Types of Astronomical Telescopes
Optical telescope
s are roughly divided into three categories: refracting, reflective, and catadioptric.
1. Refracting telescope
Advantages: The imaging is very sharp, vivid and high in contrast. Connecting the ‘right image mirror’ can make the imaging not reversed, which is beneficial to the observation of the ground scene. It is easy to maintain and has a long service life, but care must be taken not to let the lens mold.
Disadvantage: The price is high, the same price can buy a larger diameter reflecting telescope. Ordinary achromatic telescopes still have some residual chromatic aberration. And the ‘apochromatic telescope’ whose residual chromatic aberration is negligible is extremely expensive.
2. Reflecting telescope
Advantages: low price (compared with refractor), the same price has a larger aperture and can see more and darker celestial bodies. Imaging without chromatic aberration. Cost-effective. Most of the large-diameter telescopes in professional observatories use reflective structures.
Disadvantages: Routine maintenance is relatively troublesome.
Astronomical telescope parameters
1. Aperture
Aperture refers to the effective diameter of the objective lens, which is often represented by the symbol ‘D’. The ability of an objective to collect starlight is proportional to the square of its aperture. Therefore, the larger the aperture of the objective lens, the easier it is to observe fainter celestial objects.
2. Resolution angle and resolving power
Resolution angle is usually measured in arcseconds, which refers to the two luminous points in the sky that can just be resolved by a telescope angular distance between. During visual observation, the resolution angle of the telescope = 140 (arcseconds) / D (mm), where D is the effective aperture of the objective lens. The resolving power of a telescope is measured by the reciprocal of the resolving angle of the telescope, so: the larger the aperture of the telescope, the better the resolving power.
3. Magnification
The magnification is the physical quantity of the visual telescope, that is, the magnification of the angle. The magnification of the visual telescope is equal to the ratio of the focal length of the objective lens to the focal length of the eyepiece, so just changing different eyepieces can change the magnification of the telescope. However, due to the limitation of the resolving power of the objective lens, the magnification of the telescope cannot be increased indefinitely. When observing, the maximum magnification is by no means the best, but the clearest observation target shall prevail. If the magnification is far beyond its upper limit of magnification, the star image will appear larger, but it will not increase its sharpness, on the contrary, the star image will be blurred and dimmed, and the effective field of view will also become very cramped.
4. Field of view
The angular diameter of the sky corresponding to the area that can be well imaged by the telescope is called the field of view of the telescope. The larger the field of view, the wider the field of vision to see. The field of view of a telescope is inversely proportional to the magnification, the greater the magnification, the smaller the field of view. Different apertures, different focal lengths, different optical systems (including objective lenses and eyepieces) and quality determine the size of the field of view of the telescope. When looking at the starry sky, if you use a telescope with a large field of view and a wide-angle eyepiece, you will feel like walking in the starry sky.
5, limit magnitude
‘Magnitude’ is an indicator of the brightness of a celestial body, often represented by the symbol ‘m’. The smaller the magnitude, the brighter the star; the higher the magnitude, the dimmer the star. The magnitudes differ by one magnitude, and the brightness differs by 2.512 times. The limit magnitude is the magnitude of the faintest object that can be observed with a telescope. The visual limit magnitude of the telescope can be roughly estimated by the following formula: m=2.1+5logD??????? In the formula, D (diameter) is in millimeters.
Note: Light pollution in big cities can seriously affect the magnitude of the limit magnitude, so the best observation locations are in the suburbs away from urban light pollution.
Telescope eyepiece
1. Huygens eyepiece (H)
It was developed by Huygens in 1703. It consists of 2 convex lenses with a small field of view, about 25-40 degrees, and its exit pupil distance (the distance from the eyepiece to the eye) is relatively short, so when observing with a short focal length Huygens eyepiece, the eyes need to be tight Attach the eyepiece. Because Huygens eyepieces are easy to manufacture and inexpensive, they are often used on small entry-level telescopes.
2. Ramsden eyepiece (R or SR)
It was successfully designed by Ramsden in 1783, and it is also a two-piece convex lens structure. Conversely, it consists of two plano-convex lenses with the same focal length. The field of view is about 30-45 degrees. The spherical aberration, axial chromatic aberration and distortion of Ramsden eyepieces are smaller than those of Huygens eyepieces, and the field curvature is also significantly reduced. But the exit pupil distance is also shorter. Both Ramsden and Huygens belonged to the first generation of eyepieces.
