During visual observations, the surface brightness in the telescope cannot be greater than that of the free-eyed . It is maximum when the exit pupil AP of the eyepiece corresponds to the pupil diameter of the dark-adapted eye .
Dark adaptation is hardly possible near the city because of the light pollution . Therefore, the observation of astronomical objects is only possible up to a certain surface brightness. This mainly affects faint galaxies and gas nebulae .
The surface brightness is the ratio of the apparent brightness H to the surface F :
or logarithmic in magnitudes:
If the apparent brightness of the object is replaced by the apparent brightness m in magnitudes, it follows:
The surface brightness is measured in cd / m². The unit mag / arcsec² is common in astronomy ; The unit S 10 describes the brightness as the number of stars of brightness 10 mag within one square degree .
- in cd / m²
- in mag / arcsec²
- 1 S 10 = 0.69 · 10 −6 cd / m².
Example: Lowest brightness of the night sky under optimal conditions: 21.6 mag / arcsec² = 2.5 · 10 −4 cd / m² = 370 S 10 .
- Other notation:
- Converter cd / m² - mag / arcsec²