Optics & Image Formation: Structures & Vocabulary

Astronomical optics collect electromagnetic radiation and redirect it to form images or feed instruments. Lenses use refraction, mirrors use reflection, and both can bring nearly parallel rays from a distant source toward a focus.

Key elements include the objective or primary mirror, focal plane, secondary optics, eyepiece or detector, and baffling. Image scale depends on focal length and detector geometry, while field of view depends on both optics and sensor size.

Ray tracing predicts image positions and aberrations, while laboratory star tests and measured point-spread functions show how an actual optical system performs across the field.

The major structural elements of an imaging system are the collecting optic, focus, field stop or baffles, filters, detector, and often corrective optics. A longer focal length spreads a given angular field over a larger physical detector distance; a shorter one gives a wider plate scale. Real optical systems must manage spherical aberration, coma, astigmatism, field curvature, chromatic aberration in refractors, scattering, and alignment errors.