![]() ![]() Where as, if the signal increases by 400 times – the noise will have increased by a value of 20 times, the square root of the signal increase (giving only an increase of 5% of the signal). In other words, if the exposure signal increases by 100 times, the noise would increase by 10 times – the square root of the signal increase (giving an increase of 10% noise). As the signal is increased with longer exposure times, the noise will increase to the value of the square root of the signal. This is because of something called the square route rule. The longer the exposure time, the less overall percentage of noise will be present (higher signal to noise or SNR). The main thing we can do about reducing the effect of noise is to record more signal in the imaging process. The ideal would be to take a dark sub straight after every light sub, but this will loose you precious clear sky time! For this reason, a lot of people tend to take the dark frames at the end of their imaging sessions. This is an effective method but it’s difficult to keep up with the correct temperatures during an imaging session.Īs you can imagine, a session running over a period of 2 hours for example could experience quite high temperature variations. Then the information recorded from these dark frames can be subtracted from the light frames in the stacking process. These must have the same exposure time and temperature as the light frames taken (the subs). These dark frames must be taken with the camera lens or scope cap fitted so no light can enter the cameras chip. Take dark framesįor example, thermal noise can mostly be removed by taking dark frames. Here are the main steps we can take to remove a good amount of this unwanted noise. What can we do about all this noise then? Where as cooled CCD cameras will remove almost all of this type of noise. This will be increased significantly when using an uncooled DSLR camera. Thermal noise, sometimes referred to as dark noise or dark current is created by thermal variations in the cameras chip. This is due to a number of processes which happen between the chip receiving the photons & converting them into the final digital value which forms the image. Read noise is caused by the internal electronics of the camera. This randomness of photons received produces this type of noise. This means that each sub taken at the same length of time would have received slightly different amounts of photons. In other words over a set period of time the packets of photons received, by nature will actually vary in quantity as they travel through space and arrive at your camera. This is because the photons received by the cameras chip are arriving at random intervals. Photon noise or “shot noise” is actually created by the object itself. This noise comes in three main forms, photon noise, read noise and dark noise. When this mixes in with the good information (the signal), it degrades the image. Unfortunately the reality is that there will be other information (the noise) recorded at the same time. In an ideal world we would only wan to receive the pure photons of light from the object and that’s it!. When we talk about noise, we mean the elements of an image recorded by the cameras sensor (chip) which we don’t want. This will help you understand how we arrive at the final answer (don’t cheat by skipping to the end!). So the simple question is this, which method would give you the most complete image with the most detail and the least noise?įirst I’m going to go through a few fundamentals, explaining how an image actually builds up onto the cameras sensor during an imaging session. 2 subs at 60 seconds each (again totaling 120 seconds). Or you can collect the same overall exposure time by using fewer long exposures e.g. So I decided to put together the following article, so that anyone else wondering the same thing will also have the answer! So what do we mean by “more short exposures or fewer long exposures?”įor example, you can take 12 subs at 10 seconds each, giving a total exposure time of 120 seconds. Would more short exposures or fewer long exposures be the best way to go for deep sky imaging? There were so many different opinions, depending on who I asked, and at the time I thought I’d never get to the bottom of this!! Eventually after doing some homework and lots of research, I eventually managed to get the answers I was looking for. When I first started out with deep sky imaging, I too wanted to know the answer to this question. ![]()
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