A fisheye lens is a type of wide-angle lens that captures a very wide field of view, typically around 180 degrees or more. This results in a distorted, curved image that resembles the shape of a fish's eye, hence the name "fisheye".
Alt/az (altitude/azimuth) and RA/Dec (right ascension/declination) are two different coordinate systems used to locate celestial objects in the sky. Alt/az coordinates are based on the altitude (angle above the horizon) and azimuth (angle along the horizon) of an object, while RA/Dec coordinates are based on the object's position relative to the celestial equator and the vernal equinox.
A fisheye lens can be converted to flat coordinates, such as RA/Dec, using mathematical transformations. These transformations involve correcting for the distortion caused by the lens and adjusting for the specific field of view captured by the lens.
Fisheye lenses are useful for capturing wide fields of view, making them ideal for astrophotography and surveying large areas of the sky. They also have a shorter focal length, which allows for shorter exposure times and reduces the effects of atmospheric distortion.
While a fisheye lens can provide accurate coordinates when converted to flat coordinates, it is not suitable for precise astrometry. This is because the lens introduces distortions that can affect the accuracy of measurements. For precise astrometry, a flat field lens or telescope with a longer focal length is recommended.