temperature. You set the number of degrees above ambient to heat the scope and it does whatever is needed to maintain that temperature. I think "DewBuster" was the first company to produce a controller that does this ... but there are now others who copied the design.

There are some controllers that are even more advanced... and basically amount to a weather station. They monitor the temperature, relative humidity, and actually calculate the dew-point for you... and then automatically keep the optical tube just a few degrees above the dew point. Astro-Smart makes dew controllers that do this. I first learned about them because they used to make an "integrated" dew controller that was molded to fit into a battery bay of the Meade LX200 telescopes (nobody actually puts batteries in the battery bays anyway.)

But the most basic controllers typically have a simple dial that sets the percentage of the time "on" vs "off". A very simple controller is typically about $100. Advanced controllers can run a few hundred dollars depending on the features.

Dew controllers usually have multiple "channels". This lets you apply more heat to large surfaces ... but use a different channel set to a much lower amount of heat for smaller surfaces that are easier to keep warm.

[JayTee]

And just in case you wanted even more info, here is an S&T article on this topic which expands on what Kathy and Tim are saying: https://www.skyandtelescope.com/astrono ... -with-dew/

Cheers,
JT

[UlteriorModem]

I make my own dew heater strips using parallel resistors.

I dont know if you are the diy type but if interested when I get home I can post more info. In the meantime I do have a thread on that other forum

[pakarinen]

Picked up something like 40 handwarmers from Costco last night for $12 plus tax. That should last through fall and winter given how cruddy the weather usually is here.

[raybiker73]

I make my own dew heater strips using parallel resistors.

I dont know if you are the diy type but if interested when I get home I can post more info. In the meantime I do have a thread on that other forum

Yes, please! I do ham radio and model railroads, so I have loads of resistors laying around.

[JayTee]

Yes, please! I do ham radio and model railroads, so I have loads of resistors laying around.

Just to throw in my 2¢ worth, building a resistor heater is MUCH more difficult to both build and use than a simple nichrome wire setup. Trust me on this.

Cheers,
JT

[UlteriorModem]

Jaytee has a valid point, but since I also do Ham radio and used to do trains pretty handy with a soldering iron

http://www.dewbuster.com/heaters-330ohm-resistors.html

Don't know the 'feelings' or rules about cross posting but...

http://www.astronomyforum.net/astronomy ... bands.html

[JayTee]

IRT cross-posting, I don't think we are too hard over on that if the cross-post is to AF.net. We need to pull as much info as we can off that site before it goes tango uniform!

Cheers,
JT

[gregl]

I made some dew heaters with nichrome wire and some Chinese PWM controllers, all off Ebay. Wrapped the wire in duct tape straps which then wrap around the scope. There are calculators on the net that will tell you how much wire and what size to use for a given wattage, and while I checked them, I didn't over-think the whole thing and they work fine. Just be advised that you can't solder to nichrome so the connections will have to be mechanical.

[JayTee]

My nichrome wire is connected to my copper wire that leads to an RCA plug (on both ends). This was done by doing several wraps of the copper around the nichrome and then each wire been back on itself. This wire wrap was then set in a small wooden trough and liquid solder was dumped on top of it to basically encase the whole thing in solder. Granted the solder is not adhering to the nichrome but since the copper and nichrome are touching (making contact) and it's encased in solder it works.

Cheers,
JT

[pakarinen]

So those Costco chemical warmers I bought measure 3.5 x 2 inches. I opened one around 0230 today and put it on the dew shield of my 80mm. The heat pack is sitting on my desk right now and feels hotter than a couple of hours ago. I'm impressed. And I should probably have some coffee today or I'll be going to bed at 6 tonight...

TIAIHTS.

[OzEclipse]

Nichrome wire can be silver soldered - the type with a brush on flux paste and silver solder rod as used by plumbers. Apply the flux then heat to red heat before applying the silver solder rod. Due to the amount of heat involved, solder a short piece of bare solid copper wire to the end of the nichrome, then conventionally soft solder a plastic coated copper wire to that. Alternatively, use a screw terminal. Nichrome is very hard and does not crimp so I don't recommend crimp connectors.

Joe

[raybiker73]

I think I may try the nichrome wire DIY. I have a partial spool of 26 gauge nichrome from when I built a foam cutter, so I can put it to use.

[Gordon]

IRT cross-posting, I don't think we are too hard over on that if the cross-post is to AF.net. We need to pull as much info as we can off that site before it goes tango uniform!

Cheers,
JT

Considering what's going on over there now. I would suggest the original content be copied and pasted by the original poster. We could easily lose everything if we just 'link'.