# A few reminders: Flux vs brightness in radio-astronomy

Subsections

- Antenna power pattern, solid angles, associated efficiencies
- Source and observed flux density
- Observed brightnesses
- Basic relation between received spectral power and sky brightness
- Brightness temperature
  - Brightness temperature
  - Interpretation
- Effective aperture and aperture efficiency
- Relationship between effective aperture and antenna solid angle
  - Statement
  - Demonstration
  - Interpretation