Telescope Buying Guide
Types of Astronomical Telescopes
Telescopes usually consist of a long focal length objective (primary ) to focus the image of the celestial object and magnify the image with a (short focal length) eyepiece near the focal point. Generally speaking, telescope
s can be divided into three categories: refracting telescopes, reflecting telescopes and catadioptric telescopes.
Refractor
The objective lens of a general refractor is composed of two glass lenses with different refractive indices to reduce chromatic aberration and make the The images of red and blue are gathered at the same focal point, and this type of lens is called an achromatic lens. Strictly speaking, there is still a very lavender halo around the image of this type of lens.
a. Blue light focus.
b. Yellow focus.
c. Red focus.
The chromatic aberration of the red, green, and blue colors of the refracting telescope.
In order to reduce the spherical aberration (Spherical aberration), coma (Coma) and astigmatism (Astigmatism) of the lens, the focal ratio value can generally be increased, so the aperture and focal length ratio of the general refracting telescope ( focal ratio) at least between f10 and f16.
More advanced lenses, consisting of three glass lenses with different refractive indices or made of lower dispersion glass (ED) or even fluorite crystals, can eliminate the red, green and blue chromatic aberration. These lenses are called Apochromat. Their aperture to focal length ratio can reach f5. The length of the telescope is shortened, the weight is lighter, and the use is more convenient, but the price is very expensive.
Because the refracting telescope tube can be sealed, it is more convenient for maintenance, and it is more suitable for use in the field, and it is also not affected by the airflow in the lens tube.
Because the lens consists of at least two pieces of glass, the cost (four mirror surfaces to be ground) is more expensive than that of a reflecting telescope of the same diameter. The small astronomical telescopes generally sold on the market are mostly refracting telescopes.
Reflector
Reflector uses a piece of concave glass coated with metal (usually aluminum) to focus. Since the focus is in front of the mirror, Therefore, another mirror must be used to reflect the image out of the lens barrel before the focus of the objective lens, and then use the eyepiece to magnify.
Reflecting telescopes have no chromatic aberration (no dispersion because they do not pass through glass), but have other types of aberrations. If the reflective concave surface is ground into a parabolic shape, the spherical aberration can be eliminated, but it is seriously affected by the coma aberration, so the edge part is still loose.
At present, there are two types of small and medium-sized reflecting telescopes:
Newtonian
Using a piece of light and light A flat or diagonal mirror with an axis of 45 degrees is used as a secondary mirror to reflect the image to the front side of the lens barrel. This kind of structure is the simplest, the image contrast is higher, and it is also used by the most people. Usually, the focal ratio is between f4 and f8.
Cassegrain or Cassegrain for short
Using a Convex hyperboloid as a secondary mirror, at the focal point of the primary mirror The light is collected at the front, passed 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 narrow, the astigmatism is more serious than the Newtonian type, and there is a little curvature of field.
Because the reflecting telescope only needs to grind one optical surface, it is cheaper than the refractor in terms of the same diameter. Ordinary astronomy enthusiasts, there are quite a few with 150mm and 200mm diameters, and reflecting telescopes can be polished by themselves at the same time.
Because the lens barrel cannot be sealed, the main mirror 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 the airflow in the lens barrel. The position of the secondary mirror and the correction of the optical axis are also quite complicated, which is inconvenient to carry. In addition, the diffraction effect of the secondary mirror base will cause cross or star-shaped diffraction patterns in the star image of the brighter star, which also reduces the image contrast.
Catadioptric telescope
This is a kind of telescope that uses the principles of refraction and reflection at the same time. It was first introduced by Schmidt in 1930. ) invented for astrophotography. It mainly uses a spherical concave mirror as the main mirror to eliminate coma aberration, and at the same time uses an aspheric lens (Aspheric Iens) at 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 (up to 40-50 degrees) field of view can be obtained without the spherical aberration and coma aberration that common mirrors often have, and only the slight chromatic aberration made by the corrective mirror. The focal ratio of the Schmidt telescope for photography can be very small (usually between f1 and f3, with a minimum of 0.6), so it is very suitable for star field and nebula photography. However, the only disadvantage is that there is a certain field curvature, so the negatives must also be deformed to adapt (with a special film holder), and the negatives are placed in the telescope barrel, so they can only be put in one by one.
General astronomers use a Schmidt-cassegrain, which uses a convex mirror as a secondary mirror to gather light before the focal point of the primary mirror and pass through the primary mirror. The circular hole is focused behind the primary mirror. The lens barrel can be shortened because it undergoes a single reflection, and the focal ratio is usually between f6.4 and f10.
In addition to the Schmidt-cassegrain and the Maksutov design, they all use corrective lenses and a convex mirror as a secondary mirror, which will The light is concentrated, passes through a circular hole in the primary mirror and is focused behind the primary mirror. In recent years, very popular catadioptric telescopes such as ‘Celestron’ and ‘Meade’ are constructed using the Schmidt-cassegrain principle, while ‘Questar’, ‘Meade’ ETX series and ‘Intes’ use Maxus. Tof’s principle.
The mirror body of the catadioptric telescope is short, the focal length is long, the focus is behind the main mirror, and the field of view is quite flat. (but not as good as Newtonian, especially in terms of contrast).