Observation 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, more precisely to the Greek astronomer Hipparchus of Nicea, by the 2nd century B.C. Stars were then divided into six classes. The brightest stars are in the first class, faintest (those at the limit of being visible with the naked eye) 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). A estimation, more generally, may be that there are about 7,646 stars which are visible naked-eye simultaneously in both celestial hemispheres. As far as the sky in the large cities is concerned, a mere 100-150 stars are observable at night only. Observing with an instrument yields a deeper limiting magnitude. Greeks likely conceived their system of a logarithmic scale as they had instinctively understood a physical law which was rediscovered in the 19th century A.D., that the more a physical stimulus is, the less one can perceive small variations of it, a law to which the human retina complies. 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

check a table of instruments limiting magnitude