Knowledge about telescopes
30 Jun 2022
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The telescope is not actually "looking far", but "looking dark", that is, the telescope can make us see some dark celestial objects. Telescopes can "look dark" because they enlarge our viewing angle. It can be imagined that telescopes make our eyes larger to the size of the aperture of the telescope. Our eyes are "larger" and have an increased ability to receive light, so we can see dimmer celestial objects.
To understand what is the focal length (Focal Length), let's first introduce the definition of focus (Focus). The focal point refers to the concentrated point formed by the reflection of light by a concave mirror or by the refraction of a convex lens. As for the focal length, it is the distance along the optical axis from the center of the lens or single-sided mirror to the focal point. The effective focal length commonly used in astronomical telescopes refers to the distance between the focal length from the entrance pupil to the focal point.
The focal ratio is the "speed" of a lens or telescope mirror, expressed as the ratio of the focal length to the aperture of the telescope. The larger the focal ratio value, the shorter the time required to photograph the celestial object. In general, telescopes used for astrophotography should have a focal ratio lower than f/10, as this will greatly shorten the exposure time.
Formula: Focal Ratio = Telescope Focal Length (mm) / Telescope Diameter (mm) f=F/D
Magnification
Also known as magnification, it refers to how many times the opening angle of the observed object is enlarged after passing through the optical system of the telescope. If a 1-meter-sized object is 1 km away, when it is directly observed by the naked eye, its opening angle is about 0.001 radian. If a telescope with a magnification of 10 times is used for observation, its opening angle is 0.01 radian, which is equivalent to observing the object with the naked eye from 100 meters away. The magnification of the telescope depends on the focal length of the telescope and the eyepiece. Generally speaking, the focal length of the telescope is fixed, but the magnification can be changed by replacing the eyepieces with different focal lengths.
Formula: Magnification = Telescope focal length (mm) / Eyepiece focal length (mm)
Effective Magnification is the effective value within the highest magnification that the telescope can magnify. In theory, each telescope can be used for high magnification, but it is subject to factors such as telescope aperture, structure and materials used. If the magnification exceeds the highest effective magnification, the image will be blurred.
Formula: Effective magnification = Telescope diameter (inch) X 50
Resolving Power
The detail resolution capability of the telescope can be called the resolution power, which mainly depends on the diameter of the telescope, which can be understood as the light-gathering power. If we use a telescope with a resolution of only 2 arcseconds to observe two stars that are 1 arcsecond apart, we will see only one star. Of course, the formula of the resolution force does not introduce parameters such as atmospheric conditions, so the actual resolution force is not as good as the theoretical value.
Formula: Resolution = 138.87"/Telescope Diameter (mm)
Limiting Magnitude
The limiting magnitude here refers to the dimmest magnitude that the telescope can see under the best conditions. Of course, like the resolving power, many subjective factors such as atmospheric conditions, observers' vision and experience are not introduced into the formula, and the resulting values cannot represent the actual situation.
Formula: limit magnitude=6.9+5log telescope diameter (mm) M=6.9+5logD
Field of View
The field of view refers to the angular diameter of the range that can be observed by a telescope or other optical device, usually expressed as ω. The field of view of the telescope is generally fixed when the telescope is designed and cannot be changed. The field of view has an inverse relationship with the magnification, the larger the magnification, the smaller the field of view.
There are two common ways of representing the field of view:
(1) Degree: field of view=7* Indicates that the entire diameter of the field of view has a 7-degree field of view. Taking the moon as an example, its apparent diameter is half a degree, that is, 14 moons can be accommodated in a line in a 7* field of view.
(2) Expressed in feet: Field of View = 373ft/1000yards. That is, when viewing a scene from 1000 yards away, the visible field of view is 373 feet. Using simple trigonometry, divide 373 feet by 52.5 to find degrees.
Eyepiece
There are many types of eyepieces, which can be divided into positive eyepieces and negative eyepieces according to the focal position. The positive eyepiece has the focus in front of the field lens (the front lens of the eyepiece), and the negative eyepiece has the focus between the two lenses of the eyepiece.
