After several frustrating CLEAR nights when my telescope's corrector plate
fogged up in spite of a dew shield (and my Telrad finder was
completely fogged), I looked into buying a dew control system. However,
after seeing that I would have to shell out $300 plus, I decided to
see what I could find on the net about making my own dew heater system.
I owe credit in my resulting design and construction (total cost
under $60 - see parts list) to a couple of
people whose information I found insightful. Those who had especially
helpful sites were Mark Kaye
on whose Astronomy pages I found great info on heating my Telrad,
dewheater & controller pages of backyard-astro.com. Finally,
Lawrence Lile of Lile Engineering
was very helpful with information on nichrome wire, which his
business carries and offers over the net. Note: I have
since found out the the links to Lile Engineering are no longer functioning. However,
an assortment of nichrome wire is available online or on ebay from
The design criteria was simple - a 12V adjustable heating system that could heat my corrector plate enough to keep it dew free. Also, I wanted several controls in one unit, since I also wanted to heat my finderscope, my eyepiece, and my Telrad finder. Simple enough.
My research led me to some pages about soldering multiple resistors together to supply heat around the optical tube - something I plan on doing for my finderscope and my eyepieces, but around 37" circumference optical tube!!?? I wanted to be dew free THIS year. Then I found Mark's pages about using nichrome wire as a heating element - it's simple and it doesn't look like it got hit too many times with the old ugly stick.
Being a long-time electronics hobbyist, I decided to take some
ideas and run with them. Temperature control can be achieved by
varying the duty cycle of the DC voltage - PWM (pulse width
modulation), as it's called. My original thought was to house a
4-channel heater controller INSIDE my Telrad finder. However, I
decided against that idea after I etched the circuit boards and saw
just how tight the sucker was going to be (and I didn't want to turn
my Telrad into swiss cheese just to find out it didn't QUITE fit).
I also contemplated using a PIC microcontroller - which I may still do in a future project. In the end, I opted for (2) 556 dual timers, with power MOSFET outputs. To the left is the schematic I came up with (click it for a larger view).
The 556 is a dual timer, set up in astable mode, meaning that it's essentially generating two independent pulse width modulated square waves, ad-infinitum. My controller has TWO of the circuits shown in the schematic - to give a total of FOUR independent control channels. Note that you CAN run multiple lower power heaters from a single channel - i.e. - eyepiece, finder, and telrad heaters could be run in parallel off a single channel, sacrificing independent temperature control for each - and thus you would need only ONE circuit board module (one channel serving the "main" corrector plate heater and the other serving the combined other smaller heaters).
The period of each channel is just over one second, which seems to work well, AND gives the option of adding a "status" LED for each channel (in parallel with each heater) to give a visual indication of its duty cycle. The key is not to overpower the heating process - but to keep it just at the point where you need it - thereby preserving battery life, minimizing thermal layering and air currents, and keeping your viewing at optimum quality.
The choices for driving the output were either relays, power transistors, or power MOSFETs. Relays were too noisy and their mechanical nature may spell difficulties down the road for a constant cycling application such as this. Power transistors are not energy efficient enough (they get hot and need a heat sink - wasting heat and power where you don't need it). Power MOSFETs, if chosen properly, can do the trick nicely, with very little power wasted (and no heatsink). Keep in mind that MOSFETS are sensitive to static discharge, so you should wear a ground strap while handling them and assembling this circuit!
foil side vs component side
|After completing the circuit design, I set forth laying out a printed circuit board design. For this, I used "ExpressPCB", a free printed circuit design program available from expresspcb.com. It was adequate for the job, although it's not really for someone who is going to etch their own board. This program doesn't give you a "foil side view" so you're on your own trying to lay out the backwards image of the copper traces. I printed my resulting layout on a transparency, and photocopied it (reversed) onto paper. Here's what the copper layout looks like (foil side view) and the component layout (top side view, but you can "see through" the board to the traces underneath). Click either image for a larger view.|
ready to etch
Now that the layout was done, I had to copy it onto copper clad
circuit board. To do this, I used Radio Shack rub-down transfers,
Part #276-1577. I had these from a LONG time ago, so they may be
discontinued - if so, something similar should be available from
a good electronics supplier. When laying out a board,
make CERTAIN that you clean your board well to start
with, and KEEP all natural oil from your hands from getting on the
board. Latex gloves are helpful for this - or just wash your hands
with soap frequently. IF you get a fingerprint on the copper board,
it will prevent the etchant from working equally on all parts, and
you will end up eating through some traces because of the time it
takes to etch through the fingerprints. Here are pictures of my
board after the rub-down transfers were done, and after the board
was etched in a room temperature ferric chloride bath. If you want
details on the etching process, it's already been done online so
I won't regurgitate it here.
