Do It Yourself SCT Cooler

You've probably seen those ads in Sky & Telescope Magazine for the SCT Cooler from Cosmic One? You know, the one that costs $129.95? Well, I thought that was a good idea, and decided to steal, copy, purloin the idea and make one for myself, for a lot less then $129.95.

Here's my finished SCT Cooler and the total was about $15.00!

The concept behind one of these coolers is that the scope heats up in it's case or car, or sitting out in the sun waiting for darkness. When the sky is dark enough to observe, the scope and primary mirror are still cooling down through natural convection and tube currents occur in the optical path, making seeing poor. To facilitate the cooling process, the SCT Cooler was invented to blow filtered, cooler, outside air into the scope and assist in the cool down process, with the warmer air exiting out around the pipe at the back of the scope. Not a bad concept if I say so myself. Here's what you'll need to build one of these...

2", 12VDC, Computer fan, preferably with a fully round shroud

2" PVC pipe coupling

2" to 1 1/2" PVC Bushing

1 1/2" to 1 1/4" PVC Bushing

1 1/4" to 3/4" PVC Bushing

2' of 3/4" Class 200 PVC Pipe

3/4" PVC Pipe Plug

Furnace filter material

Two Conductor wire and power connector

PVC Cement

The fan is the single most expensive piece of material which will probably cost more then $10.00, depending on where you get it. It doesn't have to be new, so look for electronics dealers that deal in used equipment. The PVC pieces are available at Home Depot and should be only a couple of dollars.

The first step is to cut the flanges off of the fan so that it becomes a 2" diameter circle, I used a hacksaw and a Dremel tool. This is the reason for trying to find a fan with the shroud a complete circle, like above. You'll also need to solder some wire to the fan wires, that will be your connection to your power source. Once the fan fits snuggly into the 2" coupling, you will need to cut a small slot in the side of the coupling to allow a place for the wires to exit, make sure you get the fan positioned so that it blows in and not out (look for arrows on the side of the shroud). If you look closely below, you can see the notch I made in the coupling for the wires.

Now that you have the fan fitting nicely into the coupling, you can glue the first adapter bushing (2" to 1 1/2") into the coupling. Don't glue all the adapters together, you'll need a way to take it apart to have access to the filter!

Cut a piece of fine furnace filter into a circle that will fit inside the two inch coupling to be the air filter. The filter should fit in the middle of the 2" coupling, between the fan and the first adapter. The next step is to determine the length of your scope baffle tube. I did this by taking a straight piece of wire, bending a small hook on the end and inserting it into the back of my scope until I could hook the end of the baffle tube inside the scope. Be careful, you don't want to take a chance on scratching your secondary mirror! Now mark the wire at the end of your scope and measure the length from the mark to the end of the hook. This is the length you want the finished 3/4" pipe to be, plus an inch or two, from the last adapter to the end of the plug. My pipe length, for my 10" f10 LX200 is 15", not counting the end plug, measured from the last adapter edge. Some people have asked about the 3/4" pipe, this is a thin wall PVC used in sprinkler systems, not the heavy wall Schedule 40 more commonly used. The plug in the end may not fit this pipe properly, so I just glued it in with some epoxy, the goal is just to plug the end of the pipe. Now glue the 1 1/2" to 1 1/4" adapter to the 1 1/4" to 3/4" adapter and the 3/4" pipe into that assembly. Do not glue this assembly into the fan assembly, this is your access for getting to the filter or pushing out the fan. Now measure from the edge of the adapter to the end of the pipe and cut it to the length previously measured, and glue the pipe plug into the pipe.

Now drill some 1/4" holes, into the end of the pipe all around the plug, trying to angle the holes such that air coming out of the holes will be blown back towards the primary mirror. I then took a file and tried to round off the edges of the plug and pipe to make for smoother insertion into the scope without scraping on the inside of the baffle tube. Now solder your power connector to the ends of the wires, making note of polarity, you don't want the fan running backwards, and you're ready to go. I don't use my cooler too often, only when the scope's been sitting in the sun for awhile, or if the atmosphere is cooling very rapidly. You may feel that the amount of air coming out of the openings in the end as not being very strong, I measured the amount of flow from mine, and found it to be about 2-3 cubic feet per minute.  More then enough to exchange the air in the OTA a couple of times a minute.  We don't want a hurricane in there, just a gentle breeze that will help the scope cool down quicker. If you have any questions about the use or assembly of this SCT cooler, please contact me and I'll try to answer them. 

Clear Skies!

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