As Astrophotography for the Amateur explains at length, few telescopes produce a sharp, fully illuminated image over an entire 35-mm film frame or even an entire DSLR sensor. The reason is that telescopes are designed to work with eyepieces, and with an eyepiece, what you want is maximum sharpness at the very center of the field.
Except for a few astrographic designs, telescopes just don't work like camera lenses. Of course, if you put an eyepiece on even the finest camera lens, you'd probably find it falling far short of diffraction-limited resolution. Photographic objectives and telescopes are different instruments. Telescopes will generally show some coma or astigmatism away from the center of the field.
A more noticeable problem is vignetting, or lack of full illumination away from the center. An APS-C-size DSLR sensor is slightly larger, corner to corner, than the inside diameter of a standard eyepiece tube. Thus, to reduce vignetting, you should avoid working through a 14-inch eyepiece tube if possible; switch to a 2-inch focuser or a direct T-adapter. But even then, a telescope with glare stops designed to work well with eyepieces will not fully illuminate the edges of the field.
Vignetting can be corrected by image processing (p. 188), but I prefer to take a more optimistic approach. It's not that the image is small; it's that the sensor is big. A DSLR sensor has far more pixels than a typical astronomical CCD, and that means the picture is croppable. You can use a couple of megapixels from the middle of a 10-megapixel sensor and get a very fine image.
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