DIY dew heaters

[JamesF]

It's about that time, really. I need some dew heaters. Life would be simpler with them.

I've followed threads about DIY dew heaters for some time but doing so has not really led me to any definite conclusion regarding the "right" way to do it so I'd appreciate people's thoughts. The main candidates for heaters are, I guess, a selection of camera lenses, my Mak, SCT and perhaps the ED80 and ST80. So...

What are good ways to make them and from what materials?

How do you calculate the requirements for given lens sizes?

Do they need to be temperature controlled and if so, how?

What are good was to supply power to them?

Do you still use a dew shield with them?

I feel sure there are other things I've not thought of, but I think that's a good start.

James

[Tinker1947]

I made a dew heater for my 250PDS Secondary, 20 X 330ohm 1/4 watt resisters formed up like a horse shoe to go round the secondary stem, used a e-bay voltage regulate, i think all the bit never cost £10 works ok after 3 hours no dew on the mirror.......the ST80 i use a Revelation strip keeps the ST80 main piece of glass clear with just the ST80 Shield in place.......needs some tidying up but the 2 heaters are run from a Maplin 7A supply and the voltage needs to be around 5 volts on both heaters.....

[faulksy]

hi james. i used maplins pwm controllers aboy 15 quid each part no 919d2p, and a small enclosure from them about 5 quid. i used nichrome wire for the heating elements you can either measure 33 ohms which is what i did but later i used this method, set a length of nichrome wire in the vice or similar put a 12v fully charged battery to 1 end then the other terminal to the other end adjusting the length of wire to suit. i.e. if the wire was to hot then lengthen it to cold shorten it. measure the length then cut as many as you need to complete your application. i then covered them in heat shrink and then wraped them in neaoprene. i used 3 for my collector, 1 for my view finder and 1 for my eye pieces. battery drain appears to be very light around 2 amps on full chat. hope this helps regards mike

[badgerchap]

http://www.deepskywatch.com/Articles/newtonian-dew-heater.html

Good instructions for calculations here

[JamesF]

Quite astonishing the variation on pricing for those dimmers on ebay. Even considering only those from the UK they seem to go from less than £5 to £13.

James

[JamesF]

Aha! I've just found this page which appears to have some useful information on, too:

http://www.iceinspace.com.au/63-292-0-0-1-0.html

James

[starnut]

I used this site when I made mine

http://www.dewbuster.com/heaters-330ohm_resistors.html

[JamesF]

That looks rather densely packed I shall have a read of it later. Presumably there's very little difference as regards the maths and design characteristics whether I use resistors or nichrome wire. I quite like the idea of using nichrome wire. If something is to be wrapped and unwrapped from the telescope or camera lens on a regular basis then I think avoiding as many joins as possible seems like a good plan.

James

[Tinker1947]

That looks rather densely packed I shall have a read of it later. Presumably there's very little difference as regards the maths and design characteristics whether I use resistors or nichrome wire. I quite like the idea of using nichrome wire. If something is to be wrapped and unwrapped from the telescope or camera lens on a regular basis then I think avoiding as many joins as possible seems like a good plan.

James

I believe its best to put the dew heater in place then leave it, unwrapping and wrapping will lead to a broken wire as this is single strand....

[IanL]

James,

You have probably see the thread, but here is a link if not:

http://stargazerslou...s-sanity-check/

Summary of the calculations is below. It should give you enough to go on even if you decide on resistors rather than nichrome as the calculations are the same. Resistors may be a better bet if you are making some for eyepieces or secondaries, otherwise nichrome is less soldering overall and less likely to break with use. Your target is a power output of 0.3 watts per cm of band, which is plenty for dew prevention above zero Celsius and works fine a few degrees below zero (haven't been out in Arctic conditions).

With a metal dew shield on a refractor the approach is to heat it from outside just in front of the objective. The heat will conduct through and then radiate on to the lens to keep it clear. You need to keep the temp down to a minimum to avoid thermal currents in front of the lens.

If you are using a foam shield with a Mak or SCT wrap the band around the metal at the edge of the corrector and slide the shield over it if poss. May be possible to make a radiative heated shield somehow; metal would work but foam would block the heat reasonably well unless you crank it right up (and set fire to the foam maybe!) The other option is to make a tube that sits on the corrector surround in an SCT, don't know if the same is possible for a Mak.

The one difficult bit is working out what the resistance of the Nichrome is. You can't really rely on what people advertise for short lengths on eBay. I just spend a few quid on getting a couple of different gauges that looked reasonably close to what I needed and then measured the resistance per metre using a multi-tester.

The only note of caution I would sound (as per below) is to use a cheap 12v laptop power supply to do your build and testing. If you get the maths wrong, hopefully something (the fuse or the supply) will blow before you have a disaster. If you are using a power tank or other battery supply and miscalculate, you could end up with a fire on your hands (literally your hands if unlucky) or damaged equipment. Nichrome melts at a vastly greater temperature than insulation and skin does, so play carefully.

