Attaching a Cooling Fan to Newtonian Telescope
Cooling a reflector telescope is extremely important. As long as the primary mirror is warmer then ambient temperature (and it 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.
Mirror temperature with and without fan
Even if you take a telescope out 1-2 hours before observing session begins - a mirror will usually stay warmer than the ambient temperature. That is because it takes longer for glass mirror to cool down, than 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 (scale is arbitrary):
The Solution - Cooling Fan
The 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 its advantages and disadvantages:
Reflector telescope active cooling methods and different fan placements
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 its 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 vibration. 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.
Attaching the Fan - a Simpl Example
A fan attached to a 8" Newtonian telescope
For my 8" Orion Newtonian (and later for the 10" Skywatcher as well) I've simply attached a large rear fan to blow on the back of the primary mirror. I've used an Antec 3 speed 120mm fan from a "silent" PC case, which have proven to be vibration free. I did the attachment the simplest way possible - using a thick double sided adhesive tape (the same type which comes with a Rigel quickfinder). It is surprisingly strong and held the fan very firm for over 4 years. A fan's native 3 speed controller and a power connector were attached the same way.
A cooling fan, a dew controller , a Li-ion battery and mirror/ambient thermometer mounted on a 10" Newtonian telescope
In my 10" Newtonian in order to avoid routing the power to the telescope from an external battery - I've attached, using a velcro tape, a power source to the telescope's body as well. It is a small Li-Ion battery which powers the cooling fan and a dew controller box.
Probably a better (and a more complex) way would be to use some sort of dampening rubber, to fight the vibrations. Like suspending the fan on long rubber bands. However in my case I found out that at low speed settings the vibrations are negligible, and I couldn't notice any effect from them even during planetary astrophotography. At maximum speed on a 8" telescope - the vibrations did blur the stars into long streaks, but I only use this speed for initial cool-down. In my 10" telescope the same fan didn't affect the picture in any way even on high setting.
It is curious that an attempt to mount the fan using an external "cap" (with a central hole) to the telescope's body, which was supposed to improve airflow efficiency, actually caused really bad vibrations to my 10" scope, as opposed to attaching the fan directly to the mirror cell. Maybe this introduced some sort of resonance specific for my setup and the fan speed.
A dual display thermometer
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 its "outside" sensor fixed to the back surface of the mirror (you can see it in a previous picture). This way I can easily know when my telescope is ready for observing session.