The catadioptric telescope is an optical system that combines the refraction system with the reflection system. It can be used to correct off-axis aberrations. It first appeared in 1814.
The catadioptric telescope first appeared in 1814.In 1931,GermanyThe optician Schmidt used a unique aspherical thin lens close to the parallel plate as the correction mirror, and cooperated with the spherical mirror to make a Schmidt type catadioptric telescope that can eliminate spherical aberration and off-axis aberrationtelescopeWith strong light power, large field of view and small aberration, it is suitable for taking pictures of large areas of the sky, especially for weak nebulae.Schmidt telescopeHas become an important part of astronomical observationtool。
In 1940, Marksutov used a crescent shaped lens as the correction lens to manufacture another type of catadioptric telescope. Its two surfaces are two spheres with different curvature, which are not different, but the curvature and thickness are both large.All its surfaces are spherical, which is easier to grind than the correction plate of Schmidt telescope, and the lens tube is shorter, but the field of view is smaller than Schmidt telescopeGlassThe requirements are also higher.
This kind of telescope is characterized by strong light power and large visible sky area.So we can see very dark celestial bodies, which is especially suitable formeteor，comet, nebula observation and large-scale sky survey photography.The catadioptric telescopes often used are:Schmidt telescope，Maksutov telescope。
Because the catadioptric telescope can take into account the advantages of both refraction and reflection telescopes, it is very suitable for amateur astronomical observation and astronomical photography, and has been loved by the majority of astronomers.
Structural principle
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1、 Catadioptric telescope and shading system.
The external lens barrel can provide proper shielding for the telescope group, and isolate the light to directly illuminate the primary reflector and secondary reflector, so that the source of stray light is mainly limited to direct and scattered stray light from small angle light sources, becauseopticsThe system still has the problem of direct light source beyond the design perspective value.As mentioned earlier, the light outside these viewing angles is not affected by the primary and secondary mirrors, but directly passes through the central hole of the primary mirror and directly reaches the focal plane.At this time, the most effective way to eliminate the direct light effect of the light source is to add a light barrier to directly block the direct light. The light barrier rudder can give full play to its function of suppressing stray light. Especially for the scattered stray light generated by the light source at a large off-axis angle, the stray light energy of one to three orders can be reduced after adding a light barrier rudder.
2、 Beam tracing diagram of characteristic beam.
The size and position of the light barrier should not only consider its effectiveness in suppressing stray light, but also consider the impact of its system imaging quality. Too much shielding will reduce the system's light input and reduce the imaging contrast. If the system's viewing angle is blocked, there will be uneven energy distribution on the imaging plane.Generally speaking, the main light shield extending from the central hole of the primary reflector is called the primary light shield, and the secondary light shield surrounding the secondary reflector is called the secondary light shield.It is worth noting that the main light baffle shall not be obstructed by the reflection from the main reflector to the secondary reflectorlight, or the imaging light reflected by the secondary mirror;The secondary light barrier is the light reflected from the primary mirror to the secondary mirror that cannot be blocked, and the central shielding rate must be large enough to prevent the edge of the secondary light barrier from imaging on the focal plane.
The position, shape, size and opening size of the light barrier are related to the optical design andSensorIn practice, we can first use the initial order of the incident beam on the axis to calculate the reserved length required by the light baffle and take it as the initial design value.Then, according to the angle of view of the optical system and the specifications of the sensor, cooperate with the beam tracking of the characteristic light, such as the light incident at the most edge of the main reflector and the light incident just passing through the edge of the secondary light barrier to inspect the contour of the secondary light barrier and the primary light barrier, and estimate the assembly and manufacturing tolerances to determine the correct size.
Basic structure of catadioptric telescope
3、 Compare the PST of the system before and after adding the light barrier.
After the light barrier is determined, PST (point source transmission) can be used to evaluate the effectiveness of the light barrier.PST is the most commonly used method to evaluate the stray light characteristics of optical mechanical systems.
In short, it is a discussionlight sourceThe energy when entering the system and the proportion of the energy that finally falls on the focal plane can aggregate the stray light information of the entire optical and mechanical system into a single data to facilitate the comparison between systems.
