Telescope Reviews: Temp-EST

The C11 fans are actually the largest (25mm). All the other tubes have to have smaller fans including the C14HD which uses 20mm fans. The small fans tend to be quieter in part because they are less powerful. All the smaller fans are actually 5VDC fans (with a converter installed) while the larger fans are 12VDC. The sound is likely from one of two things. It is likely, particularly with the exhaust fan on the C11, that the fan is rubbing against the filter screen and causing the sound. It is possible that one of the fans is bad, but it is unlikely. One of the fans has to have its blades next to the filter screen which does have the potential for rubbing and, while I don't send them out that way, shipping can be pretty rough.

With the vent assembly removed from the scope, using a small blunt object, you can very gently push the filer screen out from the fan just a little to see if that solves the problem. You can obviously send them back to me and I will fix them as well.

Other people have tried "ducting" larger fans and it would be nice if the vent holes were larger (especially on the C14). But you have to remember that you can only push so much air through a small hole, so even if you used larger fans you would quickly reach a point where the air pushes back through the fan since the hole is too small to allow it to flow through the system. While it might be possible to develop a system to use a slighter larger fan with the "ducting" inside and use of more of the original filer area of the vents, I avoided this because of the much greater difficulty in machining such a system and, more important, the fact that it would likely require the disassembly of the OTA to install them and I really wanted to avoid that. I also certainly did not want to hang something large on the outside of the OTA. The saving grace is that you are only having to exchange a fairly small amount of air inside these tubes (1 to 2 cubic feet) so that an air flow of 0.5 to 3.0 cfm is plenty sufficient to do the job effectively for both initial cool down, and more important, long term equilibration. I only wish that they had used the same general vent assembly (i.e. the one on the C11) for all the tubes. The smallest fans actually cost 3 times as much as the largest fans and the small ones are very hard to get and more difficult to install (your eyes and fingers don't get better as you get older).

Regarding the lessening of the vent area, that is necessary in order to support the flow of air through the tube rather than preferentially right back out the vent that it came in. While it may reduce passive cooling slightly, the passive cooling is not much to start with. The fans require so little battery power to run, that power should never be an issue for them.