![]() ![]() The trailing might be too small to see in a subframe, but the combined image reveals the movement. Visually, the stars in each sub might appear round, but they are not round. Skipping this step means the stars in each subframe appear in the combined image at their original positions, so any movement that occurred from one sub to the next shows up as trailed stars. This is a stack of 17 5-minute subframes, combined without "registering" the individual subs to align the stars.Įxplanation: Registration shifts individual subframes so the stars line up. The autoguider keeps the guide star centered, but the image on the main camera moves relative to it, resulting in smeared stars and fuzzy details. Some items sag more than others, so the guide scope and the main scope no longer point precisely at the same spot in the sky. As the telescope tracks across the sky, gravity can cause equipment to sag very slightly. These parts may be the guide scope, the guide scope focuser, the guide camera, the main telescope, the main focuser, and/or the main imaging camera. The answer is differential flexure – bad news for anyone trying to guide a telescope with a separate guide scope and camera instead of with a second sensor inside the main camera, or an off-axis guider attached to the main camera.ĭifferential flexure occurs when part of the imaging system shifts slightly, while other parts do not. Why isn't the premium image as sharp or sharper than the SCT image? Both telescopes were precisely focused before exposing these images. The left image was taken with a basic 9¼" Schmidt-Cassegrain telescope, while the image on the right was taken with a premimum 5" refractor. These two images show part of the Crescent Nebula. ![]()
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