Articles about astronomical telescopes

Reflecting telescope uses a concave mirror called the primary mirror instead of the objective lens. There is also a small mirror called the secondary mirror, which reflects the light collected by the primary mirror to the outside of the lens barrel, and the light image reflected by the secondary mirror is amplified by the eyepiece. The biggest advantage of reflection type is that because the main mirror is a mirror, the light does not need to pass through the glass, so there is no chromatic aberration at all, and it is unlikely to absorb ultraviolet light or red light, which is very suitable for physical observation such as spectroscopy. Although there is no chromatic aberration, there are other kinds of aberrations. If the reflecting concave surface is ground into a parabolic shape, the spherical aberration can be eliminated. Because the lens barrel can't be sealed, the main mirror is easily affected by smoke and dust, and it is difficult to maintain. At the same time, it is greatly influenced by the air temperature and airflow in the lens barrel, so it is easy to move the positions of the primary mirror and the secondary mirror when carrying, and the optical axis correction is quite complicated and inconvenient to carry. In addition, the diffraction effect of the secondary mirror base will make the star image of the brighter star appear cross-shaped or star-shaped diffraction patterns, which will also reduce the contrast of the image, and the stability of the image is not as good as that of the refractive telescope.

Mirror.

At present, there are five well-known reflecting telescope designs ... I will only list two small and medium-sized reflective telescopes on the market.

(1) Newton formula (Newton)

1668 was invented and designed by Newton, which consists of a parabolic primary mirror and a planar secondary mirror. The plane secondary mirror is installed slightly in front of the focus of the light reflected by the primary mirror, and forms an angle of 45 degrees with the optical axis (as shown in the above figure). This structure is the simplest and the image contrast is high, which is also the choice of most people. Usually the focal ratio is between f4 and f8.

(2) Ka seglin or Ka seglin for short.

Using a hyperboloid as a secondary mirror, the light is focused in front of the main focus, passes through a round hole of the main mirror, and focuses behind the main mirror. Because after a reflection, the lens barrel can be shortened, but the field of view is narrower, the astigmatism is more serious than Newton's, and there is a little field curvature.

Refractive telescope is a kind of telescope. Its objective lens is a lens, which focuses the light from distant objects and displays real images. It can focus light from a distance. The advantage of refractive telescope is that it is easy to use, and it will not be unclear if it is slightly neglected in maintenance. Because the inside of the lens barrel is sealed by the objective lens and eyepiece, the air will not flow, so it is relatively stable. In addition, the image quality deterioration caused by misalignment of optical axis is better than that caused by reflective telescope. Small-caliber lenses are all spherical, which can be mechanically ground and mass-produced, so the price is relatively high.

(1) galileo telescope

The first human telescope used a concave lens as an eyepiece, and the images seen through the telescope were the same as those directly seen with the eyes. The ground is convenient, but it can't expand the field of vision. At present, this design is no longer used for astronomical observation.

(2) Kepler telescope

Using a convex lens as an eyepiece, all refractive telescopes are of this type now, and the imaging is up, down, left and right, but it has no effect on our celestial observation, because the eyepiece is a convex lens, and more than two lenses can be put together to expand the field of vision, which can improve the aberration and eliminate the chromatic aberration.

The advantages of reflection and refraction are basically the same as those of reflection, but they also have the disadvantages of reflective telescopes. In order to eliminate the comet aberration whose field of view deviates from the optical axis, a lens is used, and the main mirror is a spherical mirror, which is easier to grind than the reflective one. Only one kind of refraction and reflection telescope which is widely used is introduced.

Schmidt card type:

It was invented by Schmidt in 1930. The spherical concave mirror is mainly used as the main mirror to eliminate coma, and at the same time, an aspheric lens is placed in a proper position in front of the main mirror as a corrector to correct the spherical aberration of the main mirror. In this way, a wide-angle field of view (up to 40-50 degrees) can be obtained, without spherical aberration and coma, which are common in general mirrors, only slight chromatic aberration produced by the correction mirror. Schmidt telescope for photography can achieve a very small focal ratio (generally between f 1 and f3, and the lowest can reach ÷0.6), so it is very suitable for star field and nebula photography.