How to classify the types of telescopes

General astronomical telescopes are classified by structure and can be divided into three categories: refractive astronomical telescopes, reflective astronomical telescopes and catadioptric astronomical telescopes.

1. Refracting astronomical telescope

The so-called refracting astronomical telescope is a telescope that uses a lens that condenses the light of distant objects to show a real image as the objective lens. It refracts the light coming from a distance. Focus on the focus. The advantage of the refracting astronomical telescope is that it is easy to use. Even if you neglect maintenance, you will not be able to see clearly. 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, due to the optical axis The image deterioration caused by staggering is also better than that of reflecting telescopes, and the lenses with small apertures are all spherical, so they can be mechanically ground and mass-produced, so the price is cheaper.

(1) Galileo-type astronomical telescope:

Man's first astronomical telescope, using a concave lens as the eyepiece, the image seen through the telescope is the same as what is actually seen directly with the eyes It is an upright image. It is convenient for observing objects on the surface but cannot expand the field of view. This type of design is no longer used in astronomical observations.

(2) Kepler-type astronomical telescope:

Use a convex lens as the eyepiece. All refracting telescopes today are of this type. The image is tilted up, down, left, and right, but this is harmful to us. There is no impact on astronomical observation, because the eyepiece is a convex lens that can put two or more lenses together into a group to expand the field of view, and can improve aberrations and eliminate chromatic aberration.

2. Reflecting astronomical telescope:

Reflecting astronomical telescopes do not use objective lenses but use concave reflectors called primary mirrors. In addition, there is a small mirror called the secondary mirror that reflects the light collected by the primary mirror out of the barrel. The light image reflected by the secondary mirror is magnified with the eyepiece. The biggest advantage of the reflective type is that the primary mirror is a mirror. Light does not need to pass through the glass, so there is no chromatic aberration at all, and it does not absorb ultraviolet light or red light. Therefore, it is very suitable for physical observations such as spectroscopy. Although there is no chromatic aberration, there are other types of aberrations. If the reflective concave surface is ground into a parabolic shape, the spherical aberration can be eliminated. Because the lens barrel cannot be sealed, the primary mirror is easily affected by smoke and dust, so it is difficult to maintain. At the same time, it is greatly affected by temperature and airflow in the lens barrel. It is easy to move the positions of the primary mirror and the secondary mirror during transportation, and the corrected light The shaft is also quite complicated and not very convenient to carry. In addition, the diffraction effect of the secondary mirror base will cause cross or star-shaped diffraction patterns to appear in the star image of the brighter star, which will also reduce the image contrast. In addition, the image stability is not as good as that of the refracting telescope.

At present, the designs of well-known reflecting astronomical telescopes are roughly divided into five types. We only list two types of commercially available small and medium-sized reflecting telescopes.

(1) Newtonian astronomical telescope:

Invented and designed by Newton in 1668, it consists of a parabolic primary mirror and a planar secondary mirror. Install the plane secondary mirror at an angle of 45 degrees to the optical axis slightly in front of the focus of the light reflected from the primary mirror (as shown in the picture above). This structure is the simplest, has high image contrast, and is the most popular choice. Usually the focal ratio is between Between f4 and f8.

(2) Cassegrain or Cassegrain telescope:

Using a hyperboloid convex mirror (Convex hyperboloid) as a secondary mirror, in front of the main focus The light is gathered through a circular hole in the primary mirror and focused behind the primary mirror. Because of one reflection, the lens barrel can be shortened, but the field of view is narrower, the astigmatism is more severe than Newtonian, and there is a little curvature of field.

3. Catadioptric telescope:

Using the advantages of reflection and refraction, it is basically the same as reflection. It also has the disadvantages of reflective astronomical telescopes. In order to eliminate The cometary aberration of the field of view deviating from the optical axis uses a lens, and the main mirror is a spherical mirror, which is easier to grind than the reflective type. Only one of the most widely used catadioptric astronomical telescopes is introduced.

Schmidt telescope

Invented by Schmidt in 1930 for astronomical photography. Mainly, a spherical concave mirror is used as the primary mirror to eliminate coma aberration, and an aspheric lens (Aspheric Iens) is placed at an appropriate position in front of the primary mirror as a corrector to correct the spherical aberration of the primary mirror. In this way, a wide-angle (up to 40 to 50 degrees) field of view can be obtained without the spherical aberration and coma aberration commonly found in ordinary reflectors, and only slight chromatic aberration caused by the corrective mirror.

The Schmidt telescope used for photography can achieve a very small focal ratio (usually between f1 and f3, with a minimum of "0.6"), so it is very suitable for starfield and nebula photography