Shorter scopes have faster focal ratios and more distortion of the image at the edges of the field of view. The longer you can make the focal ratio, the easier it is to get a flat distortion free field of view over the magnification range. The ATACR has superior optical performance compared to the NX8, mostly due to the appochromatic glass and the longer focal ratio giving a nice flat, sharp field of view compared to the NX8. So there is a very valid reason the ATACR is longer than the NX8.
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