How is the aberration formed?

Aberration is a phenomenon in optics, and many people don't know much about the causes of aberration. Let me give you a detailed introduction to the causes of aberration.

Aberrations are generally divided into two categories: chromatic aberration and monochromatic aberration. Chromatic aberration, referred to as chromatic aberration, is an aberration caused by the fact that the refractive index of lens material is a function of wavelength. It can be divided into position chromatic aberration and magnification chromatic aberration. Monochromatic aberration refers to the aberration that occurs even when light is highly monochromatic. According to the effect, it can be divided into two categories: blurred image and distorted image. The former includes spherical aberration, coma and astigmatism. The latter category has image field bending and distortion.

In practical work, the image formed by optical system is different from that obtained by paraxial optics (Gaussian optics), and there is a certain deviation. The deviation between optical imaging and paraxial imaging is called aberration.

Because of the aberration, the image is different from the original shape. Chromatic aberration caused by polychromatic light is referred to as chromatic aberration for short; Nonparaxial monochromatic light causes monochromatic aberration. Primary aberrations can be divided into five types: spherical aberration, coma, astigmatism, image field bending and distortion.

Due to inaccurate production or artificial damage, the camera head can't focus all the light emitted from one point on the same position of the negative film, which makes the image distorted or out of focus.

There are various aberrations in the actual optical system. The image formed by the object point is the result of various aberrations; In addition, the actual optical system can be completely defocused on the ideal image plane, and the aberration (referring to the image spot on this real image plane) will of course change. In astronomy, the actual aberration is often represented by a point-by-point diagram of ray tracing; Aberration can also be expressed by wave aberration. The light wave emitted by an object point is a spherical wave. After passing through the optical system, the wavefront is generally no longer spherical. Its deviation from the spherical surface centered on the reference point is multiplied by the refractive index of the medium there, which is called wave aberration.

The wavefront aberration of human eyes comes from the imperfect surface of cornea and lens, and its surface curvature has local deviation.

Cornea is different from lens and vitreous body;

The contents of cornea, lens and vitreous body are uneven, which makes the refractive index deviate locally.

These structural deviations make the light passing through the deviation part deviate from the ideal optical path, so that a point on the object is not an ideal image point, but a divergent spot. As a result, the contrast of the whole retinal image decreases and the vision is blurred.

How is the wavefront aberration formed? Practice has proved that the traditional evaluation method based on the principle of geometrical optics has great limitations. Modern physics research has found that light has wave-particle duality. According to the wave theory of light, the imaging deviation of human eyes can be completely evaluated and described, which is called wavefront aberration.

Influence of wavefront aberration formation in human eyes

The latest research shows that wavefront aberration has a serious impact on human imaging, especially on myopia. In 40% myopia, the influence of average wavefront aberration is equivalent to 150 degree myopia. This is why it is always difficult for many nearsighted people to achieve the same visual acuity as normal eyes when wearing glasses. Because the existing lens method only corrects defocus, but can't correct wavefront aberration at the same time.

In addition, recent theoretical studies on myopia have shown that wavefront aberration of human eyes is a risk factor for myopia. Because wavefront aberration blurs the retinal image, animal experiments have proved that no matter what method is used to blur the animal fundus image, it will lead to myopia.

Types of Aberrations In order to explain the causes of aberrations conveniently, we only discuss their differences in geometrical optics of parallel incident light. In fact, the target of astronomical observation is distant stars, which basically conforms to the parallel light hypothesis.

Spherical aberration (symmetrical aberration): When the parallel incident light along the optical axis cannot be fully focused, we call it "spherical aberration".

Spherical aberration of lens

Spherical aberration of mirror

Coma (asymmetric aberration): The situation that parallel incident light is tilted to the optical axis and cannot be fully focused is called coma.

Chromatic aberration: If different colors of light have different focal points, we call it chromatic aberration. Usually the focal length of red light is larger than that of blue light.

Curved image field: Even if the optical system can focus perfectly, it often happens that its focusing plane is different from our expected imaging plane. So the lens will have a curved design.

Astigmatism: Because when an object is imaged through a lens, the focal points of the X axis and the Y axis are often inconsistent.

Deformation: basically, the occurrence of deformation cannot be regarded as complete aberration. It's not caused by poor image focus. On the contrary, it is a clear image, but it does not match the original appearance.

