The light frames are captured with the telescope pointing at the subject, the mount tracking the sky, and autoguiding engaged. This light frame is a four minute exposue at ISO 400. You can see many stars in the image, but not much else. There is, however, a large, beautiful nebula filling this entire frame. With a practiced eye you may pick up some hints of it, but for the most part it is invisible in this frame, being quite faint and diffuse. It's there, however, and subsequent processing will bring it out. The muddy red color is the actual color of the night sky. Our eyes are not very sensitive to color in low light - thus the old expression "all cats are gray in the dark" - so our eyes see the night sky as black or dark gray.

You might ask why I didn't use a higher ISO setting to achieve the same exposure value in a shorter time. The answer is that longer is always better. The image "signal", i.e., the light contributed by the subject, increases in direct proportion to the exposure time, but "noise", i.e., unwanted image components caused by the camera electronics and by the nature of light itself, are either constant regardless of exposure time or increase more slowly than signal. Thus the signal to noise ratio is better for longer exposures. The trade-off is that while longer exposures are better, more exposures are also better, and the number of exposures that can be acquired in a given length of time obviously depends on the length of each exposure. Also, if an exposure is too long, the signal will saturate the camera's sensor and data will be lost. So exposure time is a compromise, best determined by the nature of the subject and the sky conditions, and tempered by the astrophotographer's experience.