Here's a picture of the board with the rub-down transfers in place,
ready to be etched.
the board after etching
|After the board was etched, it was cut to size, cut in half (I made
two on a single piece of board), and the holes were drilled through
with a VERY small drill bit (made for a dremel). Here's what they
came out looking like. I was rather pleased with the result, since
I hadn't done this for years.
Note that the boards were not identical - I experimented a bit with layout - although the circuits are the same, I used slightly different pads and paths.
the case, ready to be assembled
The next step was assembly. At this point, I had given up on putting
the guts into the Telrad finder, since I could see that with the
pots and plugs, it would be very tight if not impossible.
My first assembly also used
power jacks/plugs for the power and heater connections, but I
quickly changed these to RCA type jacks/plugs because they offer a
much more reliable, low resistance connection (this
is the type of connector used on commercially available dew heaters).
Note: When dealing with a 10
ohm heater, 0.5 ohm losses at connectors add up quickly and waste
precious battery power.
Assembly was straight forward - the MOSFETs were installed back-to-back but NOT touching (this is critically important). I drilled the holes in the case for the pots, switches, and jacks, after figuring on clearance from hot-gluing the circuit boards to the bottom of the case. My first choice for MOSFETs were el-cheapo IRF510's - however, these got too hot for the BIG heater, so I had to change the MAIN heater controller MOSFET to a IRFZ34N, which was about twice the price.
After squishing everything into the box, I succeeded in getting the
bottom fastened on... and here's how it looked:
The next part was uncharted territory for me - creating my own dew
heater out of nichrome wire. Nichrome has a characteristic resistance
per linear foot. I knew I wanted about 15W maximum of heat, so working
backwards from that, P = 15Watts = VI = V²/R = 144/R.
Thus R = 144/15 = 9.6 ohms . . . I needed to create a 9.6 ohm heating element out of nichrome. Now, I also wanted to go around the telescope tube a fixed number of times - not 1.25 turns, not 3.6 turns. Upon measuring my scope circumference, it was 37", or 3.08 feet. So I had to find a nichrome wire with a characteristic resistance such that one, two, or three loops of 3.08 feet would provide just over 9 ohms of total resistance. I thought that to heat evenly and "gently", two loops would be better than one, since it would transfer the heat over a larger surface area and reduce the "point" wire temperature. As it turns out, this was a good idea, since part of the wire is not in contact with a good thermal conductor.
The nichrome I used was from Lile Engineering, who offer it at a very reasonable cost and were very helpful with suggestions on end point terminations. In the end, I opted to use nichrome with a characteristic resistance of 1.47ohms/ft - which resulted in a total resistance (Rh) of 9.06ohms. Bingo. Close enough for me. Here's a picture of the wire, and Lile Engineering's part#.
The 9.06 Ohm length results in a current of 1.32A, and a heating power of 15.8W.
first strip of double-sided tape
first strip in place
both strips of tape, backing removed
The next step was to figure out some way to fasten the uninsulated
nichrome wire to the telescope. I thought it would do best if it was
as close to the corrector plate as possible, AND to have as thin of
an electrical insulator as possible (from the
metal telescope tube) for maximum heat transfer.
My wife made a brilliant suggestion, of using double-sided sticky tape that's normally used for insulating windows with plastic in these cold Canadian winters. Since I had 2 loops of wire to make, I used 2 "runs" of this tape beside each other.
Then I tucked the nichrome wire conveniently into a little ridge in the corrector plate housing, looped it once, crossed it over the center line, and looped it again tucked in the opposite ridge. I left about 6" of spare on both ends, so I could terminate on a terminal strip.
Before I could terminate it on the screw terminals, I had to somehow bring the LEFT side end out across the other conductor - so I cut a small square of a plastic food container and drilled two holes in it to act as an insulator, and ran both end wires UP through the holes, and over to the terminal block.
I ran it through both sides of the terminal strip, under BOTH screw terminals, and connected solder-tinned wires (to the plug) under ONE side's screw terminals. I then tightened the screws VERY well to minimize resistance of the connection.
wire crossover & closeup
white duct tape to protect the heater
and the final product!