Stand (Tripod)
Mounts are very important for telescopes, because the instability of the mount can make observations difficult. Mounts are roughly divided into theodolites and equatorial mounts. The theodolite has a relatively simple structure and can rotate the lens barrel vertically and horizontally. Easier to use, generally not used for tracking and careful observation. The structure of the equatorial mount is more complicated, with a polar axis telescope, and the polar axis should point to the north celestial pole (the southern hemisphere points to the south celestial pole). And there are at least two scale rings, and the scales of the two scale rings are right ascension and declination respectively. The equatorial mount has an additional motor for tracking celestial bodies.
Due to the development of science and technology, some equatorial mounts are now equipped with automatic star finding systems. As long as the celestial object you want to find is input into the automatic star finder system, the motor of the orbiter will rotate quickly, completing the dream of aiming at the celestial object you want to observe, and you can find the nebulae, star clusters, and galaxies that can be observed in the telescope more easily and quickly. Any celestial body can also be connected to a personal computer and controlled by the computer.
Star finder
The role of the finder mirror has been mentioned in the name, and its main role is to find celestial bodies. Because the primary mirror generally has a large magnification, it is not easy to find the celestial object to be observed through the primary mirror, so the astronomical telescope is generally equipped with a finder mirror. The finderscope generally has a small magnification (2-3 times), a wide field of view, and a crosshair.
Zenith Prism (90 degree Erecting Prism)
Since the eyepiece position of the telescope may be placed at the end of the lens barrel, it will be more troublesome to observe near the zenith, so it is generally observed by using the zenith pole. The zenith pole is composed of a right-angled triangle pole, and the direction of the light path is changed through the pole to make the observation posture more convenient.
Barlow Lens
The function of the Barrow lens is to extend the focal length of the objective lens and increase the magnification. Barrow mirrors generally say 2X, 3X, etc., that is, the focal length of the objective lens is changed to 2 times, 3 times, and so on. However, it is better to double the size, because the aberration will be more obvious when the size is more than 2 times.
refracting telescope
The refracting telescope mainly focuses the light into a real image by the objective lens, and concentrates it in the focal point, and then uses the eyepiece to enlarge the image. Refractor telescopes are divided into "Galileo type" and Kepler type. The "Galileo type" is the oldest type of telescope, but has a narrower field of view and is now only used in some small binoculars. The difference between the "Galileo type" and the Kepler type is that the eyepiece of the former is a biconcave lens and is placed in front of the focal point, so the image formed by the Galileo type is an erect image, while the Kepler type image will be upside down and left and right upside down.
Kepler-type telescopes are the most common type of refracting telescopes. Since the objective lens (convex lens) of the refracting telescope is prone to chromatic aberration due to wavelength, the objective lens group is generally composed of 2 to 3 concave-convex lenses to reduce chromatic aberration, which are divided into Achromatic lens and Apocromatic lens. . Although the chromatic aberration problem of telescopes made of fluorite crystal lenses, ED lenses, and SD lenses has been overcome, they are not popular due to the laborious manufacturing and high price.
The advantages of the refracting telescope are that the optical path is simple, the imaging is sharp, the optical axis is not easily skewed, it is easy to use and maintain, and the interior is sealed, the air will not be disturbed, so the image is relatively stable. However, the large aperture of the objective lens or the poor quality of the lens are prone to aberrations. In addition, refractor telescopes are generally limited by the optical path. The larger the diameter of the objective lens, the bulkier the body, and the effect of short-focus refractors is not good, so good refractor telescopes are generally more expensive. It is more troublesome to use a refractor telescope to observe the vicinity of the zenith, and it must be observed with a zenith pole.
reflecting telescope
The reflecting telescope does not use the lens as the objective lens, but uses the concave mirror as the reflector (also called the primary mirror). There is also a secondary mirror that reflects the light collected by the primary mirror to the eyepiece, which magnifies the image. There are two types of reflecting telescopes, including: Newton and Cassegrain.
In a Newtonian telescope, the primary mirror collects light and converges it to the focal point, but a flat mirror at an angle of 45 degrees to the optical axis is placed in front of the focal point as a secondary mirror, and the secondary mirror reflects the light to the eyepiece for magnification. The advantage of the Newtonian method is that the eyepieces all point in the same direction no matter which direction is observed, so it is more convenient. But sometimes the position of the eyepiece will be higher.
