Before buying binoculars, you should first understand its characteristics and specifications. The most important thing to pay attention to when buying binoculars for astronomy is the "caliber."
Aperture refers to the diameter of the front lenses. The larger the aperture, the brighter the image will be. For astronomical binoculars, the lens diameter should be at least 40mm.
The small 20mm to 30mm binoculars are suitable for viewing scenery during the day, but they are not suitable for astronomical purposes because they cannot gather enough light.
How do I know the diameter of the binoculars with camera?
Very simple.
Each pair of binoculars with camera is marked with a set of numbers such as 7x50. The first number "7" on the specifications of binoculars is the "magnification", and the second number "50" is the lens diameter.
The seven-fold model is a best-selling model that will make every item viewed seven times closer.
You can also choose 10x and 16x. Maybe you think high magnification is necessary for astronomical purposes, but it is not. A 7x binoculars is good enough, and then we will talk about the advantages of 7x over most high-magnification models.
Field of View of binoculars with camera
You will see a set of data like "367 feet @ 1000 yards" or "120 m @ 1000 m" and so on on almost every pair of binoculars with camera manuals.
This series of numbers represents how wide you can see through the eyepiece at a distance of 1,000 yards (or 1,000 meters). This is one of the ways to measure the size of the field of view.
It is not appropriate to use the method of "a few feet within 1,000 yards" to measure the view of the sky. Instead, astronomers use degrees to measure the field of view.
It was equivalent to twice the diameter of a full moon
Seven degrees is equal to the diameter of fourteen full moons, and it is the typical field of view of binoculars. The high magnification model sees a smaller sky (3 to 5 degrees), and the wide-angle model sees more (8 to 10 degrees).
Just divide the "feet @ 1000 yards" specification by the number of feet (feet) 52. 5 can be converted into degrees. For the "meters @ 1000 meters" specification, divide the number of meters by 17. For example, a pair of binoculars with a field of view of 367 feet @ 1000 yards will have a field of view of 367/52. 5 degrees, which is about 7 degrees.
The imaging of the star point in the peripheral field of the wide-angle model is usually a bit distorted and blurred, which reduces the field of view, which is difficult to balance. In addition, wide-angle models generally shorten the viewing distance.
There is a correlation between actual field of view and effective field of view (actual and apparent fields of view). Take the effective field of view (for example, 70 degrees and divide by the magnification (for example, 10x)) and then you will get the value of the "actual field of view", which is 7 degrees in this example. So the effective field of view increases, and the actual field of view will increase accordingly. But increasing the magnification will decrease the actual field of view.
Exit pupil
You will also find a specification called "exit pupil" in many binoculars with camera manuals. It is the width of light coming out of the eyepiece.
You can calculate the exit pupil by dividing the aperture (in mm) by the magnification. For example, the 7x50 type has a 50/7 (7mm) exit pupil. All 7x50 models have an exit pupil of 7mm.
All 7x42 models have an exit pupil of 6mm, and so on. You will find that many binoculars that are recommended for astronomy have 7mm exit pupils, which makes sense.
When your eyes are used to the darkness of the night, the pupils will expand to let more light in. Human pupils can expand to 7mm at most. So as long as the binoculars shoot out a 7mm cone of light into your eyes, you are likely to see the brightest image when the light collection power of your eyes can be used to the fullest.
All binoculars with camera designed to cope with low-light environments have 7mm exit pupils. Including: 7x50, 8x56, 9x63, 10x70 and 11x80 models. Usually most suitable for astronomical observation. However, when you are old, the pupils of your eyes cannot be opened so open.
The largest pupil in the early thirties is about 6mm
Over forty years old, it falls between about 4.5mm and 5mm. If your pupil size is limited to 5mm or 6mm, using binoculars with a 7mm wide output cone will waste some of the incident light.
Part of the light can't enter your eyes, and the binoculars' light-collecting power cannot be fully utilized. For older people, the 5mm or 6mm exit pupil of the binoculars is a better choice. These models include the popular 7x42 and 10x50.
Eye Relief
Manufacturers are now paying more and more attention to the "good viewing distance" specification.
This specification refers to how close your eyes need to be to the eyepiece to see the entire field of view. The data of good visual distance is more important for myopic glasses.
Although people with myopia can take off their glasses and adjust the focal length of the binoculars with camera to compensate, it is quite inconvenient---wear the glasses to see the night sky with a clear line of sight, and quickly adjust the distance without shifting the target.
It is indeed a pleasure to bring the binoculars with camera in front of you and maintain a complete view. In addition, if you have several levels of astigmatism in one or both eyes, your glasses cannot be taken off. To see the complete eye with glasses, a good visual distance of at least 14mm to 15mm is required.
The good visual distance is measured from the lowered eye cup. Models with a good viewing distance of less than 8mm or 9mm may be difficult to observe, even without glasses.
You have to keep your eyes close to the eyepiece to see all the field of vision. Looking at it for a long time will cause a sense of pressure, and the grease and dust on the eyelashes are guaranteed to be contaminated with the eyepiece.
Prisms
The body of the binoculars with camera contains a set of synthetic prisms that can refract the light path and make the image stand upright.
Binoculars with camera can be divided into two camps --- Porro prism system and Roof prism system. Porro prisms have a Z or N shape appearance. The Roof prism system is straight.
Porro prism models are generally cheaper than Roof prism models. Roof prisms must be done well, processing and fixing must require more precise tolerances, and the production cost is high. However, Roof prism models (structurally) tend to be denser.
Porro prism models are mostly used for astronomical purposes
The Porro prism system often produces a slightly sharper image. But if you use cheap models (translation: refers to the Roof prism system) to aim at bright stars or planets, the stars will emit annoying spikes of light.
These nail-shaped diffractions are due to the Roof prism splitting the image in half and then combining them. But you will never find this effect on advanced Roof prism models.
If the prisms of Porro prism binoculars with camera are made of barium crown glass (BaK4), the field of view will be more complete. If the prism is made of borosilicate glass, the grade is lower, but the brightness difference of the image is very small in practice.