Formulae in case anyone needs them in future:

Rt - Total resistance of band (ohms)

Rm - Resistance of wire per metre (ohms)

C - Circumference of OTA (cm)

O - Overlap of band needed (cm)

T - Number of turns of wire (for a single run of wire)

R - Number of runs of wire (for a parallel set of wires)

V - Power supply (volts)

L - Load on power supply (amps)

Ot - Total output of band (watts)

Oc - Output of band per cm (watts)

For a single run of wire:

Rt = ( (C + O) * Rm / 100 ) * T

Or for several parallel runs of wire:

Rt = ( (C + O) * Rm / 100 ) / R

N.B. In both cases the calculation assumes that the turns or runs of wire are the same length and thus the same resistance.

The load on the power supply is:

L = V / Rt

The total power output of the band is:

Ot = V * L

The power output per cm is:

Oc = Ot / (C + O)

N.B. You are aiming for an Oc of about 0.3 watts per cm, a bit lower or higher will not do any harm but please be aware that nichrome wire can get very hot. Basically it is the glowing stuff you find in your toaster, and if you screw up your calculations you can easily burn your fingers, damage your equipment or burn down your house.

The biggest risk is if you short-circuit the heater band and cause its resistance to drop dramatically. For example my SCT band using 12 ohm/metre nichrome is perfectly safe at 73cm length and draws a load of about 1.25 amps. If I managed to short it out so the effective length is now only 1cm of nichrome, it would draw 24 amps = DANGER. Nichrome wire can withstand very high temperatures and there is a risk of fire in this scenario.

If you are using a mains power supply, something will either blow or trip fairly quickly (unless it can handle such a high load of course), but if you are using a battery, you may well get all the amps you asked for and some fireworks.

I'd always recommend using a fuse in series with the heater band that is rated slightly more than the value of L that you calculated above. Even using a fuse a short can damage your power supply or heater controller, so if you are really concerned then do your own research on wiring in an 'overvoltage crowbar' which will also protect the PSU in case of a short.

[JamesF]

Thank you for that Ian. It all makes good sense. I'm off to measure some tubes and do some maths

James

[JamesF]

Hmmm. It looks like some lenses and telescopes work out much more nicely than others...

My 127 Mak needs a resistance of about 10.4 ohms at 12V by the looks of it. It's 46cm in diameter, so a loop of 28SWG (I think) would appear to do the job quite neatly. On the other hand my kit Canon lens requires a resistance of about 25.5 ohms with a 12V supply over a length of about 19cm which is a bit more tricky. Need to think about that one a little more...

James

[JamesF]

Tinker, are those dimmers PWM units supplying the full input voltage for the pulse, or are they limiting the voltage? What are the LED displays that you have hooked up?

James

[Squagnut]

One method I've seen to make the heater element is to get a load of 150 ohm resistors (it's early and I'm not doing the exact maths!) and solder them in parallel to two strips of de-soldering braid, so it looks like railway tracks. This has enough flexibility to survive being taken on and off repeatedly.

If you use Arduinos then, with the aid of a TIP120 transistor, it's not difficult to use one as a PWM for any 12v application.

[JamesF]

I think that might be a later enhancement, to measure the temperature around the protected surface and adjust the heat level accordingly.

James

[IanL]

Tinker, are those dimmers PWM units supplying the full input voltage for the pulse, or are they limiting the voltage? What are the LED displays that you have hooked up?

I built a three channel controller using the cheapo eBay LED dimmers. They supply the full 12V so you don't need to do any complicated maths to work out the power output when they are on full. As you turn down the dimmer, the 12v is chopped in to shorter 12V pulses/bigger gaps, so the power output goes down due to the heater being off rather than the voltage being reduced. It's a more efficient way to use the power (especially if running on battery), rather than using a variable resistor or a regulator, which basically turns the unwanted power in to heat thus wasting battery life.

The only caveat is that the eBay dimmers apparently fail quite regularly (mine are fine but haven't had the chance to use them that much since they were built). Either buy some spares or invest a bit more in building a better quality PWM controller yourself.

[Squagnut]

I think that might be a later enhancement, to measure the temperature around the protected surface and adjust the heat level accordingly.

James

That too is straightforward enough - a DHT11 temp/humidity sensor feeding data to a simple (ha!) dew point calculator in the sketch, in turn controlling a PWM-assigned pin to run the heater.

[ben72]

ive looked into making a multi-channel controller, but after working out circuit boards, parts (and the time to do it) it was easier to buy the hi tec astro 4 channel one for £50.

NEVER LET ME DOWN YET!!

[IanL]

ive looked into making a multi-channel controller, but after working out circuit boards, parts (and the time to do it) it was easier to buy the hi tec astro 4 channel one for £50.

NEVER LET ME DOWN YET!!

Okay, but:

- They currently retail for £75 not £50

- I built my three channel dimmer for less than £25, and the most expensive part was a plastic project box. Could've recycled something and saved even more.

- This is the DIY forum so if it lets me down I WILL FIX IT MYSELF!!

-

[JKB]

Hi

I built one for the ST80 using the resistor method and popped the ring of resistors under the dew shield with a 2.1mm power jack coming out of shield. Powered using an old 12V power supply (for a radio i think)

Works a treat and cost practically nothing.

Cheers

Jamie