Dual-barrel telescope literacy

I have bought several telescopes, but I recently discovered that some basic concepts are not understood. After best practices, I wrote this post, hoping to help cainiao who have the same experience as me.

I. Telescope exit pupil diameter and brightness

The telescope never saw the nebula. I mistakenly thought that the magnification of the telescope was not enough. I recently learned that many clouds are actually very large. The area of the big Nebula in the fairy tale is equivalent to several suns. The sky is really small. If the brightness is enough, you should see it clearly. The reason is that the brightness is too low. As long as the telescope brightness is sufficient, you can discover them without the need to zoom in multiple times.

 

Of course, a planet like Saturn and Jupiter can also see the surface details with a common telescope. On the Internet, several friends used the Shooting Method to combine images after thousands of Blurred photos to produce a photo containing certain details.

 

How to determine the telescope brightness? The answer is the aperture of the telescope.

 

The diameter of the telescope's exit pupil is the diameter of the image after the light passes through the eye lens. Generally, the diameter of the image is smaller than the diameter of the human eye pupil. The closer the pupil diameter is to the pupil diameter of the human eye, the greater the brightness of sensory imaging. However, if the diameter of the pupil exceeds the diameter of the human eye, the light will be wasted, but this will look much more comfortable. Generally, the pupil diameter is about 7mm at night, and the adult pupil diameter is generally less than 5mm.

 

How to calculate the pupil diameter?

 

Generally, the specifications indicated on the telescope are similar to 7x50, which indicates that the magnification of the telescope is 7 times, and the diameter of the objective lens is 50mm. The formula for calculating the exit pupil diameter is:

 

Pupil diameter = objective lens diameter/magnification.

 

In this case, the diameter of the 7x50 mirror pupil is 50/7 = 7.1 (mm ).

In addition to the pupil diameter, the lens quality also varies greatly. I compared two 7x50 telescopes in a franchise store. The Brightness Difference is still very large. Of course, the price is also a dozen times lower. In fact, this difference is mainly caused by lens quality.

Ii. Telescope field of view

Some telescopes look wide and comfortable, and some look like looking at things from a thin tube, which is depressing. Why? This indicates that the former has a relatively large field of view and the latter has a relatively narrow field of view.

There are two parameters for measuring the telescope's field of view.

One is the actual field of view angle: This is the field of view angle actually seen by the telescope objective lens. Generally, telescopes indicate the actual field of view. For example, a telescope may directly indicate that its actual field of view is 7.1 degrees.

What affects our subjective experience is the visual field of view. For example, if the actual field of view of a 7x50 telescope is 7.1 degrees, the visual field of view should be 7.1x7 = 50 degrees.

There is a very interesting phenomenon. The higher the magnification of a telescope, the more likely the visual angle of view is to be larger. The visual viewing angle of a telescope is relatively small. Why?

In fact, the principle is very simple. The actual field of view angle of the telescope is basically subject to the focal length of the objective lens and the diameter of the mirror tube optical path. The shorter the focal length, the thicker the mirror tube, and the easier the actual field of view angle to grow.

The actual field of view of a Low-magnification telescope cannot be larger if it is unwilling to be larger. When the magnification is small, the product of the Two-that is, the visual field of view is very small.

In turn, in the case of the focal length of the lens and the diameter of the lens tube, the high-magnification telescope will have a large visual angle of view due to the high magnification.

For example, if the 7.1 and 7.1 telescopes share the same actual field of view angle of degrees, the visual angle of the 7x50 mirror is 7 x = 50 degrees, the visual angle of the 10x50 mirror is 10x7.1 = 71 degrees. Although this area is 20 degrees, the former field of view belongs to the ordinary level, while the latter belongs to the ultra wide angle, and the degree of comfort varies greatly.

3. Lens Coating

The purpose of lens coating is to reduce the reflection of lenses and increase the light transmittance. If the lens has obvious reflection, the light transmittance of the lens is not very high.

 

The telescope I bought a few years ago has a red coating, indicating that the lens of the objective lens has a lot of red light reflected, so that the scenery looks more blue and green, that is, the color is green. Generally, you can use this kind of mirror during the day, and the effect is not good at night.

At present, most people think that green or blue coating will be better, which is conducive to night observation. Some heroes say they will never buy red-coated mirrors.

 

After knowing the role of the coating, we will understand that regardless of the color of the coating, the darker the reflection, the better.