![]() ![]() The Abbe diagram is created to compare various glass types based on Abbe number and refractive index for a certain wavelength. A high Abbe number indicates low chromatic aberration, and a low Abbe number indicates high chromatic aberration. The Abbe number is inversely related to the amount of chromatic aberration. Also known as the V-number, the Abbe number combines the refractive indices of a material at three wavelengths (486.1 nm blue from hydrogen, 589.2 nm yellow from sodium, and 656.3 nm red from hydrogen) to approximate the amount of chromatic aberration a material will have when visible light passes through it (Figure 3). In the late 1800s, German physicist Ernst Abbe, a colleague of Carl Zeiss, created the Abbe number, a measure of the magnitude of chromatic dispersion for the visible spectrum. Calculation of the Abbe number to describe the amount of chromatic dispersion for a medium. This can be visualized on images at bright to dark boundaries as “fringes” of color. Each wavelength of light will thus be focused at a different distance away from the lens. In Figure 2, a single lens demonstrates chromatic aberration as different wavelengths of light have different focal lengths after passing through the lens. As a result, the index of refraction for a given medium will vary based on the wavelength of light going through it. When a single beam composed of multiple wavelengths of light passes through a lens, each wavelength of light will change direction and speed to different extents. ![]() ![]() The interaction of the particular wavelength of light with electrons in the medium gives rise to this phenomenon. Chromatic DispersionĬhromatic dispersion, also referred to as chromatic aberration, is a phenomenon where the speed and direction of light propagation in an optical medium depends on the wavelength of light. Chromatic aberration results in a single beam of light to have a different focal point for each wavelength. Lens Material Properties Index of Refractionįigure 2. Critical lens properties include refractive index, Abbe number (chromatic dispersion), specific gravity, and ultraviolet absorption. Lenses can be produced using a variety of materials and designed with several optical profiles to optimize use in specific applications. Eyeglasses also serve an important role in protecting the eyes from physical trauma and harmful radiation. Both lenses and prisms are also frequently used to improve eye alignment and treat diplopia in strabismus. You can also in post erase the diffraction spike.Corrective spherocylindrical lenses are commonly used to treat refractive errors such as myopia, hyperopia, presbyopia, and astigmatism. This can also work if the diffraction spike is interfering with the object you are trying to image. If you have a reflector telescope or using Fastar/ Hyperstar, you can minimize diffraction spikes by taking multiple images while changing the angle of your telescope and then overlay the images. If you want additional diffraction spikes in your reflector or Schmidt Cassegrain, lay string across the optical tube. In addition, when you use astroimage cameras, you have cables going in and out of the camera that go across your optical lens preventing light from coming in and creating a diffraction spike. If your DSLR is bulky and is bigger than the secondary mirror, it blocks light coming into your mirrors and therefore creates a diffraction spike in your image. You can manually create diffraction spikes when using Fastar/ Hyperstar with either your equipment (bulky cameras or Astroimaging cameras) or with strings you place on the front part of your optical tube. With the secondary mirror being held up by the lens it eliminates diffraction spikes. This spike isn’t present with refractors as it doesn’t have a secondary mirror.Ĭelestron Schmidt Cassegrain and EdgeHD optical tubes have an optical lens at the front-end where the secondary mirror is held in the middle. A diffraction spike is caused by how light bends or diffracts around an object or in this case the support beams in your secondary mirror in reflecting telescopes. A diffraction spike is the light you see extend from a star in your astro-images. ![]()
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