The histogram for a test exposure image is a great way to determine the correct exposure for an astrophoto. The "mountain" of the histogram represents the sky background. Most astrophotos are correctly exposed when the mountain is at about 30 percent and fully separated from the left wall of the histogram box. This ensures that the critical faintest detail in the image is up out of the readout noise of the camera. At this exposure, multiple frames can be stacked to improve the signal-to-noise ratio in the final image. Longer exposures will reduce the dynamic range of the image by overexposing the stars in the image, causing loss of star colors.