How to tell a good telescope from a bad telescope?

To evaluate the quality of a telescope, we should first look at its optical performance, and then look at its mechanical performance (pointing accuracy and tracking accuracy).

The optical performance of an optical telescope is generally measured by the following indicators:

1. Aperture of objective lens (d)

The objective aperture of a telescope generally refers to the effective aperture, that is, the transparent aperture (not the simple lens diameter), which is the main symbol of the telescope's focusing ability and also determines the resolution of the telescope (in layman's terms, it means to see clearly). It is the first factor in all performance parameters of a telescope. The larger the aperture of the telescope, the stronger the light gathering ability, and the darker the celestial bodies can be observed, and the brighter the celestial bodies can be seen more clearly, which reflects the telescope's observation ability on celestial bodies. Therefore, if the economic conditions permit, enthusiasts should try to choose a telescope with a larger caliber.

2. focal length (f)

The focal length of a telescope mainly refers to the focal length of the objective lens. The optical system of a telescope is often composed of two systems with limited focal length, in which the image focus of the first system (objective lens) coincides with the object focus of the second system (eyepiece). The focal length of the objective lens is often expressed by f, while the focal length of the eyepiece is often expressed by f'.

For example, the focal length (f) of the objective lens of the F700'60 astronomical telescope is 700 mm. The focal length (f') of the eyepiece PL9 is 9 mm. The focal length f of the objective lens is the main sign of negative scale in astrophotography. For the same celestial body, the longer the focal length, the larger the image of the celestial body on the negative.

3. Relative aperture (a) and focal length ratio (1/A)

Relative aperture A, also called light force, is the ratio of effective aperture D to telescope focal length F, that is, A = D/F ... Its reciprocal (1/A) is called focal ratio (that is, f/D, which is called aperture number in camera). For example, the relative aperture of 70060 astronomical telescope is a (= 60/700) ≈112 and the focal ratio is f/d (= 700/60) ≈1.67. The larger the relative aperture, the more favorable it is to observe the extended celestial bodies such as planets, comets, galaxies and nebulae, because their imaging illumination is proportional to the square (A2) of the relative aperture of the telescope. However, the imaging illumination of so-called linear celestial bodies such as meteors or artificial satellites is directly proportional to the product (D2/f) of relative aperture A and effective aperture D. Therefore, attention should be paid to choosing the appropriate A or focal ratio when taking celestial photography.

Generally speaking, the relative aperture of refraction telescope is relatively small, usually between115 and 1/20, while the relative aperture of reflecting telescope is relatively large, often between 1/3.5 and 1/5. When observing a celestial body with a certain viewing surface, the line size of the viewing surface is proportional to F, and its area is proportional to f2. The brightness of the image is directly proportional to the amount of light collected, that is, it is directly proportional to D2 and inversely proportional to the area of the image, that is, it is inversely proportional to f2.

4. Magnification (or multiple) (g)

For a visual telescope, the magnification (multiple) is the angular magnification of the observation target (equivalent to bringing the target closer by a multiple). It is equal to the ratio of the focal length f of the objective lens to the focal length f' of the eyepiece, that is, the magnification (G)=f/f'. For example, the H20 eyepiece for 70060 astronomical telescope has a magnification of 700/20 = 56'. For the same objective lens, the magnification of the telescope can be changed by changing the eyepiece. The shorter the focal length of the eyepiece, the greater the magnification, so it is not difficult to improve the magnification. But as we introduced in the chapter "How to choose binoculars", the higher the magnification, the more blurred and unstable the image is. Because astronomical telescope is different from microscope, the effect of astronomical observation on the ground is influenced by many factors such as the clarity and tranquility of the earth's atmosphere and the environment of the observation site. Generally, each telescope is equipped with several eyepieces with different focal lengths, that is, there are several different magnifications to choose from. When observing, the maximum magnification is by no means the best, and the clearest observation target shall prevail. Moreover, the maximum magnification of astronomical telescope cannot be increased at will. Due to the limitation of the resolution of the objective lens, the clarity and tranquility of the atmosphere and the diameter of the telescope's exit pupil, the maximum magnification is generally controlled to 1 ~ 2 times of the millimeter number of the objective lens according to the actual situation of the observation target and the atmosphere. For example, when the atmospheric tranquility is excellent, the maximum effective magnification of 70060 astronomical telescope should not exceed 2 ′ 60 =120 ′ (times). In general, when the magnification exceeds 1 times of the aperture of the objective lens, the imaging quality is not ideal.

