Telescope Astigmatism: Definition and Explanation
Astigmatism is a common optical error that can affect the performance of telescopes. While many telescope designs are relatively free from noticeable astigmatism, it can still be a problem in certain types of telescopes, especially at high magnifications. Astigmatism in telescopes is typically characterized by stars appearing as lines rather than points of light. It can be caused by various factors, including the design of the optics, the quality of the components, and atmospheric conditions. To fix astigmatism in telescopes, it may be necessary to make adjustments to the optics or purchase new components, depending on the specific issue.
What is astigmatism in telescopes?
Astigmatism in telescopes is an optical aberration caused by the incident light beam hitting the telescope obliquely, distortions of the main mirror, or two different curvatures of mirrors or lenses generating different focal lengths. Most telescope designs, such as Newtonians and Cassegrain telescopes, are free from noticeable astigmatism, but they may suffer from other aberrations like coma. Astigmatism can be seen in the Airy disk as an image distortion where it is longer in one axis than that perpendicular to it. The aberration can be minimized by stopping down the telescope. Astigmatism in the telescope is more easily seen at high magnifications and can cause stars to appear as lines rather than points of light. Eyepiece designs, especially inexpensive wide-field designs, can also suffer from astigmatism, which is often mistaken for coma in the telescope design.
What causes astigmatism in telescopes?
Astigmatism in telescopes can also occur due to distortions of the main mirror or different curvatures of mirrors or lenses generating different focal lengths. Astigmatism can be seen in the Airy disk as an image distortion where it is longer in one axis than that perpendicular to it. Poor lens centration in the objective or poor alignment between the objective, the intermediate optics, and the eyepiece increases astigmatism. Astigmatism errors are usually corrected by design of the objectives to provide precise spacing of individual lens elements as well as appropriate lens shapes, aperture sizes, and indices of refraction. Most telescope designs are free from noticeable astigmatism, but many eyepiece designs, especially inexpensive wide-field designs, suffer from fair amounts of astigmatism. The correction of astigmatism is often accomplished in conjunction with the correction of field curvature aberrations.
How Does Astigmatism Affect Astronomical Viewing
Astigmatism appears differently depending on the quality, magnification and focus of your telescope. When stars are in-focus, they appear as a blurred cross. When they are out of focus, they will resemble an ellipse. With astigmatisms in the tangential plane, light is stretched horizontally. Sagittal astigmatisms appear to stretch light vertically.
Astigmatism can cause light to focus on one axis ahead or behind the other, resulting in blurred or distorted images in telescopes. Stars may appear as a blurred cross when in focus, and as an ellipse when out of focus in a telescope due to astigmatism. Telescopes with astigmatism in the tangential plane stretch light horizontally, while those with sagittal astigmatism stretch light vertically. Some telescope designs, like the basic Ritchey-Chrétien, are expected to suffer a degree of astigmatism, as they are designed to eliminate coma and spherical aberration instead.
How to Fix Astigmatisms in a Telescope
To reduce astigmatism, it is typically necessary to purchase a new lens for the telescope. Regular astigmatism at the focal plane primarily affects Newtonian reflectors and can be caused by a poor-quality primary mirror. Astigmatism in a telescope can also be a result of the viewer’s own eyeball, which can be corrected through glasses, contact lenses, or refractive surgery.
Which Types of Telescopes Experience Astigmatism?
While most telescopes are designed to reduce astigmatism, it can still occur in any optical device. This is because any eye, lens or mirror is prone to damage or error. Some telescope designs, like the basic Ritchey-Chrétien, are expected to suffer a degree of astigmatism. This is because they are designed to eliminate coma and spherical aberration instead. Astronomers using these designs prefer astigmatism over coma because it still allows you to find accurate positional measurements.
