Boundary layer fan mod

Something I've been wanting to have a go at for ages was installing a fan to help with cooling the primary mirror and especially a side mounted fan to hopefully eliminate the boundary layer of warm air that sits over the mirror that can affect seeing - see here. The normal way to minimise this effect is to just try to cool the mirror as close as possible to the ambient temperature. The problem with this though is that it does need to be very close indeed to the ambient temp which is difficult to achieve with fans blowing on the back of the mirror alone.

I did have a try using a Peltier cooler and fan combination but this still struggled to cool the mirror to ambient and I think would have needed a lot more engineering to go on around the mirror cell for it to be really effective. Anthony Wesley in Australia has done a lot of work on this using multiple fans and Peltier coolers, but his mirror cell (and the whole scope) is custom built and beyond my expertise and budget. My Orion Optics SPX250 already has a small cooling fan in the mirror cell, to which I have already added a baffle (made out of a rubbery Ikea placemat), to stop the air from just coming straight back out of the cell again and instead go to the edges of the mirror and up the tube. I also have an indoor/outdoor thermometer velcroed to the OTA to measure the ambient and mirror temps.

Baffle

Indoor/Outdoor Thermometer

A small, side-mounted fan can gently blow away the layer of warm air that sits above the mirror. Run at full speed and suitably filtered, it can also help with cooling the mirror itself.

So I set about removing the primary mirror and cell, and marking and cutting the necessary hole in the OTA.

I fitted the fan, oriented so that it blows air into the tube, and covered it with pc fan filter foam and made a grill to secure it out of an old oven-chip crisper. This was all secured by four small nuts and bolts. I added a 3.5mm sound jack (rescued from an old 56k modem) and wired it to the fan.

Fan inside

Fan outside

I have the fan plugged in to my 12V supply via a variable resistor so I can adjust the fan speed - full to help with the initial cooling, down to a gentle whisper across the surface of the mirror for observing.

12v Supply and speed control

The primary is back in the scope, it's all collimated again so I need some clear sky now to test it.