Astronomical telescope purchase guide

Astronomical Telescope purchase guide

Types of astronomical Telescope

The Telescope is usually focused by a telephoto mirror (main mirror) to focus the image of the celestial body, and then uses a (short focus) eyepiece to enlarge this image near the focus.Generally speaking, Telescope can be divided into three categories: refraction Telescope, reflecting Telescope and refractive Telescope.

Refractor

Generally, the objective mirror that reflects the Telescope is composed of two different discounted glass lenses to reduce the color difference and make the red and blue images gather on the same focus. This type of lens is called Achromatic lens.Strictly speaking, there is still a very lilac halo in the periphery of such lens images.

a. Blu -ray focus.

b. Huang Guang's focus.

c. Red light focus.

The color difference between the red, green and blue color of the Telescope.

In order to reduce the spherical difference of the lens, the comets and the AstigMatism can generally increase the focus ratio, so the caliber and focal length ratio (coke) of the longoscope generally reflect the at least from F10 toBetween F16.

The higher -level lens is composed of three different discounted glass lenses or made of lower -color (ED) or even fluorite crystals to eliminate the color difference between red, green and blue.These lenses are called apochromat.Their caliber and focal length can reach F5.The length of the Telescope is shortened and the weight is light, it is more convenient to use, but the price is very expensive.

Because the refraction of the Tube can be sealed, it is more convenient for maintenance, and it is more suitable for moving to the wild without being affected by the airflow of the lens barrel.

Because the lens is composed of at least two pieces of glass, the cost (four mirrors) is more expensive than the reflected Telescope with the same caliber.The small astronomical Telescope sold on the market is mostly refracted Telescope.

Reflector

The reflection Telescope is to focus on a concave glass plated (usually aluminum) plating (usually aluminum). Because the focus is in front of the mirror, it must be used to reflect the image outside the lens barrel with another mirror before the focus of the object mirror, and then use the eyepiece to put it large.

There is no color difference in the reflection Telescope (not scattered because it does not need to be through the glass), but there are other types of aberration.If the reflection concave surface is grinded into a parabolic, the spherical difference can be eliminated, but it is seriously affected by the comet shape, so the edge part is still loose.

At present, there are two types of small and medium -sized reflecting Telescopes:

Newtonian

A 45 -degree plane mirror (Flat or Diagonal) with a piece of light shaft is used as a secondary mirror to reflect the image to the front side of the lens barrel.This structure is the simplest, the image contrast is high, and the most people are selected. Usually Job is between F4 and F8.

Cassegrain (Cassegrain)

Use a Convex Hyperboloid as a secondary mirror. Before the focus of the main mirror, the light is gathered, and the main mirror is focused on the main mirror through the main mirror.Because after a reflection, the lens barrel can be shortened, but the field of view is narrow, like a lot of dispersion than Newton. At the same time, there are a little Curvature of Field.

Because the reflective Telescope only needs to grind a optical surface, in terms of the same caliber, the price is cheap than the refractive mirror.Ordinary astronomical enthusiasts have a lot of 150mm and 200mm caliber, and reflex Telescope can be worn at the same time.

Because the lens cylinder cannot be sealed, the main mirror is very easy to be 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 of the inside of the lens.The light shaft is also quite complicated, and it is not convenient to bring.In addition, the diffraction effect of the auxiliary mirror seat will make the star image of the light stars appear a cross or star -shaped diffraction pattern, and also reduces the image contrast.

CatAdioptric Telescope

This is a type of Telescope that uses the principles of refraction and reflection at the same time. It was used as astronomical photography by Schmidt in 1930.The main thing is to use a spherical concave mirror as the main mirror to eliminate the comet shape difference, and at the same time, a non -spherical lens (ASPHERIC INS) is appropriately placed in front of the main mirror as a correction mirror to correct the spherical difference between the main mirror.In this way, you can get a wide -torment (up to 40 to 50 degrees) fields without the common spherical difference between the spherical difference between the common reflex mirror. Only the mild color difference made of the correction mirror.The Schmidt Telescope for photography can be very small (usually from F1 to F3, minimum can reach 0.6), so it is very suitable for Hoshino and Nebula photography.However, the only disadvantage is that there is a certain venue, so the negative must also be changed to adapt (undertake with a special negative seat), and at the same time, the negative is placed in the long -distance lens barrel, so it can only be put in.

Generally, astronomical enthusiasts use Schmidt-Cassegrain (Schmidt-Cassegrain). It uses a convex mirror as a secondary mirror to gather the light before the main mirror focus.after.Because of a reflection, the lens barrel can be shortened, usually between F6.4 and F10.

In addition to the Schmidt-Cassegrain, there are also Maksutov designs.A round hole focuses on the main mirror.In recent years, the popular spinning Telescope such as "Celestron" and "Meade" are composed of Schmidt-Cassegrain principles, while "Questar", "Meade" ETX series and "Intes" use Mark Su.The principle of Toff.

The mirror body of the refractive Telescope is short, the focal length, and the focus is behind the main mirror, and the field of view is quite flat.In terms of contrast).