Limitting stellar magnitude. 

In astronomy, the magnitude refers to a logarithmic measure of the brightness of a celestial body. The larger the magnitude number, the fainter the object.

The limiting magnitude of a telescope is the brightness of the dimmest object that can still be seen with a particular telescope. A substantial value of limiting magnitude of a telescope means that even a very faint object can be seen.

The value of the limiting magnitude depends on:

• the telescope’s objective diameter and
• observer’s eye entrance pupil. 

Limiting magnitude can be calculated using the formulas listed below:  

( GL) → Light grasp; brightness increase (how much more light can telescope gather compared to the human eye)

• D = objective diameter (mm)
• d = eye pupil diameter (mm)

Brightness increase in the terms of magnitudes:

The maximum diameter of the human pupil is 7 mm. The faintest magnitude our eye can see is magnitude 6. This value has to be added to Gmag  to get the value of the faintest magnitude that can be seen with a particular telescope (m). 

We must take into consideration that this equation doesn’t include:

• the light that is lost within the telescope,
• seeing conditions and
• telescope’s age.