3. Kellner eyepiece (K)
Appeared in 1849, it consists of three lenses. It is developed on the basis of Ramsden eyepieces. The main improvement is to change the single-piece eyepiece to a double cemented achromatic lens, which can eliminate the chromatic aberration of magnification, and can also effectively reduce the positional chromatic aberration, astigmatism and distortion. Improve the image quality of the edge, the effective field of view reaches 40-50 degrees. At medium and low magnification, the image quality of the Kellner eyepiece is better than that of the Huygens and Ramsden eyepieces. In addition, the field lens of the Kellner eyepiece is close to the focal plane, so that the dust on the field lens is easy to image and affects the observation, so special attention should be paid to cleaning.
4. Abbe’s undistorted eyepiece (OR)
Designed by German Abbe in 1880, it is a four-piece, two-group structure, in which the field lens is The triplet lens, the eyepiece is a plano-convex lens, the eyepiece successfully controls chromatic aberration and spherical aberration, and reduces ghost images and field curvature to an imperceptible level. It also has a flat field of view of 40-50 degrees and sufficient output. Pupil distance, which has good performance at various magnifications, has been widely used. But the price is relatively high.
5. Prussell eyepieces (PL or Plossl)
Also known as ‘symmetric eyepieces’. It is a four-piece, two-group structure, consisting of the same two sets of double cemented achromatic lenses. Its parameter performance is comparable to that of the OR eyepiece, but the PL eyepiece has a larger exit pupil distance and field of view. Since the two achromatic cemented lenses in the PL eyepiece can be exactly the same, the cost is lower than the OR, and it is the most widely used eyepiece at present. K, OR, and PL belong to the second generation of eyepieces.
6. About the eyepiece interface size
There are roughly three types of interface specifications for telescope eyepieces: 24.5 mm (0.965 inches), 31.7 mm (1.25 inches) , 50.8 mm (2 in). 24.5mm is mainly used for small entry-level telescopes, and 31.7mm is the current mainstream size. 50.8 mm is mainly used in high-end large-aperture telescopes.
Selection guide for astronomical telescopes 1. Comparison of types of astronomical telescopes
1) Refractor astronomical telescopes are more convenient to use, with a larger field of view and bright star images, but there is chromatic aberration, which reduces the resolution. Objectives for high-quality refractors are either two double-separated achromatic objectives or three apochromatic objectives. However, achromatic or apochromatic does not completely eliminate chromatic aberration.
2) The advantage of the mirror astronomical telescope is that it has no chromatic aberration, but the coma and astigmatism of the mirror is large, which makes the image quality at the edge of the field of view worse. Commonly used mirrors are Newtonian and Cassegrain. The former optical system is simple and cheap, the spherical mirror is at the back end, and the eyepiece is at the front side; the latter optical system has aspherical primary and secondary mirrors, and both the primary mirror and the eyepiece are at the back, with better image quality and higher price. .
3) The catadioptric astronomical telescope mirror takes into account the advantages of the refractor astronomical telescope and the reflector astronomical telescope: large field of view, good image quality, short lens barrel, and easy to carry. Compared with the same focal length and the same aperture refracting telescope, the price is less than one-third. There are two types of catadioptric mirrors: Schmidt-Cassegrain type and Maksutov-Cassegrain type, the latter also known as horse-card mirror. There are two-piece and three-piece mirrors. For example: Boguan BOSMA1800150 astronomical telescope and BOSMA2400200 astronomical telescope are three-piece type, because the image quality is better than the two-piece type, which is very popular among domestic and foreign astronomers.
2. Reasonably choose the focal length of the astronomical telescope
Choose the focal length of the astronomical telescope, which is related to the celestial body you want to observe. If you want to observe nebulae and look for comets, choose a short focal length astronomical telescope; if you want to observe the moon and planets, choose a long focal length astronomical telescope; if you want to observe double stars, multiple stars, variable stars and star clusters, it is best to choose a medium focal length astronomical telescope telescope. Medium focal length lenses can take care of both ends and are more popular. Usually a short mirror means that the ratio of focal length to aperture is less than or equal to 6, and a long mirror means that the ratio of focal length to aperture is greater than 15, and the middle focal length mirror is called in between.
