1. Specifications of telescopes

The specifications of telescopes are expressed by magnification and objective lens diameter, such as 7x35, which means that the magnification of this telescope is 7 times and the objective lens diameter is 35 mm.

2. What does the magnification of a telescope refer to?

The magnification of a telescope refers to the ability of the telescope to bring objects closer. For example, if you use a 7x telescope to observe an object, the effect of observing an object 700 meters away is similar to that of an object 100 meters away observed by the naked eye (of course, the effect is worse due to the influence of the environment). Many people always think that the higher the magnification, the better. Some manufacturers also attract consumers with false high magnifications. Some telescopes on the market are even marked as 990 times! In fact, the reasonable magnification of a telescope is related to the diameter of the telescope and the observation method: the larger the diameter, the higher the magnification can be, and the higher the magnification can be when observing with a tripod than when observing by hand. The larger the magnification, the worse the stability. The larger the magnification, the smaller and darker the observation field, the greater the jitter it brings, and the greater the impact of breathing airflow and air fluctuations on it. For handheld observation binoculars, 7-10 times is the most suitable, and it is best not to exceed 12 times. Most military binoculars in various countries around the world are also mainly 6-10 times, because clear and stable imaging is very important.

3. What does the aperture of a telescope refer to?

The aperture refers to the diameter of the telescope objective lens. The larger the aperture, the larger the observation field and brightness, which is conducive to observation in dim light, but the larger the aperture, the larger the volume. Generally, it can be selected between 21-50mm according to needs.

4. What is the field of view of a telescope?

The field of view refers to the size of the range observed within a certain distance. The larger the field of view, the wider and more comfortable the observation range. The field of view is generally expressed by the horizon at kilometers (observable width) and converted into angles. Generally speaking, the larger the aperture, the lower the magnification, and the larger the field of view, but the more critical thing is the design of the eyepiece group.

5. What is the exit pupil diameter?

The exit pupil diameter is the size of the light spot formed on the eyepiece after the image passes through the telescope. The exit pupil diameter can be obtained by the following formula: objective lens/magnification=exit pupil diameter. From this, we can see that the larger the objective lens and the lower the magnification, the larger the exit pupil diameter. Theoretically, the larger the exit pupil diameter, the brighter the observed scene, which is conducive to observation in dim light. Therefore, when purchasing a telescope, you should try to choose a larger exit pupil diameter. So is the larger the better? Not really, because we normally use telescopes mostly during the day, when the pupil of the human eye is very small, only about 2-3 mm. At this time, if you use a large exit pupil diameter such as 4 mm or more, most of the useful light is not absorbed by the human eye, but wasted. The human eye pupil can only reach about 7 mm at dusk or in darkness. Therefore, in general, it is enough to choose an exit pupil diameter of not less than 3 mm. So the exit pupil diameter is also called the dusk factor.

6. What is the role of coating

If you pay attention, you will find that the outside of the objective lens of the telescope will have different colors, red, blue, yellow, purple, etc. This is what is usually called coating. So what is the role of lens coating? The role of lens coating is to nourish reflection, increase light transmittance, and improve observation effect. The color of the coating should be determined according to the optical material and design requirements. The lighter the coating and the smaller the reflection, the better. Blue and red films are usually used most, but now there are many telescopes with strong reflections and bright colors on the market to attract consumers. In fact, the coating of the telescope is matched with the structure of the telescope. These inferior coatings reflect and lose a lot of light, which reduces the observation effect. Some unscrupulous businessmen even call the red-film-coated telescope "infrared night vision telescope" and say that it can be observed at night and in dim light. In fact, the real infrared night vision device is a photoelectric tube imaging, which is completely different from the structure and principle of the telescope. It cannot be used during the day and requires power for observation.