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A flat field can be made by imaging a uniform field, such as a plain wall, the gray sky, or in this case, a specially made light-box. The result can be applied to an image to correct for the difference in sensitivity of each pixel on the chip. A flat field taken at the same time as the image can be used to correct problems such as vignetting and dust particles on the telescope's optics. A light box can be used to make a flat field at any time. This is a typical light box; it fits over the end of the telescope. The important thing to note in building a light box is that the light level must be absolutely uniform. For this reason there is a double layer of diffusion built into the box, and the lights are not seen directly. They must bounce off the end of the box before getting to the first level of diffusion material.
These plans show a very simple but sophisticated approach to building a light box.
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