Attaching a cooling fan to Newtonian
Cooling a reflector telescope is extremely important. As long as the primary mirror is warmer then ambient temperature (and it's usually is) - several processes occur: Warm air rises from the mirror, causing currents and turbulence inside the tube. A boundary layer of slightly warmer air forms right over the primary mirror surface. These layers and currents degrade the image, making it "dance", and blurring the fine details. Also the mirror itself while it constantly cools down - changes its properties (focal length) which can be a pain for photographers.
Even if you take a telescope out 1-2 hours before observing session begins - a mirror will usually stay warmer then the ambient temperature. That is because it takes longer for glass mirror to cool down, then for air temperature to drop during the night. This effect is especially severe in case of cold weather, and large mirrors.
See an illustration which shows mirror temperature behavior, as a function of time:
A solution is active cooling of the primary mirror.
There are several ways of doing the job, while most popular of them involve attaching some type of PC fan to back of the OTA (which is often fitted with screw holes for this purpose). This can be either intake, or exhaust fan. One bigger fan, or several small ones. Additional option is to attach side fans. Each way have it's advantages and disadvantages:
Generally - rear mounted fan which is blowing on the mirror's surface is most effective for cooling the mirror. It's a good idea to mount it on some sort of circular mask - which will force the air into the OTA, and increase fan's efficiency. However it's often not enough to completely eliminate tube currents.
Exhaust rear fan is often used instead of intake one because it doesn't suck up dust into the telescope from behind, and *probably* creates more laminar airflow inside the OTA. Another reason for avoiding intake method is that every fan heats up the air flowing through it (which gets even worse if there is a dust filter mounted on it). However exhaust fan can be slightly less effective for cooling the mirror itself.
Side mounted fans are especially effective for blowing off hot turbulent boundary air layer off the mirror's surface. which prevents turbulent flows. They should be used all the time during the observing session. Note that for side blowing fans - holes should be drilled on opposite side of the OTA, with overall area comparable to area of the fans. A clear disadvantage to this method is that not everyone is enthusiastic about the idea of drilling holes in their new and shiny OTA. Especially if one is unsure of it's rigidity.
A combination of rear and side fans can be used both for effective cool-down, and for controlling the boundary layer of hot air.
There are more exotic methods - like suspending a small fan in front of the mirror, or attaching metal cooler or peltier devices on the mirror's back end, or using a rear intake fan with a diaphragm around the main mirror (which forces the air to blow on mirror's surface).
While choosing the fan which is turned on during the observing - important consideration is the fan size and quality: A a quality "silent PC" type of fan from a known manufacturer (with ball or oil bearings) usually will do the job.
A large fan is capable of producing the same airflow as a small one, while working at lower RPM speed - Therefore theoretically it should produce less vibrations. However in practice it not always true, since with one of my fans I found out that lower frequency vibrations of a large fan (a quality Noctua model) actually affect my telescope more then higher frequency vibrations of a smaller fan. Also not every "quiet" fan is necessarily vibration free. Therefore it's best to experiment with several different fans. In any case it's a good idea to limit RPM speed below the nominal level by supplying a lower voltage (or adding a series resistor). Some "quiet" fans come with such option.
For my 8" Newtonian - I decided to simply attach a large rear fan, with 3 speeds (from Antec "silent" PC case) to blow on my primary mirror surface:
I did the attachment the simplest way possible: Using double-sided adhesive tape (same type which comes with Rigel quickfinder). It is surprisingly strong and held the fan very firm (for over 3 years). A fan's native 3 speed controller, and a power connector were attached the same way.
Of course a better way of attaching it would be to use some sort of dampening rubber, to fight the fan vibrations. A most effective (and the least aesthetic) way is to suspend the fan on long rubbers bands. In my case I found out that on maximum speed - the vibrations blur the stars into long streaks, and I only use this speed for initial cool-down. At lowest speed however, the vibrations are almost negligible, and have amplitude of approximately 0.5 arc second, which is acceptable.
Note: nowadays it's very cheap to buy an electronic thermometer, with external sensor, which is supposed to show "inside" and "outside" temperature. I've attached such device to my telescope, with it's "outside" sensor attached to the back of the mirror. This way I can easily know when my telescope is ready for observing session.