The PST curve can clearly compare the difference in the proportion of stray light before and after the light barrier of the device with the function of the internal light barrier to block the direct light source.The red curve shows the proportion of stray light obtained when the primary and secondary light barriers are not increased. It is obvious that the proportion of stray light can be effectively suppressed within the angle range where there would be direct light source. Taking the peak value at 10 degrees of incidence as an example, PST is suppressed from the original 1.8% to 5.19 × 10 − 3%.
So primary and secondaryLight barrierThe direct light source can be prevented from entering the rear end assembly of the telescope group as expected.In addition, it is worth noting that within the range of 4 ° – 5 °, the PST value is not as expected, instead of falling, it rises.After clearing all stray light paths, it is found that the main gearLight plateInstead, it is the key mechanism to induce stray light, which means that more attention must be paid to the scattering characteristics (BSDF) of the surface when manufacturing the baffle and blackening the structural surface, so as to prevent the light from being reflected to the focal plane through the main baffle.
The outer lens tube and the primary and secondary lens baffle are the most typical shading system of the telescope system. The comparison before and after adding the shading system is provided, hoping to provide a reference for optical or optical machine designers to understand the importance of the shading system.
major function
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1、 It must be able to eliminate the direct light effect of the light source and prevent the main mirror, secondary mirror and the rear correction mirror group in the telescope group from being directly exposed by the light source outside the angle of view, such as sunlight.
2、 Blocking the view or not followingDesignLight path refers to the light passing through the central hole of the main reflector.
3、 Reduce scattered stray light in the system, or double pass ray focal plane imaging between the primary mirror and the secondary mirror.
4、 Standardize the viewing range of the telescope set.According to the above conditions, the shading system can be classified into sun shield, primary baffle located at the central hole of the primary reflector, secondary baffle around the periphery of the secondary reflector, and other shading systemselementAnd the secondary structure on the mechanism to prevent the structure surface from scattering stray light, which is called the light blocking rudder plate.
Advantages and disadvantages
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Advantages of catadioptric telescope
Catadioptric telescope
1. A short lens barrel can be used to obtain a long focal length, so it is easy to obtain a high system magnification with the eyepiece of a general focal section;
2. Relative toRefracting telescope, the same cost gets a larger caliber;Compared with the reflecting telescope, the same aperture and overall size can obtain longer focal length;
3. The mirror cylinder seal reduces the corrosion of the reflection coating by the air, thus prolonging the service life of the system;
4. Small size, easy to carry, easy to realize field observation;
5. Simple maintenance, almost no maintenance required;
6. Same ascaliberCompared with the refracting telescope,PriceLower.
Disadvantages of catadioptric telescopes
1. Compared with the reflection telescope of the same aperture, the price is higher.
2. Because the second lens is used to reflect the light, part of the light will be lost.
3. The shape may not be the same as the telescope you imagined.
Research and development
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When observing the sky with a large aperture reflecting telescope, large aberrations are inevitable, and only a small part of the center can achieve good resultsobservationeffect.That is to say, the range of the observed high resolution sky becomes smaller.In order to solve this problem, German Russian photologist Schmidt (1879-1935) invented a new type of telescope to overcome this shortcoming.People call itSchmidt telescope。
In his early years, Schmidt put gunpowder into aSteel pipeLi ignited it and accidentally blew off his right hand and forearm.Therefore, when he grinds the mirror surface, he can only work tenaciously with one arm.In 1930, he made a catadioptric telescope, which was made between the reflection telescope andRefracting telescopeThe compromise between the two is to take the advantages of the two and overcome the disadvantages of the two.Schmidt selected concave spherical surface as the primary mirror. In order to overcome the spherical aberration caused by the imaging of the primary mirror, it added a "correction lens" with a strange shape, thick in the middle and thin on both sides.The function of the correction lens is that when the light passes through, the light refraction can just compensate for various aberrations generated by the reflector, so that the telescope can obtain a larger and clearer field of view.Use this telescope to conduct celestial objectsPhotography, can obtain a large area of photos of the sky, known as the "Sky Patrol Police".At present, the largest Schmidt telescope in the world is located in the Schwarzschild Observatory in Thuringtaudenburg, Germany. Its main mirror is 200 cm, the diameter of the correction mirror is 134 cm, and the field of view is 3.4 ° × 3.4 °.