5. Angle of view (ω)

The angular diameter of the sky area where the telescope can image well is called the field of view or viewing angle (ω) of the telescope. The field of view of a telescope is usually determined at the time of design. The field of view of a telescope is inversely proportional to the magnification. The larger the magnification, the smaller the field of view. Different optical systems, different imaging quality (caused by aberration), different aperture and different focal length determine the size of different fields of view of the telescope (for celestial photography, the size of the negative or CCD will also restrict the size of the field of view). Reflective telescopes have the smallest field of view, generally less than 1 degree; Refraction telescope is large, which can reach several degrees; The reflective telescope has the largest field of view, which can reach more than ten degrees or even dozens of degrees.

6. Discriminatory ability

The resolving power of the telescope is measured by the reciprocal of the resolving angle (δ) of the telescope (1/δ). The resolution angle is usually in angular seconds, which refers to the angular distance between two luminous points on the celestial sphere that the telescope can just resolve. For the visual telescope, according to the diffraction principle of light, the theoretical resolution angle of the telescope (λ=555 nm relative to the most sensitive wavelength) is: δ "= 140/d (mm) (where d is the effective aperture of the objective lens).

Due to the influence of atmospheric tranquility and the aberration of the telescope system, the actual resolution angle of the telescope is much larger than the theoretical resolution angle (a better telescope can only be between 0.5 and 2 seconds).

The higher the resolution of the telescope, the darker and more celestial bodies can be observed, and the clearer the image. Therefore, high resolution is one of the most important performance indicators of the telescope.

7. Extreme order of magnitude (penetration)

Magnitude is a numerical value used to represent the relative brightness of celestial bodies (that is, the brightness observed on the ground in clear weather, not their true brightness). The greater the star equivalence, the smaller the brightness. For example, the sun is about -26.7, the full moon (average brightness) is about-12.7, Sirius is about-1.6, Vega is about 0. 1, Altair is about 0.9, and Polaris (α Ursa minor) is about 2./kloc-0. On a clear moonless night, if we aim the telescope at the zenith, the magnitude of the darkest star we can see is called the extreme magnitude of the telescope (also called penetration). Generally, the darkest magnitude that the human eye can see is about 6, and the darkest magnitude that the telescope can see is mainly determined by the effective aperture of the telescope. The larger the aperture, the higher the magnitude you can see (for example, 50 mm telescope can see 10, 500 mm telescope can see 15). Of course, in fact, the limit magnitude is not only related to the effective aperture of the telescope, but also related to the absorption coefficient of the telescope objective, the atmospheric absorption coefficient and the brightness of the sky background. For photographic observation, the limit magnitude is also related to exposure time and film characteristics.

What do the numbers in the astronomical telescope model mean?

Different from binoculars, the magnification does not appear in the astronomical telescope model, but the focal length of the objective lens. For example, "70076" means that the focal length of the telescope objective lens is 700mm and the aperture of the objective lens is 76mm；; "1800 150" means that the focal length of the objective lens of the telescope is 1800mm, and the aperture of the objective lens is150 mm. It also means that the aperture is placed in front of the focal length. For example, the above two telescopes are also marked as "76700" and "15065438+". Anyway, it's not easy to make a mistake. The big number is the focal length, and the small number is the objective aperture.