Cassegrain-style telescopes have a hole in the center of the primary mirror, which collects light and reflects it on a convex secondary mirror. The secondary mirror reflects light and passes through the central hole of the primary mirror and converges at the focal point, and the eyepiece is placed at the focal point to magnify. The advantages of the Cassegrain type are that the focal length is large, the forming magnification is large, and the lens barrel is short and easy to carry. The disadvantage is that the field of view is narrow and the production is difficult (mainly to open a hole in the center of the primary mirror), so small-diameter telescopes generally do not use this format, on the contrary, large-diameter telescopes generally use this format.
Reflecting telescopes are generally less expensive and easier to make. However, frequent maintenance is required, and the optical axis is easily deviated, and the image quality is easily affected by the air disturbance in the lens barrel.
catadioptric telescope
A catadioptric telescope is a telescope that combines the refraction of a lens and a reflector, also known as a compound mirror. Since it is manufactured using a refractor and a reflector, it has advantages and disadvantages of both. Catadioptric telescopes are divided into Schmidt. Cassegrain (Schmidt.Cassegrain) and Makasutov. Cassegrain (Maksutov.Cassegrain).
Schmidt. The Cassegrain telescope is composed of a correction lens, a primary mirror and a secondary mirror, and the correction mirror is an aspherical lens. Makasutov. The Cassegrain telescope also has a correction mirror, which is a meniscus lens, but its center is a convex spherical mirror as a secondary mirror. In both cases, the light passes through the correction mirror, the main mirror (central opening) reflects the light to the center of the correction mirror, and then reflects through the central hole of the main mirror and converges at the focal point, and the eyepiece is placed in the focal point and enlarged. Makasutov.
Cassegrain telescope
The catadioptric telescope is characterized by large mouth, long focal length, short lens barrel, easy to move, wide field of view, strict structure, and the optical axis is not easy to be tilted. However, due to the closed design, in order to avoid turbulent flow in the tube caused by temperature changes, it is recommended to move the telescope outdoors half an hour before use.
1. Basic parameters of the telescope
Focal RetioTo understand what is the focal length (Focal Length), let's first introduce the definition of focus (Focus). The focal point refers to the concentrated point formed by the reflection of light by a concave mirror or by the refraction of a convex lens. As for the focal length, it is the distance along the optical axis from the center of the lens or single-sided mirror to the focal point. The effective focal length commonly used in astronomical telescopes refers to the distance between the focal length from the entrance pupil to the focal point.
The focal ratio is the "speed" of a lens or telescope mirror, expressed as the ratio of the focal length to the aperture of the telescope. The larger the focal ratio value, the shorter the time required to photograph the celestial object. In general, telescopes used for astrophotography should have a focal ratio lower than f/10, as this will greatly shorten the exposure time.
Formula: Focal Ratio = Telescope Focal Length (mm) / Telescope Diameter (mm) f=F/D
Magnification
Also known as magnification, it refers to how many times the opening angle of the observed object is enlarged after passing through the optical system of the telescope. If a 1-meter-sized object is 1 km away, when it is directly observed by the naked eye, its opening angle is about 0.001 radian. If a telescope with a magnification of 10 times is used for observation, its opening angle is 0.01 radian, which is equivalent to observing the object with the naked eye from 100 meters away. The magnification of the telescope depends on the focal length of the telescope and the eyepiece. Generally speaking, the focal length of the telescope is fixed, but the magnification can be changed by replacing the eyepieces with different focal lengths.
Formula: Magnification = Telescope focal length (mm) / Eyepiece focal length (mm)
Effective Magnification is the effective value within the highest magnification that the telescope can magnify. In theory, each telescope can be used for high magnification, but it is subject to factors such as telescope aperture, structure and materials used. If the magnification exceeds the highest effective magnification, the image will be blurred.
Formula: Effective magnification = Telescope diameter (inch) X 50
Resolving Power
The detail resolution capability of the telescope can be called the resolution power, which mainly depends on the diameter of the telescope, which can be understood as the light-gathering power. If we use a telescope with a resolution of only 2 arcseconds to observe two stars that are 1 arcsecond apart, we will see only one star. Of course, the formula of the resolution force does not introduce parameters such as atmospheric conditions, so the actual resolution force is not as good as the theoretical value.
