While experimenting with a Keplerian telescope, William Gascoigne discovered that an image viewed with a telescope became clearer when the optical focal points of two lenses was combined. Realizing that a telescope could be more accurately used when using a line to guide its direction, he created a new telescopic sight. He then created a sextant, similar to the ones used by Tycho Brahe, to measure the distance between celestial bodies with a stunning level of accuracy for the time. This was due to Gascoigne adding two adjustable points and a screw in order to increase the accuracy of the measurement. This would eventually be called a micrometer, and was one of the most common tools used for astronomical research for more than a century. Image of Robert Hooke's drawing of an early micrometer. The image is from Wikimedia and is public domain.