Perfect imaging: parabolic mirror

Mathematical definition: y2= 4 F.x F: the focal length of the mirror.

Mirror features: the incident light parallel to the optical axis can be perfectly focused on the focus. At the same time, because it is a reflective surface imaging, there is no color difference. It is a very good choice to use paraboloid as the main mirror of astronomical telescope. Not only the weight of the optical system and the imaging quality can be considered. Unfortunately, if non-parallel incident light comes in along the main axis, there will be a symmetrical "spherical aberration". If the parallel incident light is inclined to the main axis, asymmetric coma will occur. Therefore, parabolic mirrors are most suitable for astronomical telescopes with long focal length, and are not suitable for observing ground scenery.

The mirror of paraboloid is not easy to manufacture, and the curvature of paraboloid must be gradually approached by grinding methods of many spherical mirrors, so the price is naturally higher. For an 8-inch, F/4 mirror, the gap between the middle mirror and the spherical mirror is actually very small, only a few wavelengths away. Although this is only a slight difference, it can improve the image quality a lot.

In order to obtain a high-precision paraboloid, it is necessary to grind the spherical surface many times.

Parabolic mirrors can be regarded as multiple spherical mirrors because they are ground by spherical mirrors many times. Using this optical characteristic, it can be a simple method to detect parabolic mirrors, which we call "knife edge inspection".

Conclusion: The production cost of reflector is much lower than that of refractor, so almost all large-mouth telescopes use parabolic mirrors. If the use is limited to astronomical photography, it is a good choice to buy this type of telescope. Especially in the shooting of star clusters and nebulae, the super-large diameter parabolic mirror is almost the only choice.

Coma-free: spherical mirror

Mathematical definition: y2= 4 F2- x2 F: the focal length of the mirror (R=2F).

Characteristics of spherical mirror: Spherical mirror is geometrically symmetrical, so parallel incident light along the optical axis or inclined optical axis has the same "spherical aberration". However, no coma is its advantage. Because the production cost of spherical mirrors is low, most of them are made into super-large caliber to gain their advantages. The parabolic mirror, which is also a reflector, has gradually replaced the spherical mirror because of its perfect imaging quality in the middle of the mirror.

Elastic image correction: refractor

Because the refracting mirror is composed of multiple lenses, each side of the lens is a spherical mirror. At present, due to the progress of lens grinding technology, a few lenses can also be made into aspherical mirrors. In order to eliminate "aberration" and "chromatic aberration", the material of lens is very important. Generally, a lens group consisting of two lenses is called achromatic.

The cost of lens grinding is high, and the lens group is heavy, which is very unsuitable for large-aperture astronomical telescopes. However, because the refractor can eliminate chromatic aberration and aberration by matching lenses with different materials and curvatures, it can be used for both astronomical observation and ground scenery viewing, so it can be regarded as an all-round telescope. Many people call it "fluorite mirror" or "ED mirror" because the first lens in the lens group is a high refractive index and low dispersion lens, while the second lens still needs a high dispersion lens. Ordinary optical glass has high refractive index and large dispersion, so "fluorite mirror" or "ED mirror" is precious. Because the incident light of the astronomical telescope is almost parallel light (distant star) and the field of view is narrow (high magnification), it is enough to use only the main mirror of the three-piece lens. Of course, if you want to watch it on the ground, the effect will definitely be discounted!

Conclusion: If you want to buy an extra one that can be used for both ground landscape viewing and astronomical observation. That refractive telescope is your first choice.

optical system

In a simple reflective astronomical telescope (Newton telescope), the starlight is often+shaped due to the tilt of the mirror.

Coma will appear at the edge of the image and its shape will be oval. If all the stars are oval, it means that the telescope is tracking the photographic error, not the aberration.

The large aperture advantage of reflective telescope can make dark stars fully appear. This is beyond the reach of ordinary refractive telescopes. Therefore, the mirror is an essential tool for nebula photography. At present, almost all large observatories use reflective telescopes to capture the darkest stars for research.

Refractive telescopes will have obvious chromatic aberration. In order to avoid tiny chromatic aberration, we often have to pay a high price.

Refractive astronomical telescope with high price and good imaging is very suitable for shooting high-magnification star field photography.

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