Limiting Magnitude

The limiting magnitude is the maximum magnitude which can be reached with an instrument (or naked-eye), i.e. the fainter stars you are able to see. The magnitude scale originates back to the Greeks. Stars were then divided into six classes. The brightest stars are in the first class, faintest in the 6th. Each class represents a star brightness 2.512 times fainter than the previous: a star of magnitude 3 is 2.5 times less bright than a star of magnitude 2, and 2.5 times brighter than a star of magnitude 4. Magnitude scale starts at 0. Stars brighter than class 1 are termed negative (e.g. Sirius, the brightest star in Northern hemisphere is magnitude - 1.5). Naked-eye allows to see stars down to magnitude 6 to 6.5 (that is about 5026 stars). Observing with an instrument yields a deeper limiting magnitude. The table below gives different values of it (there is no difference due to instruments being a refractor, reflector or SCT (Schmidt-Cassegrain telescope)). Photographic limit magnitude is about 2 or more magnitudes fainter than visual one

to a table of instruments limiting magnitude