All optics are subject to manufacturing tolerances; even with perfect manufacture, optical trains experience thermal expansion.
2.
Due to the selected design, the optical train was complex and turned out to be problematic to maintain.
3.
The analytical stereoplotters viewing system has an optical train system, which usually includes the ability to change the focal length of a camera by adjusting the lens.
4.
:: A complication is that astronomical telescopes usually show an inverted image, because this requires the least amount of lens glass ( or number of mirror surfaces ) in the optical train, thus maximising the brightness of the image and minimising its degredation, given that no lens or mirror is perfect.
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