There are three types of astronomical telescopes: refraction, reflection and catadioptric:
Refracting telescope
The so-called refracting astronomical telescope is a telescope whose objective lens is the lens that condenses the light of distant objects to show the real image. It will refract the light from the distance and concentrate on the focal point. The advantage of the refracting astronomical telescope is that it is easy to use, and it does not need maintenance and maintenance. It will be difficult to see because the inside of the lens barrel is sealed by the objective lens and the eyepiece, and the air will not flow, so it is relatively stable. In addition, the image deterioration caused by the misalignment of the optical axis is better than that of the reflecting telescope, and the lens is not large. They are all spherical, so they can be produced in large quantities by mechanical grinding, so the price is cheaper.
- Galileo type astronomical telescope:
The first astronomical telescope in mankind uses a concave lens as the eyepiece. The image seen through the telescope is the same as the actual one seen directly with the eyes. It is very convenient to observe the surface of the earth but cannot expand the field of view. At present, this type of astronomical observation is no longer used. design.
- Kepler-type astronomical telescope:
A convex lens is used as the eyepiece. All current refracting telescopes are of this type. The imaging is upside down, left and right, but this has no effect on our celestial observation because the eyepiece is a convex lens, so you can put more than two lenses together. A group can expand the field of view, and can improve aberration and eliminate chromatic aberration.
Reflective astronomical telescope:
Reflecting astronomical telescopes do not use an objective lens but use a concave mirror called the primary mirror.
In addition, a small mirror called the secondary mirror reflects the light collected by the primary mirror out of the lens barrel. The light image reflected by the secondary mirror is magnified by 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 is very suitable for physical observation such as spectroscopy. Although there is no chromatic aberration, there are other kinds of aberrations. For example, the reflective concave surface is ground. In a parabolic shape, the spherical aberration can be eliminated. Because the lens barrel cannot be sealed, the primary lens is easily affected by smoke and dust, so it is difficult to maintain. At the same time, it is greatly affected by the temperature and airflow in the lens barrel. It is easy to move the position of the primary lens and secondary lens during transportation, and correct the light. The shaft is also quite complicated and inconvenient to carry. In addition, the diffraction effect of the sub-mirror will make the star image of the lighter star appear to cross or star diffraction pattern, and also reduce the image contrast. In addition, the image stability is not as good as that of a refracting telescope.
- Newtonian astronomical telescope.
- Cassegrain type or Cassette astronomical telescope for short.
Catadioptric telescope
The type with the advantages of reflection and refraction is basically the same as reflection. It also has the disadvantages of reflective astronomical telescopes. In order to eliminate the coma aberration of the field of view deviating from the optical axis, a lens is used, and the main mirror is a spherical mirror, which is easier to grind than the reflective type.