What do you think of the indicators of the telescope?

I don't know whether you are talking about binoculars or astronomical telescopes. I will give you two indicators.

Binoculars:

1. Basic knowledge

Before buying binoculars, you should know its characteristics and specifications. The most important thing to pay attention to when buying astronomical binoculars is "caliber". Aperture refers to the diameter of the telescope lens. The larger the aperture, the brighter the image. Astronomical binoculars, the lens diameter should be at least 40 mm. Small binoculars of 20mm to 30mm are suitable for watching the scenery during the day, but they are not suitable for astronomical purposes because they cannot gather enough light. How do you know the diameter of binoculars? It's simple.

Each telescope is marked with a set of numbers, such as 7x50. The first number "7" in the specifications of binoculars is "magnification" and the second number "50" is the lens diameter. The sevenfold model is the best-selling model, which will make everything you see nearly sevenfold. You can also buy 10x and 16x. Maybe you think astronomical purposes need high magnification, but it's not. A pair of 7x binoculars is enough, and then we will talk about the advantages of 7x over most high-powered models.

2. Vision

On almost every pair of binoculars, you will see a set of data like "367 feet @ 1000 yards" or "120 m @ 1000 m". This string of numbers represents how wide you can see the landscape of 1 1,000 yards (or 1 1,000 meters) through the eyepiece. This is one of the methods to measure the size of the visual field. It is not appropriate to measure the horizon of the sky by the method of "1 000 yards". Instead, astronomers use degrees to measure their field of vision. At one time, it was twice the diameter of the full moon. Seven degrees is equal to the diameter of fourteen full moons, which is a typical binocular vision. The sky seen by the high-powered model is smaller (3 to 5 degrees), and the wide-angle model sees more (8 to 10 degrees). Just divide the foot data in the "feet @ 1000 yards" specification by 52. 5 Convert it into degrees. "m @ 1000 m" specification divided by the number of meters (meters) divided by 17. For example, a pair of binoculars with a field of view of 367 feet @ 1000 yards has a field of view of 367/52. 5 degrees, about 7 degrees. The imaging of the stars around the wide-angle model is usually a little distorted and blurred, which damages the view, which is difficult to balance. In addition, wide-angle models generally shorten good sight. There is a correlation between actual field of view and apparent field of view. Divide the effective field of view (say 70 degrees) by the magnification (say 10x), and you will get the value of "actual field of view", which is 7 degrees in this example. Therefore, the effective field of vision is improved, and the actual field of vision will be improved accordingly. However, when the magnification increases, the actual field of view will decrease.

3. Exit pupil

In the specifications of many binoculars, you will also find a specification called "exit pupil". It is the width of the light coming out of the eyepiece. You can calculate the pupil by dividing the aperture (in millimeters) by the magnification. For example, the model 7x50 has an exit pupil of 50/7(7 mm). The exit pupil of all 7x50 models is 7mm. The exit pupil of all 7x42 models is 6mm, and so on. You will find that many binoculars recommended by astronomy have a 7mm exit pupil, which makes sense. When your eyes get used to the darkness at night, your pupils will open wide and let more light in. People's pupils can be enlarged to 7 mm at most. Therefore, as long as binoculars shoot a 7 mm cone of light into your eyes, you may see the brightest image when the concentrating power of your eyes is fully exerted. All binoculars designed for low light environment have 7 mm exit pupil. Including: 7x50, 8x56, 9x63, 10x70, 1 1x80. Usually the most suitable for astronomical observation. However, when you get older, your eyes are not so big. People in their early thirties have a maximum pupil of about 6 mm. After forty years old, it will drop to about 4.5mm to 5 mm. If the upper limit of your pupil size is only 5 mm or 6mm, binoculars with a 7mm wide exit cone will waste some incident light. Some light can't enter your eyes, and the light collection ability of binoculars can't be fully exerted. For the elderly, the exit pupil of binoculars is 5mm or 6 mm. These models include the best-selling 7x42 and 10x50.

