What aperture should you use? Your f-stop will be based on the ISO you select.
You might think that because the sky is so dark you need a wide aperture. Just the opposite is true. Remember, your objective is not to record the dark sky except as background. You want to record the intensely bright streaks of color. Were you to use a wide open aperture during your time-exposure, you would probably overexpose the colors. Result: They would “burn out” and lose coloration. To intensify the color, therefore, use a smaller aperture like f/8, or f/11, or even f/16. As with your choice of shutter speed, you will have to set your aperture manually. Which you should use depends upon your digital camera’s ISO setting (or the speed of your film), and the intensity of the color bursts. We suggest you bracket your shots, using different apertures.