Cool That Mirror
A simple, low cost cooling fan for your Newtonian telescope, by Maarten de Vries
I’ve got a Helios Apollo 150. It is a 6” Newtonian short tube telescope, quite a nice little instrument, with very little plastic on it. It is built quite robustly, especially the mirror cell. It has a 1/2mm thick steel plate covering on the back. Although this is ideal for protecting the mirror and keeping it in collimation, it has a disadvantage too. It acts as a big heat sink, which increases the cooling down time of the scope considerably. As a matter of fact, some of the effects can still be seen after several hours of observation.
My Apollo 150 had serious 'chimney effect'. This is when warmer air from the mirror and cell rises up in the telescope tube and gathers there. It then seeps out the front. This means that light travelling through that section of the tube gets distorted. It is easy to check if your telescope has this problem. Take a bit of card and hold it in front of the top part of the aperture while looking at an object. Does the object get sharper? If so, you've got chimney effect.
I needed to find an easy way to cool the entire mirror and cell more quickly. The answer came from my PC. I had a bit of a noisy processor cooling fan, so I decided to replace it. I kept the little 12 volt fan which came out of the PC and used it for my telescope instead. I removed the steel plate from the back of my scope and cut a nice round hole in it by first drilling a large number of smaller holes in a circle, and then using a half round file to smooth the edges. I made sure the hole was the correct size (inside diameter of the fan). I drilled 4 holes for the fixing bolts (these fans have a square outside shape with a bolt hole in each corner). From Maplins, I got a socket identical to the one I use for the power pack of my drives. I drilled another hole in the back and fitted this little socket.
When fitting the fan, make sure it is fitted so that when it runs, it sucks the air into the scope (and onto the mirror). I soldered the wires from the fan to the socket and used some tape to secure the wires tidily. I then replaced the whole thing onto the back of the scope. 
When I put my scope out, I run the fan for about 15-20 minutes on the 6v supply from the drives. This appears to be enough to cool the mirror and cell down to a very acceptable level. The chimney effect has disappeared and images from the scope are much better.
I guess that 12v processor fans are adequate for 4.5” to 6” scopes. If you have a bigger scope, I suggest you either get a bigger cooling fan, or get two. These can be obtained for a quite modest price from either PC shops or Maplins.