The Nesmith system ( Nesmith telescope), Nasmyth is the so-called three-mirror modification of the Cassegrain system , in which a diagonal mirror is installed inside the telescope tube between the main and secondary mirrors to drop the image sideways. Thus, the focus of the telescope, called in this case the focus of Nesmith , is on the side of the tube. Usually, at the exit from the tube, the optical axis is aligned with the declination axis or the height axis of the telescope. Such an optical scheme allows the telescope to be loaded with bulky observation equipment, without unbalancing the pipe. The main mirror (unlike the classic Cassegrain ) can be made without a central hole, moreover, the diagonal mirror can be mounted on a rack attached to the main mirror. Compared with the two-mirror Cassegrain or Richie-Chretien part of the world is lost on the diagonal mirror. This system was developed in 1842 by James Nesmith better known as the inventor of the steam hammer.
From the point of view of optical terminology, it is more correct to speak about a Cassegrain telescope (or another system) with observational equipment installed in Nesmith’s focus. Large telescopes often allow the equipment to be installed on the same telescope, both in the direct focus of the main mirror and in the focuses of Cassegrain (interchangeable secondary mirrors), Nesmith, and Kude.