Definition: The term "astigmatoscope" refers to a type of optical instrument used in ophthalmology, specifically for diagnosing and treating refractive errors. An astigmatoscope is an optical device that uses a special lens or prism to create two images on a screen (the "cornea" and the "iris") at different distances from the eye's retina. These images are then combined using mathematical algorithms to produce a visual image of the eye's shape, which can be used to diagnose various