Ozzzy's Place
Dew Heaters
Losing a night's observing due to dew forming on my corrector plate
and other bits and pieces of glass was rather annoying so it was time to do something. I read a few websites on how
to build heater strips from resistors and heater wire and thought that most were overly
complicated.
Why we get dew
The reason that we get dew is that those pesky laws of thermodynamics havn't been repealed. The
effective temperature of a clear night sky is 2.7K (about -271C). As we all learned in high-school
physics heat flows from hot to cold and not the other way around. Your corrector plate or objective
has a temperature somewhere around ambient air temperature... say 373K. When it's pointed at the sky
the glass will radiate away heat towards the 2.7K of outer space and start to cool down... rapidly
at times. Because the sky is colder than the air around the glass, eventually the glass will cool down
to the dew point, that point where the relative humidity is 100%. Via conduction the glass will then cool
the air in the boundary layer to below the dewpoint and any moisture OVER what the air can hold at that
temperature will end up condensing out on your glass. And that's the skinny.
We can do a few things to stop this:
- Blow ambient temperture air at the glass to 'warm' it up slightly
- Not show the glass as much of the sky.
A dew cap limits the amount of the sky that the glass can see. This means that the glass is radiating
away heat towards a few degrees of cold sky instead of 180 degrees. This will hold off the dew for a while.
- Replace the heat being lost.
This is where active dew-control comes in. We want to replace that FEW watts of heat being radiated away by
putting something warmer than the glass near it. Then thermodynamics dictates that the heat around whatever
that warm thing we have is will flow from the warm thing to the cold glass. Now, we don't want to be able to
fry eggs on our corrector plate, we want to keep it at or very near ambient temperature. This balance allows
the scope to fight off dew but won't upset our seeing with nasty thermal currents.
The second and third options are what I use and this page is on how to make yourself some heaters.
How heaters work
The theory behind anti-dew heaters is that you run a current into a resistive load and the load
'slows' the current. Doing this it has to do something with the power being applied so the load
radiates it away as heat. One doesn't want
enough heat to melt anything, just enough to keep the optics at ambient temperature with a bit
of overhead for things like 'defrosting'.
Calculating resistance, power and current
To figure out how much resistance your heater has is fairly simple.
- Resistors in series
The total resistance of a bunch of resistors wired in series is simple. Add all the values
together. For example three 20ohm resistors in series will have a total resistance of 60ohms.
- Resistors in parallel
This is a bit more complicated. Normally with dew heaters you'll have all the same value of
resistor. For example if we had 45 330ohm resistors in parallel, divide the resistance of ONE resistor
by the total number of resistors, then invert it (the 1/x button on your calculator). This
gives us 1/(45/330) or 7.3 ohms.
Power
Now that we know the total resistance of the load we can calculate the power generated. The
formula for this is P=E^2/R or 'power = voltage squared/resistance'.
In our example of the 7.3ohm load above with 12 volts bein supplied, this would come out to
144/7.3 or 19.7 watts.
Current
The current passing through the heater is I=E/R or 'amperage = voltage/resistance' and in the
above instance of 12 volts and 7.3ohms comes out to 12/7.3
or 1.6 amps.
A note on resistors
Resistors come in many values from 1/10's of an ohm to many millions of ohms. Each comes with a rating
on how much power they can safely radiate without burning up. Some common ones are 1/4W, 1/2W, 1W, 2W etc.
When building a heater strip out of resistors pay attention to the power rating. If we were using 45
330ohm resistors to deliver 20 watts then divide the power by the number of resistors to see how much
power EACH resistor is radiating. A good rule of thumb is to use a resistor rated at twice this value. In
our case of the 45 resistor heater this comes out to 19.7/45
or .43 watts per resistor. In this case I'd use 1W resistors (but most use 1/2).
Our string of 20 .33ohm resistors will radiate close to 1W per resistor so use 2W resistors.
Building Heaters
Heaters can be anything that can take electricity and convert it into useful heat. A lot of telescope
heaters seem to be a made out of a string of 330 ohm, 1/2W resistors hooked up in parallel. An 8 inch scope uses about
45 of these to deliver a maximum of 19.7W of heat. I've built one of these and they have three things that
are 'show stoppers' in my opinion.
- Work time
Laying out and soldering all of those resistors is tedious and time consuming.
- Implimentation
To work right this strip of resistors that is about 28 inches long and 1 inch wide needs to be insulated
on one side to keep the heat in the scope. This makes for a fairly large and bulky heater which also doesn't
look very pretty.
- Use
These heaters won't fit inside the corrector cell and therefore have to deliver their heat to the tube of
the scope if you use a dew cap. This isn't optimal.
My idea is to use resistors in series which means a whole lot less soldering and you end up with a heater
that will fit inside the corrector cell where it belongs. Also, I can use my dew cap with the heater in
place which means I need a whole lot less heat to keep things clear. You have to watch the power rating
of your resistors much more closely in this instance as each resistor gets very hot when the controller
is turned up to full power.
My heaters
Corrector
For my corrector plate I used 20 .47 ohm resistors and strung them together in series. This gives me about 9.4
ohms total and about 15W of heat at full power. I used 2W resistors and each radiates about 3/4 of a watt so there is
loads of headroom built in.
For my 8 inch SCT the total length was 70.5cm. Once they were soldered together I ran heat shrink over
the whole strip (don't shrink it yet) then soldered on one of my power leads. I then backed off the heat shrink
until I could solder the other lead then wriggled the shrink into the right place and shrunk it down. This gives
a nice, fairly stiff heater that can be 'molded' into shape. For a bit more 'bite' in the corrector cell I took
68.5cm of coat hanger, covered it with heat shrink and taped it to the heater strip at 5 points (between resistors)
then bent the whole thing into a slightly larger diameter than the inside of the cell. Now I compress the
whole thing, put it in the cell and it springs outward to keep the heater in place.
I've made a small cut-out in my dew cap so that it slides on over top without crimping the wires from the heater.
Because you want some way of controlling how MUCH heat is delivered you'll want some way of controlling this. You
can either use a simple in-line resistor to cut the current going to the heater or use something a bit more elegant. For discussion
about heater control see my page on my heater control unit.
Finder/Camera Lens
For the finder (a Leupold 20x50 spotting scope mounted on my piggyback attachement) I built another
heat strip out of 10 6.8ohm 1W resistors which uses the 12v straight through for just around 2W of heat.
Notice that I've built it slightly 'oversize' and used an elastic band to tighten it on the finder. This
is because it has to do dual-duty with my Nikkor 180/2.8 ED lens (my favourite for piggy-back work) which
has a slightly larger diameter.
Telrad
For the Telrad I placed a 100ohm 2W resistor under the angled glass, another under the mirror and wired
them to an RCA plug in the side. This is plugged straight into the 12v to give me about 1/3W of heat for
each element. This I've found to be lots for a Telrad. If you need more you can probably omit the resistor
in the bottom and just use the one one under the glass. Power runs in through an RCA plug I mounted in the
side of the unit.
Clear skies