Attach your camera to the telescope, center a fairly bright star in your telescope and achieve a rough focus. Then place the mask over the front aperture of your telescope. Take a short exposure. The image of the star will show diffraction spikes. The “center” set will most likely be shifted to one side or the other. This is showing the defocus. Adjust the focus of your telescope and re-expose the star, once again showing the diffraction spike pattern. Iterate the process until you get the central diffraction spike centered against the star and the other spikes. When you achieve a symmetrical pattern, you have achieved the best possible focus. Remove the mask, center your target object and take your images!
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