![]() ![]() As with most telescopes, distortion is negligible. Field curvature exists, so wide field photography may be difficult without a corrector lens, although the limiting factor with an achromat is almost always the chromatic aberration. Doublet objectives suffer from slight spherochromatism, where spherical aberration is eliminated in green light but there is slight undercorrection in red and a slight overcorrection in blue. An air-spaced objective has less coma than a cemented one. Both coma and off-axis astigmatism are present and can be problematic for wide-field imaging. The primary aberration in an achromatic refractor is longitudinal chromatic aberration. An air-spaced objective gives two additional degrees of freedom (another surface curvature and the thickness of the air space), allowing better control of coma. This type of objective will normally have more coma than an air-spaced objective and is usually only seen on designs with very small apertures and slow focal ratios. This eliminates possible degrees of freedom from the design. A cemented objective has the same curvature on the inside surfaces of the lenses (the second and third optical surfaces). There are two basic configurations for the objective: cemented and air-spaced. The two lenses together make up the telescope objective. These are sometimes called apochromatic refractors, but do not adhere to the strict definition of an apochromat as outlined below. Many telescopes now use extra-low dispersion (ED) glasses to minimize chromatic aberration. ![]() The lower the dispersion of the crown glass, the less the residual chromatic aberration (the difference between green and red/blue focus). (In the diagram above, the lens on the left is the positive crown, the second lens is the negative flint.) The green is still slightly different, but the difference is relatively small. Chosen correctly, this results in the red and blue wavelengths coinciding at the same focus. Then a flint glass element, with a higher dispersion, is used as a negative element. This glass has a low dispersion, spreading the colors out to a lesser extent. In the usual configuration a positive lens is made of crown glass. If focused for the green light to which the human eye is most sensitive, the red and blue colors will appear out of focus, yielding a blurry image.Īn achromatic refractor uses two pieces of glass of differing optical properties to focus two colors to the same point. This is longitudinal chromatic aberration. Specifically, red light is focused farther from the lens than blue light. For a review of the optical design terms, see the Optical Aberrations and Optical Design sections.Ī simple lens focuses different wavelengths of light to different points. For a more basic overview of this design please see the Refractors page. This section details the optical design and inherent aberrations of refracting telescopes. Above: Optical layout of a typical refracting telescope ![]()
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