4. Good eye relaxation

Manufacturers pay more and more attention to the specification of "good sight distance". This specification refers to how close your eyes need to the eyepiece to see the whole field of vision. The data of good sight distance is more important for myopia glasses family. Although nearsighted people can take off their glasses and adjust the focal length of binoculars to compensate, it is quite inconvenient-it is really a pleasure to put on glasses, see the night sky clearly, put binoculars in front of your eyes quickly without deviating from the target, and then keep a complete field of vision. In addition, if you have several levels of astigmatism in one or both eyes, your glasses can't be taken off. To see complete vision with glasses, you need at least 14mm to 15mm from the retired glasses. Vehicles with a good line of sight less than 8mm or 9mm may be difficult to observe even without glasses. You must put your eyes close to the eyepiece to see all the fields of vision. Looking at it for a long time will cause a sense of oppression, and the grease and dust on the eyelashes will ensure that the eyepiece will be polluted.

Astronomical telescope:

1. objective aperture

The objective aperture of a telescope generally refers to the effective aperture, that is, the aperture of light, that is, the entrance pupil diameter of the telescope, which is the main symbol of the telescope's focusing ability, not the diameter of the lens. For example, the aperture of F700x60 astronomical telescope is 60mm.

2. Focal length

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 expressed by f'.

For example, the focal length (f) of the objective lens of the F700x60 astronomical telescope is 700 mm. The focal length (f') of the H8mm eyepiece is 8 mm..

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

The ratio of the effective aperture d to the focal length f of a telescope is called relative aperture or relative aperture a, that is, a = d/f.

This is a sign of the optical power of a telescope, so it is sometimes called optical power. The illumination of visible objects such as comets, nebulae or galaxies is directly proportional to the square of the relative aperture (A2). The illuminance of so-called linear celestial bodies, such as meteors or satellites, is directly proportional to the product (D2/f) of relative aperture A and effective aperture D. When taking pictures of celestial bodies, we should pay attention to choosing an appropriate A or focal ratio 1/A (that is, f/D, which is called the aperture number on the camera). For example, the relative aperture of the F700600 astronomical telescope is A=(60/700)*0.09, and the focal ratio is 1 1.67.

4. Magnification (gram)

For a visual telescope, if the focal length of the objective lens is f and the focal length of the eyepiece is f', the magnification is G = f/f'

If the H8mm eyepiece is used in the F70060 astronomical telescope, the magnification is 700/8 = 87.5x As long as the eyepiece is replaced, the magnification of the telescope can be changed for the same objective.

Due to the influence of objective resolution and the fact that the exit pupil diameter of the atmospheric visibility product telescope cannot be too small, the magnification of the telescope cannot be arbitrarily too large. According to the actual situation of the observation target and atmospheric visibility, the magnification is generally controlled at 1-2 times of the objective millimeter. For example, the maximum effective magnification of the F70060 astronomical telescope does not exceed 2x60 = 120x.

5. Angle of view (ω)

The angle of the sky area where the telescope can directly image well in the observer's eyes is called the field of view or field of view angle (ω). The field of view of a telescope is usually determined at the time of design. Refractive telescopes are limited by mass, which limits the field of view, while reflecting telescope or catadioptric telescopes are often limited by the size of the secondary mirror. For astrophotography, the field of view may also be limited by the pixel size of the receiver.

The field of view of a telescope is inversely proportional to the magnification. The larger the magnification, the smaller the field of view.

6. Extreme magnitude

On a clear moonless night, observing the magnitude of the darkest star near the zenith with a telescope is called the limit magnitude (mB), which is related to many factors such as the effective aperture of the telescope, the relative aperture, the absorption coefficient of the objective lens, the atmospheric absorption system, the brightness of the sky background and so on. The empirical expressions given by different authors are slightly different. The simple estimation formula is mB=6.9+5lgD.

Where d (objective aperture) is in centimeters. For photographic observation, the limit magnitude is also related to exposure time and film characteristics. There is a common empirical formula: mB=4+5lgt+2. 15lgt.

Where t is the limit exposure time, regardless of the reciprocal failure of the negative film and the influence of urban lighting.

A simple way to check the limit magnitude of a telescope is to use the standard magnitude of the star selected by the center of the Pleiades or the standard magnitude of the Polaris (photographic magnitude, apparent magnitude, etc.). ) estimation or miscalculation.