Back in 1762, MV Lomonosov proposed a schematic of a reflector telescope in which the main mirror is tilted to the tube axis. This arrangement eliminates the need for a secondary mirror. It is widely used in solar telescopes. The long focal length of the lens minimizes aberration when the lens is tilted.
In modern large telescopes (for example, a six-meter one), an observer’s cabin is located in the upper part of the tube, which allows measurements and photographs to be taken directly in direct focus. The shielding of the luminous flux by the cabin is the same as from the secondary mirror.
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