Sources and standards

Sources for optical methods and safety guidance.

Primary sources

• NASA Science — Eclipse Viewing Safety: solar viewing safety.
• Celestron — All About Barlow Lenses: magnification and Barlow effects.
• Celestron — f-number: focal-ratio definition.
• Celestron — Collimation: meaning of optical alignment.

Sources last reviewed 16 July 2026. Geometric formulas are also documented on the methodology page.