Capturing Images

Quick Guide

Long-exposure noise reduction Turn long-exposure noise reduction off.

Focus Turn off autofocus. Focus on the brightest object in the sky, trying to reduce it to the smallest point that you can. Use live view and zoom in live view to make the stars as small and sharp as you can.

Exposure settings I typically expose for 20 seconds with a very wide lens (11mm on a full-frame sensor). A shorter exposure would be better but any shorter has too much noise. Lenses that have a narrower field of view require a shorter exposure. I use the largest aperture (lowest number) for which my lens is sharp—f/4. I use ISO 3200 which I know is too low. Because of ISO invariance I can brighten the image during processing and not have any more noise than if I had shot at ISO 12800, and I get the advantage of more color in the bright stars.

Capturing Light Frames To minimize camera shake set the camera to live view to lock the mirror up. To capture images quickly to minimize the movement of the stars over the course of the capture, set the camera to burst mode. Using a remote shutter release or intervalometer, hold the shutter button down long enough to capture at least 10 images. More is better. I do not know what the practical limit is, but I will try capturing stacks of at least 50 images the next time I am out.

Capturing Dark Frames In the past I have recommended capturing at least 10 dark frames. But in my recent tests, I have found that using 10 dark frames always results more noise in the final image. I have experimented with up to 55 dark frames. I don't have any examples of night images where dark frames have reduced noise, and I have several examples sent to me by users as well as my own experiments. Currently I recommend against using dark frames unless you know more about the theory and practice of image processing than I do. My tests and calculations tell me you need at least 5 times as many dark frames as light frames and for most people that is not practical. In general, you are almost certainly better off using that time for more light frames or for a single long exposure of the foreground at low ISO. If you have an example where dark frames helped, please contact me. I want to know more about how to make dark frames useful.

Capturing Flat-Field Frames Flat-Field frames are usually prepared at home before or after collecting the other images. See Preparing Master Flat Frames.