The exception to this is with refractors employing an optical design known as a ‘Petzval’, in which there are four lens elements, the front pair correcting chromatic aberration and the rear pair correcting field curvature. Short focal length telescopes – especially Newtonian reflectors – exacerbate this problem, but field curvature is present in most telescopes to some extent. However, the flat surface of a camera’s sensor is unforgiving and photographic images will clearly show off-axis stars as being out of focus. This is not such a big issue with observing as the eyepiece and the eye itself compensate to a large extent. Field curvature Field curvature can result in stars at the edge of images being blurred, as seen in this cropped corner of a frame.Credit: Steve Richards Affects: All telescope designsĬurved surfaces are employed to bend light in both refractors and reflectors, and this results in a curved focal plane where objects at the centre of the field of view are in focus on the camera’s sensor but those further out from the centre, known as off-axis, are out of focus. It uses three glass elements, one of which is commonly made from ED glass. There is also a more complex design called an apochromat that is specifically designed to bring all the wavelengths of visible light to the same point of focus, resulting in high-quality images. The inclusion of extra-low dispersion (ED) glass elements can also deliver very good results.
This design, known as an achromat, brings red and blue light to the same point of focus, but colours in between these two wavelength extremes still focus at a slightly different point.Ĭhromatic aberration also decreases with an increase in the focal length. The effect can be minimised in scopes that use two glass elements made from different types of glass (historically crown and flint glass). This shows as a blue halo around bright stars and as a yellow and blue colour cast to the opposite edges of the Moon and planets. The glass lens elements in a refractor are unable to focus all the colours of light at the exact same position because the refractive index of glass varies with the wavelength of the light passing through it, resulting in colour fringing. Chromatic aberration Chromatic aberration introduces haloes around bright stars Credit: Steve Richards Affects: Refractors, especially achromats, but can be mildly present in apochromats Such a star will show an enlarged Airy disc and close examination on either side of correct focus can reveal much about the quality of the optics by noting the formation of the concentric rings formed in the image.īelow, we’ll explain what causes the most common aberrations found in amateur telescopes. Some aberrations can be seen during a star test, which is a process in which a bright star is examined out of focus and at high magnification. There are essentially two methods of bending the light from distant objects: by refraction though lenses and by reflection from the surface of mirrors.Įach method produces a range of aberrations, although some are common to both types. Though some aberrations are barely visible when observing they become apparent once a camera is introduced. It’s the telescope optician’s job to minimise these aberrations as far as possible to produce a bright, high-fidelity view. Telescopes work by collecting light and then bending the rays to bring them to focus at the eyepiece or camera sensor, at a point known as the focal plane.The very action of bending the light introduces unwanted aberrations, and although modern telescope designs produce amazing results, they will always exist to some extent simply through the laws of physics.
Arrow corner reflector how to#
Optical aberrations: how to fix errors in your telescope
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