3. Is it good for the astronomical telescope to have a high magnification?
According to the long-term observation experience of astronomers, the maximum magnification of the astronomical telescope shall not be greater than the diameter of the 1.5x objective lens (expressed in millimeters). The telescope, observing in moderately quiet atmospheric conditions, must not be larger than 125 times. The optimum quietness can reach 190 times; when the diameter is 200 mm, it shall not be greater than 170 times when the atmospheric quietness is moderate. At the best quietness, it can reach 340 times; in fact, for astronomy enthusiasts to observe bright celestial objects, the maximum magnification can reach twice, or even 2.5 times the diameter of the objective lens (expressed in millimeters). However, too much magnification of the astronomical telescope makes the image larger and darker, and the jitter of the atmosphere is also amplified, making the image more blurred.
4. Choose an astronomical telescope with a large aperture as much as possible
1) The larger the aperture, the more light energy it receives, and the darker celestial objects can be observed.
2) The larger the aperture, the larger the maximum effective magnification V, because V = the aperture D of the primary mirror (expressed in millimeters).
3) With large aperture and high resolution, it can observe more details of planets, distinguish binary stars, and possibly find fainter asteroids and comets. In theory, the resolution is only related to the caliber, but in fact it is related to the optical design, processing, installation and calibration. The resolution of general popular science telescopes can reach 2 times the theoretical resolution angle, even if it is a high-quality telescope, but the Boguan BOSMA1800150, tested by imported measuring instruments, has a resolution better than 1??, which is close to the theoretical value.
5. How to distinguish Optical quality of the astronomical telescope
When you buy it during the day, you can use the astronomical telescope to observe a building in the distance, and move the outline of the building to 1/4 of the field of view. If the line is very bent, don’t buy it; take a look at the leaves in the distance. Generally speaking, a telescope with a diameter of 60 mm can clearly see the tendons of the leaves at a distance of 40 meters. Don’t buy it if you can’t see it clearly. The larger the aperture, the farther you can see. The Boguan BOSMA70060 astronomical telescope (60mm aperture) can clearly see the leaf ribs 85 meters away. You can see the stars at night, if the stars you see are colored and particularly obvious, or The stars at the edge of the field of view drag their tails, and their length is twice the size of the stars. This kind of astronomical telescope is not suitable for astronomical observation.
6. The best way to detect the resolution of the astronomical telescope
Observation of double stars. For example: Pi Aquila is a double star (near Altair) with an angular distance of 1′.4; ?? Aries is a double star with an angular distance of 1′.5; Delta Cygnus is a double star with an angular distance of 2′. 1; Auriga is a double star with an angular separation of 3′.0; ?? star in Leo is a double star with an angular separation of 4′.3.
7. Observation of planets
The magnification required to observe gold, wood, water, fire, and Saturn is that the small round surface of the planet in the field of view of the astronomical telescope has the same view as the full moon seen by the naked eye. Field size (31 arc minutes). Therefore, Jupiter can be observed with an objective lens with a diameter of 50mm, Venus and Mars can be observed with an 80mm objective lens, and a 280mm objective lens can be used to observe Mercury.
8. Observation of sunspots
Large sunspots can be seen with a small telescope, while some very small sunspots can be seen clearly with a large telescope. For amateur observation of sunspots, projection observation is generally used. Observing the sun and the moon should use a telescope with a smaller aperture ratio (D/F), preferably 1:15 to 1:20. You can also add a diaphragm in front of the mirror to reduce the aperture ratio. Doing so, however, reduces the resolving power of the telescope.
9. Observations about the moon
The moon has craters, chain mountains, moon seas, moon valleys, grooves (dried rivers) and bright radiation stripes. A good astronomical telescope can see very fine details on the moon. details. The best magnification for observing the moon is (1-1.5) ?? primary mirror diameter (mm).
10. About the bracket of the astronomical telescope
The bracket of the astronomical telescope has two types: the horizontal type and the equatorial type, both of which have two mutually perpendicular rotating shafts. The field of view of astronomical telescopes is generally relatively small, and the larger the magnification, the smaller the field of view, so choose a bracket that will not shake due to wind.