Astrophotography DSLR cameras are often used with telescopes but, usually, together with the shutter speed, also the thermal noise increases, an inherent feature in all digital sensors (that you can see even if the sensor is not exposed to light). In addition, in addition to the sensor quality, the noise varies with temperature and with the exposition time of the image. In addition, due to sensitivity to temperature differences, some pixels may be bright (hot pixels) and other darker(cold pixels) than the average. All this is inevitably added to our real image lowering the quality, especially in common DSLR cameras often used in astrophotography from people starting taking pictures of the Universe. Is there a way to solve this? The answer is yes: cooling system.
Given the large temperature sensitivity of digital sensors, using a special cooling system (usually with Peltier cells) you get considerable reduction of noise in the astrophotography DSLR camera. In this way it's possible to lower the sensor temperature relative to the ambient temperature and to stabilize the sensor temperature for all night long. This last aspect is very important and appreciated by every astro-photographer, because it allows you to acquire the "dark frames" at a different time from the night that instead can be entirely (or almost) devoted to record images (the light frames, that that will compose the final image). The "dark frames" are recorded at the same temperature, ISO sensitivity and exposure times but with the lens closed in the complete absence of light. It's a good idea to acquire several dark frames that then are processed by the processing software, thus creating a "master dark" with the median combination (hint: you will get good results with 9 dark although you can record more).
Let us see now some examples of thermal noise and how this varies in different environmental conditions and sensor ISO sensitivity. As astrophotography DSLR camera we used the Nikon D5500a Cooled camera with up to -27°C Peltier cell system and internal IR-Cut internal modified filter to be more sensitive to the red part of the spectrum (where there is the very important H-alpha emission line typical of nebulae). We have acquired several dark frames at various ISO sensitivity of 800 and 3200 ISO, at the sensor temperature of + 19°C and -5°C and with 300 and 600 seconds of exposure times. With this images, we performed a series of interesting comparisons using PixInsight.
EXAMPLE 1: by maintaining the operating temperature (+19°C with the cooling system switched off) and sensitivity (ISO 800), you can see that the noise increases by increasing the shutter speed. It's in fact greater in the right image (600 seconds exposition) with respect to the left (300 seconds exposition).
EXAMPLE 2: by maintaining the exposure time (300 sec) and sensitivity (ISO 800), the image on the left (without cooling) shows much more noise than the right one (with active cooling).
EXAMPLE 3: increasing even more the exposure time (600 sec) and increasing the sensitivity (ISO 3200), the difference between the images without cooling (left) and with active cooling (to the right) is even more evident.
In addition to visual feedback, the noise can be quantified in a "scientific" way to find the value of the "standard deviation" function in PixInsight in the "Statistic" tool. Analyzing it in the following two images, you can noticed a drastic reduction of the noise representative value in RGB in the case of astrophotography DSLR camera with sensor cooling. The values of "standard deviation" in the two images were practically halved cooling the sensor from 19°C to -5°C.
In conclusion, the cooling system dramatically reduces image noise improving image quality of astrophotography DSLR camera. This, combined with a good calibration with dark frames, flat and bias, allows to obtain the highest quality final image.