Coronameter (andsundialIrrelevant) is a specialtelescope。Astronomers don't have to waitsolar eclipseOccur, and use the coronagraph toCoronaTake pictures.Coronagraph telescopelensThe camera on the coronagraph can take photos of the corona after the metal disc installed on the coronagraph cuts off the light from the photosphere.The coronagraph is only effective when it is used in space (such as Ledao Observatory) or in places with thin air (such as high mountains).[1]
Chinese name
Coronagraph
Foreign name
Coronograph
Invention time
one thousand nine hundred and thirty
Place of Invention
France
inventor
B. Lyot
Properties
Astronomical instrument
Purpose
Observing the solar corona during non solar eclipse period
Astronomers are comparingTotal solar eclipseWhen taking photos, it is found that the shape of the corona changes every time there is a total solar eclipse.When there are few sunspots on the sun, the corona extends very long near the solar equator. At the poles, the corona looks like thick brush tips, so the entire corona becomes like a butterfly.When on the sunsunspotMany times, the corona becomes very large, like a wide and dazzling aperture, surrounding the sun from all around.In addition, astronomers have also determined that bright red protrusions appear from behind the moon——ProminenceThe light of the corona shines farthest.To study how the corona changes with the phenomenon on the sun, it is necessary to observe the corona for a long time. But people only encounter a dozen total eclipses in their life, and each total eclipse lasts only two or three minutes, which is too short for scientists to observe.
In 1930, French astronomer Leo invented the coronagraph (B. Lyot), which enables people to observe the light generated by the corona when the sun is shining.[2]
working principle
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The design principle of the coronagraph is to create artificial solar eclipses and eliminate the instruments as much as possibleScattered light。In order to eliminate scattered light, the thickness of the lens and the number of interfaces between glass and air should be minimized, so the objective lens O1 uses a single pieceThin lensAnd a narrowband filter F is placed behind the camera lens C to eliminate thechromatic aberration。Aperture S2 is slightly smaller than aperture S1, which can block the edge of S1diffractionAndStray light。The small stop S3 on the optical axis can block the holiday image formed in the optical path.The mirror tube is coated with black paint without gloss.On the ground, the scattering of the earth's atmosphere shines in the corona, so the coronagraph is usually placed on a mountain with thin air.[3]
Key points of instrument design
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The coronagraph enables people to observe the corona at times other than total solar eclipse.It uses man-made solar eclipses to make observations.The principle is very simple. Place a shield disk at the main focus of the telescope, which allows the coronal image to pass through while shielding the photosphere image.However, it is actually much more complicated, because there are scattered light and (or) diffracted light in the instrument, and the earth's atmosphere is still several orders of magnitude brighter than the corona, so in order to minimize these irrelevant lights, special precautions should be taken in the instrument design and operation.[4]
Structure of objective lens
The most critical precaution is the structure of the objective lens.In order to reduce the number of surfaces contained in the instrument, a single lens objective shall be used and the glass blank shall be free from bubbles, textures and other defects as far as possible.Carefully polish the lens surface to eliminate all scratches and other surface markings.If it is not operated during use, it must be sealed to prevent dust;A long oiled cylinder can also be attached in front of the objective lens as a dust cover.
Shelter disc
The shield plate is a polished metal cone or an inclined reflector, and the radiation from the photosphere can be safely reflected to a separate light and heat window by it.The objective lens is imaged with a Brie lens after the occlusion disk to eliminate the diffraction at the edge of the objective lens, and then an aperture slightly smaller than the image of the objective lens is used to eliminate the edge effect.In addition, the objective lens can be divided into many parts, so that its transparency is reduced in a Gaussian way from the center to the edge.Although some resolution will be lost, the diffraction halo is completely suppressed.The second blocking disk in front of the final imaging objective is used to eliminate the multiple reflection effect behind the first objective.The final coronal image is generated by the second objective mounted behind the diffraction aperture.
Optical configuration diagram of sundial
Observation position
Atmospheric scattering can only be reduced by selecting appropriate observation stations.Therefore, the early coronagraphs were built on observatories at high altitude, but recently they have been installed on spacecraft to completely eliminate the impact of the Earth's atmosphere.
With a single lens, the image at the main focus will have chromatic aberration, so a filter must be added to the system.This is necessary in any case, because the coronal spectrum is a large number of emission lines superimposed on the weakened solar photosphere spectrum.Therefore, choosing a narrow band filter with a central wavelength on a coronal strong emission ray can improve the contrast of the final image quite well.Using ground instruments for observation, it is only possible to occasionally image the white light or broad band of the corona, and only under the best observation conditions can you try.Therefore, it will be more common to use satellites to carry observation instruments.
Instrument improvement
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It has been proved that balloon or satellite borne instruments can improve the basic coronagraph. First, the background light at high altitude is weaker than the scattered light in the instrument under normal conditions. In addition, two different forms can be used for improvement.
First, a reflecting objective can be used, which is composed of an uncoated off-axis paraboloid.Most of the light is absorbed through the objective lens.At this time, bubbles and stains in the glass are not important, because they mainly cause scattering in the forward direction.The mirror is not coated because the metal coating is very irregular, which will cause quite serious scattering.
The second method to improve the coronagraph is quite different from the previous one. It is to directly generate an "artificial" solar eclipse outside the instrument, rather than at the main focus.The shield plate shall be placed at an appropriate position in front of the first objective lens of the original quite ordinary coronagraph, and must be large enough to ensure that the first objective lens is completely within the umbra of the plate.Therefore, the interior of the corona (image) will be seriously affected by vignetting, but this is not important because it is the brightest part of the corona;Even it may be an advantage because it will reduce the dynamic range that the detector must cover.Due to diffraction, a single disk will produce an image with a bright spot in the center, but this phenomenon can be eliminated by using a sharp toothed zigzag edge or using multiple shielding disks.With these methods, the scattered light of the instrument can be reduced to 10% of that of the basic coronagraph-4。[4]
Final imaging mode
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The final image generated by the coronagraph can be taken directly, but it is more common to send it to the spectrometer, photometer or other auxiliary instruments.On the earth, it is usually only possible to measure the corona about one time the solar radius outside the photosphere, but the coronagraph onboard the satellite has successfully measured the corona about six times the solar radius or more outside the photosphere.[4]
