What Are the Parameters of the Optical Prism of a Telescope?

Sharpness of optical prism

Modern telescopes generally have good sharpness in the central viewing field during the daytime, but the best sharpness is achieved in stargazing settings. High-quality models will present stars as dots, while lower quality models will render them as smudges. Models with substantial chromatic aberration typically perform worse in terms of optical prism sharpness. Older straight-tube mirrors without phase-correcting coatings had slightly worse sharpness and luminosity than Maksutov-Cassegrain models, but these shortcomings have largely been resolved in today's top-of-the-line mirrors.

Chromatic aberration and color reduction rate of optical prism

Different coatings can allow more of a certain wavelength of light to pass through, causing the viewing field to lean towards a certain color; for less expensive models, this might be a darker yellow. However, European high-end models tend to lean towards a brighter yellow, though in practice they actually feel brighter and provide increased contrast, giving people a feeling of the world's beauty. This is a useful attribute in European forests where light is scarce. Slight chromatic aberration is a minor issue and should not affect stargazing, but it can affect one's personal impression of certain brands. It is undeniable that chromatic aberration will reduce color reproduction, but this is a trade-off, and severe chromatic aberration will result in much lower scores.

Transparency and clarity of optical prism

Most inexpensive telescopes suffer from an ailment where there is an improvement to transparency compared to the past, but it is still insufficient and a certain level of haziness remains. The best binoculars are ones that make you feel the glass does not exist while you're using them. Transparency and clarity are crucial rating standards, and their causes are complex, including coatings, mirror design, and glass quality.

Brightness of optical prism

The brightness of an optical prism is affected by three main factors: the prism, coatings, and color. If the color skew is towards blue, the brightness will be lower, and if it leans towards bright yellow, then the opposite is true. High-quality mirrors are generally bright enough, and if two models are of the same level, issues such as distortion, light transmittance, and color differences are often more important.

Glare control of optical prism

For most pocket binoculars and medium-sized binoculars, their internal shading system cannot block stray light at low angles due to limitations in their body design, and poor coating quality causes light to scatter irregularly inside the body when viewed in the direction of sunlight, leading to the entire field of vision being covered by diffuse glare, which seriously affects the observation. Multilayer coatings reduce internal reflection and control glare to some extent, and higher-end models have longer front hoods to play the role of a sunshade, but the problem cannot be completely solved.

Optical prism aberration

• Chromatic aberration

  When light passes through glass, refraction occurs, and different frequencies of light have different angles of refraction, resulting in blue and yellow edges appearing around high-contrast images. This problem is particularly severe at high magnifications, but fortunately, low-magnification binoculars suffer less from chromatic aberration. Some binoculars use only ordinary glass but still produce excellent images. Chromatic aberration is definitely an important selection criterion.

• Spherical aberration

  The properties of ordinary spherical glass mean that the light rays at the edge cannot focus at the same focal spot, resulting in shorter focal points the closer the light is to the edge, causing the edge to blur. This also affects central imaging resolution and brightness to some extent. The solution is to install special ground aspherical mirrors (multiple curved surfaces) inside the mirror to improve edge distortion. For wide-angle models, some blurring at the edges is not a problem, but it should be avoided as much as possible.

• Field curvature

  Field curvature refers to the fact that the focal plane of the image is a curved surface. The image will not become hazy, but straight lines at the periphery will become curved. The solution is to add a flat field lens inside the mirror.