Device that reflects sunlight to a constant direction
The optical device that reflects sunlight to a constant direction consists of two flat mirrors.The first plane mirror, also known as the celestial mirror, is placed in an equatorial base frame without the latitudinal axis, and the mirror surface coincides with the instrument axis pointing to the celestial north (south) pole.
The optical device that reflects sunlight to a constant direction consists of two piecesPlane mirrorform.The first plane mirror, also known as the heliostat, is placed in an equatorial base frame without the latitudinal axis (see Figure [Structure Diagram of Horizontal Heliostat]), and the mirror surface coincides with the instrument axis pointing to the north (south) pole of the sky.During observation, the mirror is driven by a motor and rotates in the same direction as the sun's apparent motion at a uniform speed of 48 hourssunlightReflect to a fixed direction, intercepted by the second plane mirror, and then reflect the light to the horizontal (or vertical downward) direction, and then enter the horizontal (or tower)Solar telescopeMedium.The second plane mirror usually adoptsHorizontal device, and slightly rotate around two mutually perpendicular axes to correct the deviation of the sun image in the tracking process.In order to adapt to the annual change of solar declination, the sunlight on the heliostat should be avoided during the dayAngle of incidenceIf it is too large (generally not more than 45 °), and the second plane mirror device blocks the light to the heliostat, the heliostat and the second plane mirror should be able to make relative movement.
Operating principle
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40cm heliostat of Yunnan Observatory
The horizontal solar telescope uses the north-south and east-west orbits to achieve relative motion in the horizontal plane.And in manySolar towerIn order to reduce the area of the tower top, the heliostat should be able to move on the north-south guide track inclined along the polar axis or on the arc track, and the second plane mirror should be able to move up and down.
working performance
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Heliostat
When the heliostat device is used for total solar eclipse observation, the second plane mirror can be used instead of the first plane mirror to reflect to a specific horizontal direction, and its azimuth angle is determined according to the current solar declination and local geographical latitude.The advantages of the heliostat are its simple structure and stability. Especially compared with the solar telescope fixed on the ground, the reflected sky area does not rotateSolar spectrographThe application of is very beneficial.
Most solar towers before the 1960s used heliostats.The disadvantage of the heliostat is that in the tracking process of a day, the incidence angle and reflection angle of the sunlight are constantly changing, and the polarization state of the reflected light is also constantly changing, and the change rule is not a simple function. When measuring the transverse component of the solar magnetic field, the instrument deflection that is difficult to compensate is introduced.In addition, the structure is scattered and cannot be placed in the vacuum system, so the adverse effects of the hot air turbulence generated by itself on imaging cannot be avoided.
Control system
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Starlight receiving of stellar interferometertelescopeThe system should adopt the mode of horizontal heliostat plus fixed telescope.Starlight
Heliostat
When reflected to a fixed direction, the positioning and tracking of stars will be transformed into the positioning and tracking of the normal direction of the heliostat. The two are quite different.
1、 Positioning of the heliostat According to the requirements of interference, two or more heliostats should point at and track the same celestial body at the same time, and the error should not exceed ± 5 (star direction, the normal direction of the heliostat is half of it).withStepper motorAs a driving componentInductosynLevel 1 digital display meter is used as the position closed-loop detection device.This scheme can save 80 - ''90 expenses compared with the scheme using optical code disk.To meet the requirements of light interferenceVibrationSmall, the step distance of driving the celestial mirror should be as small as possible, such as less than 0.05, so that when tracking celestial bodies, the change of wave front direction is less than 0.1/for ordinary domestic stepping motors, the step distance is 1.5.In fact, to reduce the step distance to 0.05, the transmission ratio is as high as 108.Such a large multiple is impossible only by mechanical deceleration, so it is required toStepping motorElectrical subdivision is performed at the step of.If the energy and electricity are subdivided to 64 times and the mechanical transmission ratio is 2000 times, the above requirements can be met.
If the power is subdivided to 128 times, the step distance can be as small as 0.02, which is the most ideal situation (when the power is subdivided to 256 times in the future, the mechanical transmission ratio only requires 1000).Before a specified time, the normal of the celestial mirror is pointed to a certain direction in the sky so that the light of the object to be measured can be reflected to a certain fixed direction. The relationship between the normal direction of the celestial mirror and the direction of the object to be measured can be determined byAstronomyThe triangle is calculated, assuming that the azimuth and height of the celestial body at a certain time are and respectively, and the corresponding azimuth and height of the normal direction of the heliostat assume that the current position of the normal of the instrument is the sign of the difference (one) and (one.), which gives the direction of rotation of the instrument, and the absolute value will determine the maximum speed of rotation.To make the instrument run smoothly, positionaccurate, decide to adopt the method of stepwise speed up and speed down.For example, △ is greater than 10.When the speed rises to the highest speed (such as 2./), △, △ is less than 0.When operating at sub high speed (e.g./).In order to decelerate in time, after the instrument starts to accelerate, the controller queries the current position of the instrument every 50 times, and compares it with the target position to get △, △.In order to make the instrument stop at the expected position accurately and smoothly without oscillation, several speeds can be set according to the size of △ and △, for example, △ is less than 2.Change to 0 when.5。/When △, △ is less than 0.5.Change to 0/slow speed, △, △ less than, change to 20/tracking speed, until △, △ is zero, stop running.