Formula: Resolution = 138.87"/Telescope Diameter (mm)
Limiting Magnitude
The limiting magnitude here refers to the dimmest magnitude that the telescope can see under the best conditions. Of course, like the resolving power, many subjective factors such as atmospheric conditions, observers' vision and experience are not introduced into the formula, and the resulting values cannot represent the actual situation.
Formula: limit magnitude=6.9+5log telescope diameter (mm) M=6.9+5logD
Field of View
The field of view refers to the angular diameter of the range that can be observed by a telescope or other optical device, usually expressed as ω. The field of view of the telescope is generally fixed when the telescope is designed and cannot be changed. The field of view has an inverse relationship with the magnification, the larger the magnification, the smaller the field of view.
There are two common ways of representing the field of view:
(1) Degree: field of view=7* Indicates that the entire diameter of the field of view has a 7-degree field of view. Taking the moon as an example, its apparent diameter is half a degree, that is, 14 moons can be accommodated in a line in a 7* field of view.
(2) Expressed in feet: Field of View = 373ft/1000yards. That is, when viewing a scene from 1000 yards away, the visible field of view is 373 feet. Using simple trigonometry, divide 373 feet by 52.5 to find degrees.
2. The basic structure of the monocular
A single-lens telescope is generally composed of a main mirror (also known as a lens barrel), an eyepiece, a stand, a finder mirror, etc., which will be introduced one by one below. (The mirror body part will be introduced in the next section)Eyepiece
There are many types of eyepieces, which can be divided into positive eyepieces and negative eyepieces according to the focal position. The positive eyepiece has the focus in front of the field lens (the front lens of the eyepiece), and the negative eyepiece has the focus between the two lenses of the eyepiece.
Stand (Tripod)
Mounts are very important for telescopes, because the instability of the mount can make observations difficult. Mounts are roughly divided into theodolites and equatorial mounts. The theodolite has a relatively simple structure and can rotate the lens barrel vertically and horizontally. Easier to use, generally not used for tracking and careful observation. The structure of the equatorial mount is more complicated, with a polar axis telescope, and the polar axis should point to the north celestial pole (the southern hemisphere points to the south celestial pole). And there are at least two scale rings, and the scales of the two scale rings are right ascension and declination respectively. The equatorial mount has an additional motor for tracking celestial bodies.
Due to the development of science and technology, some equatorial mounts are now equipped with automatic star finding systems. As long as the celestial object you want to find is input into the automatic star finder system, the motor of the orbiter will rotate quickly, completing the dream of aiming at the celestial object you want to observe, and you can find the nebulae, star clusters, and galaxies that can be observed in the telescope more easily and quickly. Any celestial body can also be connected to a personal computer and controlled by the computer.
Star finder
The role of the finder mirror has been mentioned in the name, and its main role is to find celestial bodies. Because the primary mirror generally has a large magnification, it is not easy to find the celestial object to be observed through the primary mirror, so the astronomical telescope is generally equipped with a finder mirror. The finderscope generally has a small magnification (2-3 times), a wide field of view, and a crosshair.
Zenith Prism (90 degree Erecting Prism)
Since the eyepiece position of the telescope may be placed at the end of the lens barrel, it will be more troublesome to observe near the zenith, so it is generally observed by using the zenith pole. The zenith pole is composed of a right-angled triangle pole, and the direction of the light path is changed through the pole to make the observation posture more convenient.
Barlow Lens
The function of the Barrow lens is to extend the focal length of the objective lens and increase the magnification. Barrow mirrors generally say 2X, 3X, etc., that is, the focal length of the objective lens is changed to 2 times, 3 times, and so on. However, it is better to double the size, because the aberration will be more obvious when the size is more than 2 times.
3. Classification of monoculars
Optical telescopes are generally divided into two categories: monoculars and binoculars. Monoculars are basically divided into three categories according to optical paths and imaging principles: refracting telescopes, reflecting telescopes, and catadioptric telescopes. These three types of telescopes can be subdivided into various types. The following are introduced one by one.refracting telescope
The refracting telescope mainly focuses the light into a real image by the objective lens, and concentrates it in the focal point, and then uses the eyepiece to enlarge the image. Refractor telescopes are divided into "Galileo type" and Kepler type. The "Galileo type" is the oldest type of telescope, but has a narrower field of view and is now only used in some small binoculars. The difference between the "Galileo type" and the Kepler type is that the eyepiece of the former is a biconcave lens and is placed in front of the focal point, so the image formed by the Galileo type is an erect image, while the Kepler type image will be upside down and left and right upside down.
