1 MOA (one minute of angle or minute of angle) is 1/60th of a degree of angle.
With the help of a simple coating, this loss of light transmission (transmission loss) is reduced to 1.5%, for example. This already increases the amount of light that reaches your eyes to around 80%.
The image produced by a lens objective has a spherical error by nature. This error can be corrected using appropriate optical elements.
