Small fan assembly for cooling SCT during use
Putting a small fan on the OTA to blow air through the OTA
Some time ago I described my SCT cooler made to cool the OTA before using it. This fan based cooler pushes the air through the whole OTA and around the primary mirror. Moving fresh air limits boundary layers that form around slowly cooling primary mirror. That allows to cool a big SCT to ambient quite quickly.
At some nights the OTA would not reach ambient without such cooling as the air temperature fallen faster than SCT could cool down. When cooled with a fan the OTA would be stable for some time and after that it would start drifting and spoiling the image with air currents. To prevent that I've decided to make a small cooler to push some air around the primary mirror in between imaging runs.
To make the mini cooler I used a 12V 25x25mm standard fan and slightly bigger "generic casing" - both from electronics store. I've also added a 12V DC adapter plug to the fan cables. On the casing top side I made a hole for the fan, and on the bottom - a hole matching the hole on the OTA. Inside the casing I've put a white cloth from a vacuum cleaner bag (not paper, a better one from a while fancy cloth) to work as a air filter.
The fan was glued with "super glue" and some plastic glue with a glue gun. I've also used the glue gun to connect the cooler with the OTA by putting a ring of the plastic around the hole and sticking the cooler box to it. This also compensated for the curved surface of the SCT OTA.
After powering the fan I can sense air flowing out from the OTA through three remaining fork mount mounting screws. A small fan is able to push the air around the primary mirror, and that should be enough to limit boundary layers creating around it.