Glossary | Telescopes | Optics | Light gathering capacity

The light gathering capacity describes how much more light the telescope collects compared to the eye.

The light gathering capacity relationship = aperture² / pupil diameter²

In Astroshop, a pupil diameter of 7 mm is always assumed. This may differ from the diameter of an individual observer, however it ensures that the telescopes are comparable with one another.

The light gathering capacity in mirror telescopes (Newton, Maksutov, or SC scopes) is reduced by the secondary mirror. In practice this shadowing effect, also called obstruction, does not play a significant role, as it represents only a small portion of the total area. The obstruction is more noticeable since the diffraction rings are brightened by the secondary mirror and its mounting. This reduces the contrast compared to a refracting telescope, which does not need such a secondary mirror.

Critical for a reduction of the theoretical light gathering capacity is therefore not the obstruction, but the quality of the other optical elements, such as the diagonal mirror or eyepieces.

The maximum pupil diameter decreases with age. Thereby, the light sensitivity of the eye decreases. The following values apply on average:

• 20 years: 7.2 mm
• 30 years: 6.6 mm
• 40 years: 6.0 mm
• 50 years: 5.4 mm
• 60 years: 4.8 mm
• 70 years: 4.2 mm
• 80 years: 3.6 mm

Source: Handbuch Astronomie Oculum-Verlag, Erlangen, 2015