Chinese name: Coronograph mbth Invention Time: 1930 Invention Location: French inventor: B.Lyot Attribute: Astronomical instrument uses: instrument uses, working principle, instrument design points, objective lens structure, shielding plate, observation position, instrument improvement, final imaging mode, instrument uses Astronomers found that the shape of the corona changed with each total solar eclipse when comparing photos taken. When there are few sunspots, the corona extends very long near the equator of the sun and looks like a thick brush tip at the poles, so the whole corona becomes like a butterfly. When there are many sunspots on the sun, the corona becomes very large, like a wide and dazzling aperture, surrounding the sun from all directions. In addition, astronomers have also determined that the coronal light shines farthest where the bright red protrusion-prominence is exposed from the back of the moon. It takes a long time to study how the corona changes with the phenomena on the sun, but people will only encounter more than a dozen total solar eclipses in their lifetime, and each total solar eclipse is only two or three minutes, which is too short for scientists. 1930, French astronomer Leo invented the coronagraph (B.Lyot), which enables people to observe the light generated by the corona when the sun shines. Working principle The design principle of the coronagraph is to create an artificial solar eclipse and eliminate the scattered light of the instrument as much as possible. In order to eliminate scattered light, the thickness of the lens and the number of interfaces between glass and air should be reduced as much as possible, so the objective lens O 1 adopts a thin lens, and a narrow-band filter F is placed behind the camera lens C to eliminate the chromatic aberration of O 1. The aperture of the diaphragm S2 is slightly smaller than that of the diaphragm S 1, which can block the diffraction and stray light at the edge of S 1. The small aperture S3 on the optical axis can block the false image formed in the optical path. The lens barrel is painted with dark black paint. On the ground, the scattered light of the earth's atmosphere shines on the corona, so the corona instrument is usually placed on the mountains with thin air. Key points of instrument design Corona instrument enables people to observe the corona outside the total solar eclipse. It uses artificial eclipses for observation. The principle is simple. A shielding disk is placed at the main focus of the telescope, which shields the photosphere image and allows the coronal image to pass through. But in fact, it is much more complicated, because there are scattered light and/or diffracted light in the instrument, the earth's atmosphere will still be several orders of magnitude brighter than the corona, so in order to minimize these irrelevant lights, special preventive measures should be taken in the design and operation of the instrument. The structure of the objective lens is the most important preventive measure. In order to reduce the number of surfaces included in the instrument, Dan Toujing objective lens is adopted, and the glass blank has no defects such as bubbles and textures as far as possible. Carefully polish the lens surface to eliminate all scratches and other traces on the surface. Do not operate in use, must be sealed and dustproof; You can also attach a long oiled cylinder in front of the objective lens as a dust cover. The shutter disk is a polished metal cone or an inclined mirror through which the radiation from the photosphere can be safely reflected to a single photothermal window. After shielding the optical disc, the objective lens is imaged with Fabry lens, which can eliminate the diffraction at the edge of the objective lens, and then the edge effect is eliminated with an aperture slightly smaller than the object mirror image. In addition, the objective lens can be divided into many parts so that its transparency decreases in a Gaussian manner from the center to the edge. Although some resolution is lost, the diffraction halo is completely suppressed. The second shutter in front of the final imaging objective is used to eliminate the multiple reflection effect behind the first objective. The final corona image is generated by a second objective lens installed behind the diffraction aperture. Schematic diagram of optical configuration of sundial instrument Only by choosing a suitable observation station can the atmospheric scattering at the observation position be reduced. Therefore, the early coronameters were built on high-altitude observatories, and have been installed on spacecraft recently, completely eliminating the influence of the earth's atmosphere. With Dan Toujing, the image of the main focus will have chromatic aberration, so it is usually necessary to add a filter to the system. This is necessary in any case, because the corona spectrum is a large number of emission lines superimposed on the weakened solar photosphere spectrum. Therefore, the contrast of the final image can be improved quite well by choosing a narrow-band filter whose central wavelength is on the strong coronal emission line. Using ground instruments to observe, we can only occasionally image the white light or wide band of the corona, and we can only try it under the best observation conditions. So it will be more common to use satellites to carry observation instruments. The improvement of instruments has proved that balloon or space-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 circumstances, and the improvement can take two different forms. First, a reflective objective lens can be used, which consists of an off-axis paraboloid without coating. Most of the light is absorbed through the objective lens. At this time, bubbles and spots 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 coronal instrument is very different from the previous one. It produces an "artificial" solar eclipse directly outside the instrument, not in the main focus. The shielding disk is placed in an appropriate position in front of the first objective lens of the original quite ordinary coronagraph, and it must be large enough to ensure that the first objective lens is completely within the umbra of the disk. Therefore, the inside 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 this 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 serrated edges of sharp teeth or using multiple shielding disks. By these methods, the scattered light of the instrument can be reduced to 10 -4 of the basic coronagraph. The final image generated by the coronagraph can be taken directly, but it is more common to send it to a spectrometer, photometer or other auxiliary instruments. On the earth, usually only the outer corona of the photosphere with the radius of about 1 times the sun can be measured, but the corona instrument carried by the satellite has successfully measured the outer corona of the photosphere with the radius of 6 times or more.