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Here's a sample image from my homemade telephoto lens.
The focal length is a bit less than the 625 mm that is predicted by the formula. The size of the lunar image is slightly smaller than one I shot using a 600 mm mirror lens, so I'll estimate its actual focal length at about 575 mm. And of course, the image is not particularly sharp. To get the best possible image, I closed down the diaphragm to a point just before the diffraction became objectionable. There was a large amount of false color due to uncorrected chromatic aberration. I was able to compensate for that somewhat by using only the green channel to make a monochrome image. Finally, I did a little gentle sharpening in PS. The image is still not as good a straight-out-of-the-camera image using a commercial 600 mm mirror lens, but as a demonstration of the telephoto design, I'll take it for now.