Question: what magnitude can i see with my telescope

title: Limiting magnitude
text: As a first approximation , the gain in magnitudes of a telescope is 5 ⋅ log 10 ⁡ ( D 1 D 0 ) ( \ displaystyle 5 \ cdot \ log _ ( 10 ) ( ( \ frac ( D_ ( 1 ) ) ( D_ ( 0 ) ) ) ) ) , where D is the diameter of the telescope 's primary light gathering component , and D is the diameter of the eye 's dark adapted pupil . Both quantities must be measured in the same units . D varies from person to person but is typically 6 -- 7 mm ( ~ 1 / 4 `` ) .