I usually try to get the shot in one exposure. I am not against stacking, layering or HDR photography I just haven’t tried it much myself. Probably because I am not very good at it 😉 So when I heard that most star trails shots are stack images I decided to do some research. I have been reading about how to stack star trails but I decided I wanted to see how close I can get with a single exposure out of the camera. Nikon has two noise reduction options for pictures on is High ISO Noise Reduction and the second is Long Exposure Noise Reduction. 
I usually keep the long exposure noise reduction turned off because it takes way to long. For example, a shot that is 45 minutes long locks up camera for another 45 minutes while it reduces noise on the image so one picture can time the camera up for a full 1.5 hours. In other words, I can only take one photo every 1.5 hours; better get it right the first time 😉 One the other hand stacking multiple images in Photoshop can take even longer but a single shot in your stack won’t make the whole shoot a dud. The first night I took a 50-minute exposure without any noise reduction performed either in camera or post. The second night I wanted to include the north star but I had to shorten the exposure to 45 minutes due to the extra light in the sky from Vegas. While the two shots are not scientifically comparable it is still close enough to see the difference and decide if there is a way to get star trails in one shot/exposure. Unfortunately, the second night was windy which added motion blur and that makes it difficult to know if the in camera noise reduction also reduces the sharpness. Something interesting about the first night shot is I tried to remove the artifices/noise in Lighroom and was unable. LR is fantastic for removing ISO noise but seems to lack when the noise is caused by a long exposure.
