Why large lens diameters for twilight observations?Updated a day ago

To ensure maximum light transmission, lenses must be coated. However, due to physical limitations, it is impossible to achieve 100% transmission. As a result, residual reflections are visible on binocular lenses, appearing in various colors depending on the type of coating layer.
The coloration of an anti-reflective coating is a result of the non-linear behavior of residual reflection across the wavelength spectrum. This residual reflection is primarily influenced by the type of anti-reflection coating (single-layer, multi-layer coating, or broadband anti-reflection), the position of the reflection minimum, the composition of the coating system (material selection, layer thickness, and sequence), the refractive index of the base material, and manufacturing variations (deviations in layer thickness and positioning within the coating chamber).
Traditional single-layer coatings with MgF₂ typically exhibit a blue tint. Multi-layer coatings often appear violet but can also show other colors, including blue, if the reflection minimum is shifted towards red.
Modern broadband anti-reflection coatings are often slightly green. This is because within the wide spectral range where reflection is very low, the reflection is slightly higher in the middle range (green), which is perceived as a color tone.

(Source: A. Köhler from www.akoehler.de)