The FGS On The Hubble Space Telescope (HST)
The Hubble Space Telescope has three Fine Guidance Sensors (FGSs). Two are used to point and lock the telescope onto the target, and the third can be used for position measurements - also known as astrometry. Because the FGSs are so accurate, they can be used to measure stellar distances and also to investigate binary star systems.
The three FGSs are located at 90-degree intervals around the circumference of the telescope's field of view. To achieve the very high pointing accuracy Hubble needs, the FGSs have been constructed as interferometers to exploit the wavelike features of the in-coming starlight. With this kind of accuracy and precision, the sensors can search for a wobble in the motion of nearby stars that could indicate the presence of a planetary companion, determine if certain stars really are double stars, measure the angular diameter of stars, galaxies, etc.
Due to the sensitivity of the FGS they can not be used whilst the HST is pointed within 50 degrees of the sun.
Read more about this topic: Fine Guidance Sensor
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