Introduction: Adding a Cooling Fan to Dobsonian Telescope

About: Engineer, designer, machinist, welder, guitarist, woodworker, amateur astronomer, and general analog hacker. Based in Detroit, Michigan.

This article details the addition of a cooling fan to my Dobsonian telescope, the construction of which is detailed on a separate article. The fan helps bring the main mirror into thermal equilibrium more quickly, and keeps air circulating through the optical tube to help prevent tube-current distortions when viewing. This was a relatively inexpensive modification (under $15) and has improved the usability of the scope, as it no longer takes so long to acclimate before observing.


- Computer cooling fan (12V), generic from Micro Center

- RCA male plug (found in my big box of odd electrical bits, but ordinarily quite cheap)

- RCA female plug (same as above)

- 12V wall-wart transformer from broken humidifier

- DuPont connectors

Step 1: Cutting Out Recessed Mounting Hole

I am a big fan of the "fasteners are for fastening, features are for locating" design rule, so I wanted to route a shallow square pocket in the rear panel of the telescope to provide a solid locating feature for the fan. I did this with a 1/4" carbide bit in a router, and a few scraps of MDF for a makeshift guide. The hole came out nice and neat, and will prevent the mounting screws from being stressed or pulling out if the fan gets bumped laterally. It's probably overkill, but since I had the panel out anyway, I figured if it's worth doing it's worth overdoing.

I also bored a large hole for the air to flow through using a carbide hole saw. I cut the hole from both sides (using the same pilot hole) to avoid ugly tear-out on the panel's surface that can happen when using a hole saw.

Step 2: Counterbore Hole for RCA Jack

Mounting the RCA jack on the rear panel required a counterbored hole. I made the counterbore first with a forstner bit, then the through-hole with an ordinary twist bit. I backed up the exit side of the hole with a scrap of plywood to prevent tear-out of veneer surface of the plywood panel.

Step 3: Wiring

The RCA jack has wires soldered to it. I wanted the fan to be removable if needed, which implied that I couldn't just solder the leads from the jack straight to the fan. To solve this, I crimped on male DuPont connectors to the leads from the jack, and matching female connectors on the leads from the fan. This allows me to easily disconnect the fan if I need to remove it or the RCA jack.

I also painted the counterbored hole black, to match the rest of the panel.

Step 4: Reinstall Mirror Cell

Here, the mirror cell is re-installed in the back panel, and the panel itself is re-installed in the telescope. The little wire from the fan fits under the mirror cell with room to spare. Note that for this to work, your mirror cell needs to have a large hole behind the mirror and in-line with the fan, otherwise you'll just be cooling the mirror cell and not the mirror.

Step 5: Attach Fan

Finally, the fan itself is secured on the outside of the panel with four screws. (The three knobs adjacent the fan are for mirror collimation,) The jack is easily accessible from the outside, and allows easy connection of the 12V wall-wart transformer which provides power to the fan.

Step 6: Conclusion

The addition of the fan seems to help the mirror cool faster, which means less time between setup and viewing. The fan itself is remarkably quiet and free from vibration considering it's under-ten-dollar price point. The 12V transformer could be replaced with a battery pack if needed, as the fan draws a fairly low current. Overall, this was an inexpensive one-afternoon project that has enhanced the usability of the telescope, and I'm pleased with the results.