a photographic camera with a film holder but without an objective lens that is mounted on the eyepiece (ocular) end of a telescope, microscope, or other optical instrument.
There are several methods of using such a camera. In one method, the eyepiece part of the instrument is removed and is replaced by the camera in such a way that the emulsion of the photographic plate is located in the same plane as the real image of the object produced by the objective of the instrument. In a second method, the eyepiece of the instrument is pulled out slightly, so that its first focus is located beyond the image plane of the objective. In this case, the eyepiece functions as a projection system and forms a real image that has some additional magnification. In a third method, special projection eyepieces or other optical systems that produce a real image are used instead of an ordinary eyepiece.