A Simple 12-volt Power System for a 20-inch Dobsonian

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Last touched 2002 January 18

While building TJ2, my 20-inch Dobsonian, in 1999, I used silicon glue to attach a complete piece of American Science Center Heat Rope to the secondary mirror. Twelve volts run through this stuff heats the secondary just enough to keep it dew free for the night.

At the time, I ran two pieces of telephone wire along one spider vane from the focus cage ring to the secondary. The telephone wire was held in place with some black electrical tape. I then kept 12 feet of speaker wire stored on the focus cage during travel that I would unwrap in the field. I ran this wire to a small Radio Shack alarm battery stored in the rocker box. Clumsy, ugly, crude but it worked.

In 2000, I started thinking of improving this system. In addition to powering the secondary dew heater, there were a growing number of other accessories that were demanding 12 volts of juice. These included the Sky Commander DSC's (not really needed, but nice to have), a dew heater for the eyepiece (traditionally I store my eyepieces in various pockets to keep them warm, at the cost of lint on the lenses), and perhaps power to the laptop computer used to run Megastar. Normally, I had run power from the car battery to power the laptop. This is risky in cold weather, as you might need a jump start in the morning.

At some point, I decided the key piece would be a "power box" that would accept incoming 12v from either an automobile lighter jack or via a direct hook up to a battery. It would also be nice to remove the wires from the spider vanes and instead use the spider vanes to conduct the juice to the secondary heater. It would also be nice to rid myself of the wire hanging from the focus cage and instead use the lower set of struts to carry the current to the upper section of the scope.

After playing around with different thoughts, I decided to build a simple device in a Radio Shack project kit box. During the construction of Ellie, my 12-inch Suitcase travel scope, I also realized it would be nice if the power box could be moved from scope to scope. When I had built Ellie, I had also done similar things; use the struts and spider vanes to conduct the power.

Construction started after new moon in November 2001 as the promise of traditional DC winter weather came arrived. The focus cage was pulled inside the shop and disassembled. Cardboard spacers were installed to electrically isolate two of the spider vanes so they could carry the positive and negative current.

New electrical connections where made to the secondary heat rope and were soldered to the spider vanes. Quick disconnects were installed on the connecting wires so the secondary mirror can be removed from the spider without de-soldering the wires.

Since the current would be transmitted directly to the aluminum tubes that support the focus rings, I ran wire from the attachment bolts on the bottom ring to the bottom strut wedge clamps.

Wire was then embedded in the wedge clamps and also the lower strut clamps on the mirror box. When the struts are attached and compressed into position, electrical contact is completed between the mirror box and focus cage.

An additional set of wires were run from the bottom ring to a four port audio jack (female) thingy from Radio Shack. This roughly 1x3 piece of plastic has four standard audio jacks. I soldered wire between the four jacks so that all four jacks would carry the same current. I then attached the jack thingy to the focus board between the cage rings. Quick disconnects were installed so if I want to remove the focus board, I don't have to break the solder joints or cut wires.

I cut a piece of heat rope in half, crimped the ends to some wire that was soldered to a male audio plug. This makes up a real simple, but effective eyepiece dew heater. A hair braid "borrowed" from my wife slides up and down to squeeze the heat rope against any of my eyepieces. The plug, of course, plugs into the female audio power jacks. Since there are four power outlets, I could heat other devices on the focus cage like a unit power finder (fat luck!). I actually made two eyepiece heaters and keep the spare in my eyepiece case.

Notice of theft: the idea for the heat rope eyepiece heater and using audio jacks was completely stolen from friend Greg Dillon of the Charlottesville Astronomical Society.

The Power Box

I used a 4x6-inch plastic project box from Radio Shack. Bigger then I really needed, but lots of room to work with and room for expansion if I need it. It has two incoming cords, a cigarette lighter jack and a set of battery terminal clamps. These are, of course, 12 volt. This allows me to easily use either a battery or run the power from a car or other power supply. Both have in-line power on-off switches so I can disable the one not in use (yeah, a switch on the box would be better, but this works).

The box has four audio jacks for output. In normal operation, two of these are used, one for the focus cage systems and the other by the Sky Commander box. The others will be used by fans at some point in the future.

The box also has a single cigarette lighter (female) jack. This was purchased from Radio Shack and is meant to be mounted under an auto dash. I was able to drill out a 1 1/8-inch hole in the side of the box and slip the jack into it. Works great. This jack is used by the lapout power supply.

Future Upgrades

If I ever feel like, I might like to do the following upgrades:

The box has velco on one side used to attach it to the scope. Currently, I'm attaching it to the front of the mirror box since I tend to place the battery either in the rocker box or hang the battery from the front of the rocker box.

On Ellie, my 12-inch Suitecase scope, I attach it to one of the bearings.

Click on most images for a larger/high resolution version of the image

The Power Box
The power box attached to TJ's mirror box.

Back View of TJ Rigged up
Here's TJ all rigged up for observing as seen from the back. The computer is on the "Roster Tail and the Sky Commander is on the "Riser." You can see the interface cord between the SC and computer also.

The focuser
Here you can see the eyepiece heater and the four audio jack thingy. The eyepiece heater is 1/2 of one American Science Center heat rope. A hair braid slides up and down the heat rope to firm it against an eyepiece. You can also see the quick disconnects for the power jacks in case I want to take the focus board off the cage for any reason.

Secondary power
The secondary mirror has a complete piece of heat rope. Quick disconnects from the heat rope lead to wires that are soldered to the spider vanes. Two vanes each carry the power from the cage.

Upper Strut Wedges
This view shows the upper strut wedges and the power wires that run from the wedges to the aluminum supports for the cage rings. The struts make contact with the wires on the inside of the wedges, drawing the current up from the mirror box.

Power Box hooked up
This shows the power box hooked up to a small Radio Shack Alarm system lead acid battery. While they are heavy, the lead acid batteries are great because the don't develop a memory after a few uses.

Lower Strut Clamps
Here is one of the lower strut clamps with the power line attached. The wire is counter sunk on the inside of the clamp so as to force close contact with the struts as they are pressed into the clamp. The struts on TJ are 1 5/8-inch in diameter.


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