I’ve built a DIY dedicated flat field panel using a repurposed LED light fixture. The flat field panel is a light source with relatively uniform brightness. The panel attaches directly onto my telescope and can be used for taking flat frames.
If a telescope with camera captures a target that is known to have a uniform brightness or illumination (such as this DIY flat field panel), the unevenness in the illumination of the field such as vignetting or presence of dusts are revealed. When a flat frame is applied to an image, any variation in brightness or illumination across the frame is leveled out, thus, vignetting and dusts are removed in the image.
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Night Sky in Focus © Anthony Urbano | Bacoor, Philippines