Space is already cold, so why do some space telescopes require cooling? Ex: infra-red telescope
What kind of cryocoolers are used, and why?
Space telescopes like Spitzer, Herschel, Planck, WISE, and in few years Webb, need to observe in the mid and far infrared wavelengths. The infrared radiation of normal spacecraft temperatures, even if kept cold by our standards, would look like a bright light at those wavelengths. (Google black-body radiation to learn more.) The infrared detectors need to be kept at single-digit Kelvin temperatures in order to not radiate into themselves, especially to have the necessary signal-to-noise ratio to be able to detect faint infrared sources in the universe. Hence the need for cryo-coolers and/or liquid Helium on those telescopes.
Planck needed to be so cold that it used Helium-3 instead of Helium-4, since the boiling point of He-3 is about one Kelvin lower than He-4.
You can look at this page for more information on the thermal control of Webb. The Webb cryocooler for the MIRI instrument has been one of the most difficult developments. Cryocoolers work on the same principles as your refrigerator, using compressors and heat exchangers. However they use Helium as the exchange fluid, and have multiple stages to get the temperature to single-digit Kelvins. Here is an informative paper on the MIRI cryocoolers.
In space, the notion of temperature is not the same as on Earth:
Temperature can only apply to matter which vacuum has not. Space is near vacuum, so its temperature is irrelevant to large and dense objects.
Satellites (including telescopes) are made out of matter and therefor can heat or cool. They are full of electronics that produce heat, and receive sunlight when not on night side of earth. Because space is made of vacuum, heat can not be exchanged by contact with colder matter (convection) , and therefor they must be cooled by radiation (emitting infrared). Because this is much less efficient than traditional cooling, a complex cooling system must be added.
TL;dNR: Space is not
cold, and fans don't work well in space since there is no air.
The density of space is really really thin. In the same way that water conducts heat much better than air, so air conducts it much better than space. In fact, even though the temperature might technically be very cold, in fact it doesn't matter, because that heat isn't transferred. Most of the heat that objects feel depends on their surface area, power usage, and area incident to the Sun. An object is really cold if it isn't in the Sunlight in space. Otherwise, it is warmer, typically at similar temperatures to Earth. Basically, the object reaches a temperature known as the Thermal Equilibrium. Near Earth, that is typically around room temperature (20 degrees C)
The spacecraft that need to be really cold essentially measure temperature throughout the cosmos. Every object produces a small amount of light, although it might not be visible. You can feel a fire if you are close, even if the wind was blowing the heat away from you. This feeling of heat is infrared, which is what these cold spacecraft measure. If the temperature is too high, then the signal is confused, much in the same way that it is difficult to see out of a dirty windshield if driving towards the sun. Something can be seen, but it is much more difficult to see.