Cooling of astrophotography camera sensor chip For astrophotography using 'medium size' land telescope at good location,


How many time longer exposure, in view of boosting the system's ability to photograph even darker objects, can one gets from cooling down digital camera sensor chip?
Is there a 'sweet spot' temperature where most thermal related noise is suppressed and further lowering of temperature yields litter extra benefit.
What is the ball park temperature for low (amateur), medium and high (professional) grade cooled camera?  


Web search shows that Peltier electric cooled camera works at around "minus a few tens degrees centigrade". Does the really professional one cool to more extreme level, somewhere close to liquid nitrogen temperature? 


Does CCD, CMOS or other sensor have different 'sweet spots' temperature?
The practical length of the time exposure will depend on your equipment. Cameras used in astrophotography are generally able to be used with exposure times ranging from milliseconds to hours. As your question stands, cooling the chip will have no effect on the length of the exposure time. What it can affect is the amount of noise in the image. Cooling the sensor generally reduces the amount of noise. Cooling beyond -20°C may have limited value, particularly if you are not using research-grade instruments. While you may further reduce the noise, you may still be at the mercy of the quality of the optics, drive mechanisms, etc. and you may have less noise in the image but the limiting factor is not your image noise but the system limit for drift and distortion.

Some professional instruments use liquid nitrogen cooling while others use thermoelectric cooling, depending on the manufacturer and the intended use. The instruments that use sub-zero cooling also usually have a high quality sensor with a pixel well depth that supports longer exposures, without bleeding effects from adjacent pixels. It all depends on the sensor, camera, and other parts of the system, so your mileage may vary.