The magnitude of faintest star you can see with the telescope is called Limiting Magnitude. The limiting magnitude is directly related to aperture, where larger apertures allow you to see fainter stars. The atmopheric condition often reduce the limiting magnitude.
Photographic limiting magnitude is approximately two or more magnitudes fainter than visual limiting magnitude.
M=7.5+5logD
M: Magnitude (constant)
D: Diameter of primary mirror (cm)