Kepler-type telescopes are the most common type of refracting telescopes. Since the objective lens (convex lens) of the refracting telescope is prone to chromatic aberration due to wavelength, the objective lens group is generally composed of 2 to 3 concave-convex lenses to reduce chromatic aberration, which are divided into Achromatic lens and Apocromatic lens. . Although the chromatic aberration problem of telescopes made of fluorite crystal lenses, ED lenses, and SD lenses has been overcome, they are not popular due to the laborious manufacturing and high price.
The advantages of the refracting telescope are that the optical path is simple, the imaging is sharp, the optical axis is not easily skewed, it is easy to use and maintain, and the interior is sealed, the air will not be disturbed, so the image is relatively stable. However, the large aperture of the objective lens or the poor quality of the lens are prone to aberrations. In addition, refractor telescopes are generally limited by the optical path. The larger the diameter of the objective lens, the bulkier the body, and the effect of short-focus refractors is not good, so good refractor telescopes are generally more expensive. It is more troublesome to use a refractor telescope to observe the vicinity of the zenith, and it must be observed with a zenith pole.
reflecting telescope
The reflecting telescope does not use the lens as the objective lens, but uses the concave mirror as the reflector (also called the primary mirror). There is also a secondary mirror that reflects the light collected by the primary mirror to the eyepiece, which magnifies the image. There are two types of reflecting telescopes, including: Newton and Cassegrain.
In a Newtonian telescope, the primary mirror collects light and converges it to the focal point, but a flat mirror at an angle of 45 degrees to the optical axis is placed in front of the focal point as a secondary mirror, and the secondary mirror reflects the light to the eyepiece for magnification. The advantage of the Newtonian method is that the eyepieces all point in the same direction no matter which direction is observed, so it is more convenient. But sometimes the position of the eyepiece will be higher.
Cassegrain-style telescopes have a hole in the center of the primary mirror, which collects light and reflects it on a convex secondary mirror. The secondary mirror reflects light and passes through the central hole of the primary mirror and converges at the focal point, and the eyepiece is placed at the focal point to magnify. The advantages of the Cassegrain type are that the focal length is large, the forming magnification is large, and the lens barrel is short and easy to carry. The disadvantage is that the field of view is narrow and the production is difficult (mainly to open a hole in the center of the primary mirror), so small-diameter telescopes generally do not use this format, on the contrary, large-diameter telescopes generally use this format.
Reflecting telescopes are generally less expensive and easier to make. However, frequent maintenance is required, and the optical axis is easily deviated, and the image quality is easily affected by the air disturbance in the lens barrel.
catadioptric telescope
A catadioptric telescope is a telescope that combines the refraction of a lens and a reflector, also known as a compound mirror. Since it is manufactured using a refractor and a reflector, it has advantages and disadvantages of both. Catadioptric telescopes are divided into Schmidt. Cassegrain (Schmidt.Cassegrain) and Makasutov. Cassegrain (Maksutov.Cassegrain).
Schmidt. The Cassegrain telescope is composed of a correction lens, a primary mirror and a secondary mirror, and the correction mirror is an aspherical lens. Makasutov. The Cassegrain telescope also has a correction mirror, which is a meniscus lens, but its center is a convex spherical mirror as a secondary mirror. In both cases, the light passes through the correction mirror, the main mirror (central opening) reflects the light to the center of the correction mirror, and then reflects through the central hole of the main mirror and converges at the focal point, and the eyepiece is placed in the focal point and enlarged. Makasutov.
Cassegrain telescope
The catadioptric telescope is characterized by large mouth, long focal length, short lens barrel, easy to move, wide field of view, strict structure, and the optical axis is not easy to be tilted. However, due to the closed design, in order to avoid turbulent flow in the tube caused by temperature changes, it is recommended to move the telescope outdoors half an hour before use.