becauseSchmidt telescopeThe shape of the middle correction mirror is strange, and the grinding process is very difficult.To simplify the correctorshapeAt the beginning of 1940, the former Soviet optician Maksutov invented a new catadioptric telescope calledMaksutov telescope。The principle of this telescope is the same as that of Schmidt telescope, except that the secondary mirror is replaced by a special crescent shaped lens, also known asMeniscus correction lens。The correction mirror is ground spherical. As long as the curvature and thickness of both sides of the lens are properly selected, the meniscus mirror can produce enoughaberration。At present, the world's largest Maksutov telescope is installed at the Russian Abbas Tumani Observatory. Its main mirror aperture is 98 cm, focal length is 210 cm, and the mouth diameter of the correction mirror is 70 cm.
Basic structure of catadioptric telescope
The aperture of the telescope becomes larger and larger, but with the increase of the aperture, it is more and more difficult to make it. In recent years, with the application of computers in telescopes, a new idea of combining multiple mirrors into a reflecting telescope came into being in 1979.At present, the first combined telescope is composed of six Cassegrain type telescopes with a diameter of 1.8 meters. They are combined into a reflecting telescope with a light force equivalent to that of a single main mirror with a diameter of 4.5 meters.This new generation telescope is installed at the Madison Hopkins Observatory in Williamstown, Massachusetts, USA.European Southern ObservatoryRecently, scientists of, have successfully installed a new generation telescope, the New Technology Astronomical Telescope (NIT), at the Scylla Observatory in the Andes of Chile. Because of its simplicity, portability, low cost and high efficiency, it is favored by people.The key to the success of NTT is the application of active optical components. In the observation, the system composed of sensing elements and computer controlled motors can automatically analyze and adjust the shape of the primary mirror and the position of the secondary mirror.It can also continuously self monitor to put the whole mirror in the best state.NIT has also adopted a new system, which can translate the telescope aberration into measurable displacement in the image from the reference source point,computerThe image analyzer can then track the wrong optical path, return to the primary mirror, calculate the required correction amount, and add it to the correct optical path.
NIT uses an R-C primary mirror with a focal ratio of f/2.2.The light cast on the primary mirror is first reflected on an 88cm R-C secondary mirror, and then reflected on a movable 45 ° plane mirror, which deflects the light to one of the two second focusing points at both ends of the latitudinal axis.Different from other telescopes of the same size, NIT has no Cassegrain focus, and its main mirror is 3.56m, which canfixed starEighty percent of the starlight converges to a point only 0.096 arc seconds in size.NIT's 120 ton leveling device and 250 ton building rotate together around a rotor bearing with a diameter of 7 meters.In order to ensure that the heat generated by the telescope and its shell does not reduce the visual acuity, the observation room is cooled,air conditioning equipmentThe telescope and observation room are constantly adjusted to observe the ambient temperature at night.
To facilitate observation,European Southern ObservatoryIn 1987, a satellite communication line was set up between Syrah and Gachen near Munich, the headquarters, to realize remote observation.It only takes 30 seconds to transmit an unprocessed TV image, so that astronomers can quickly conduct analysis and research.Since its trial use in 1997, NIT has broken all astronomical observation records and has obtained hundreds of images with a resolution of 0.7 arc seconds to 0.3 arc seconds.It is conceivable that its future will be considerable.
Classification and use
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The catadioptric mirror can be divided into (1) pure Schmidt (2) Schmidt Gesellyn type (3) Maxtov type:
Catadioptric telescope
1. Pure Schmidt mirror -- astronomyPhotographyspecial-purpose
2. Schmidt Gesellyn style
3. Maxtov style has the specialty of reflector, and willaberrationThe number of problems has decreased.
So forplanetIf you are interested in observing the lunar surface, please choose a refractor and a catadioptre. If you are interested in nebulae and star clusters, please choose a reflector.If you can afford it, please buy a telescope with large aperture as much as possible, because the larger the telescope aperture, the stronger the light collection.However, we should also pay attention to the brand, because the brand is often proportional to the optical quality.Such as Nikon, ZEISS, Takahashi VIXEN（refractionMirror).