Practice has proved that this method of stepwise speed rise and fall can effectively avoidinstrumentOverrun, but the positioning is successful at one time.All the above control software is stored in a single control MCU.2、 The positioning of the tracking heliostat during observation is to use the driving and detection section above to introduce how to make the instrumentobservationPrevious receiving date: 1991 - 03 - 1265 Volume 4 astronomical literature information orientation predetermined position.At the beginning of the observation, the instrument should follow the star.Due to the use of the horizon type heliostat, the normal position and velocity of the heliostat are completely different from those of the celestial body to be measured, and the former can be calculated by the latter.According to the calculation results, the zenith distance of the measured celestial body is ≤ 45.Under the normal observation conditions of Phase 2-3.If the position and speed are refreshed once every second, the cumulative error in azimuth is less than 0.005, and the cumulative error in altitude is less than.0.01。Therefore, the position and speed of the normal of the heliostat can be refreshed once every second.The tracking calculation is divided into the following steps: In the last step,
Heliostat
PlusatmosphereRefraction correction shall be calculated at any time according to the temperature, air chamber and other parameters during observation.During driving, convert, - 1 - _+- 1, intofrequencyOutput toStepping motorTo prevent possible loss, the feedback of position reading can be used to check and timely compensate (increase or decrease the number of strokes sent to the motor) possible situations.
3、 In order to compensate for possible step loss during tracking and more importantly to correct the influence of atmospheric turbulence on wavefront tilt, a closed loop system is formed withTilting mirrorIts adjustment range is about ± 25 (in each direction).In addition to moving at the given position and speed according to the above calculation, the other function of the heliostat is to keep the tilt mirror at the center of its working range, that is, if it exceeds its central range by ± 5 in one or two directions, this information will be sent to the control machine of the heliostat, so that the heliostat can rotate correspondingly on one or two coordinates,Thus the tilt mirror returns to its central range.It can be seen here that there are two closed systems in the loop of the heliostat: the closing of the position disk and.In order to prevent possible occurrence between two systems.Push pull phenomenon, one of the two closed-loop systems should have priority, and after some judgment conditions are added, they can work in coordination.
software design
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The stellar light interferometer in the experimental stage uses the horizontal heliostat, which has the advantages of large coverage of the sky, symmetrical overall structure, flexible placement, etc.In the process of positioning and tracking, the telescope is fixed, and only the heliostat is moving. Most optical and mechanical parts and baseline reference points are basically stable in space, but the reflection effect of the heliostat magnifies the pointing error of the heliostat by twice.
When the accuracy requirement is certain, the heliostatServo drive systemThe resolution and accuracy are required to be doubled.In order to correctly find the celestial body to be measured, according to the design requirements of the overall scheme, the pointing accuracy of the celestial mirror should reach ± 5 "for each coordinate (except for mechanical system error).
We use a 1 "optical code disk and a 1" digital display meter to form a closed-loop system, a transmission system consisting of gears and copper belts with small backlash difference, and a RM5640D microStepper motorDrive.In order to reduce the impact of the vibration generated when the stepping motor rotates on the heliostat, to make the changes of the wavefront of the two coherent beams smooth, and to reduce the difficulty of maintaining the parallelism between them, it is required that the driving heel angle of the stepping motor should be<0.05 ". This resolution can be achieved by adding 1000 times the mechanical transmission ratio to the RD-053MS (0-400 subdivision) driver produced by RORZE Company[1]
Driving principle
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The servo driving principle of the celestial mirror Zhang Zhuheng, Sun Shuqin (Shaanxi Astronomical Observatory, Chinese Academy of Sciences, Lintong, Shaanxi Province, 710600)The method of speed reduction is used to reduce the vibration of the heliostat during rotation and improve the positioning and tracking accuracy. Due to the application of 400 electrical subdivision and 1000 speed ratio, the step angle of the stepper motor reaches 0.00648 °. 1 In order to ensure the stability of the optical system, mechanical system and the reference point of the king city in space,The light collecting device of the constant star light interferometry experimental system in our country also uses the horizon type celestial mirror, which is not directly pointing to the celestial body, but reflects the star light to the direction of the optical axis of the interferometer, so it has the advantages of large coverage of the sky, symmetrical structure, easy to determine, etc. This paper introduces the celestial positioning servo drive subsystem of the experimental system
The composition and working principle of the fixed weather servo subsystem The baseline of the optical interferometer is determined by the mirror surface center points of the two heliostats. Each heliostat has two setsServo drive system, namely azimuth drive system and altitude drive system, which are respectively controlled by an 8098 single board computer. 8098 single board computer passes through RS232 standard serial interface[2]
Related introduction
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The solar tower of Nanjing University is located near Xiaolingwei at the southern foot of Zijin Mountain in the eastern suburbs of Nanjing. It is a tower type solar telescope. The telescope was completed in 1979, passed the national appraisal in 1982, and was officially put into operation. The height of the observation tower is 21 meters. The aperture of the celestial fixing mirror is 60 centimeters. The aperture of the imaging mirror is 43 centimeters. The focal length is 21.7 meters. The main observation instrument is a multi bandimaging spectrometer At present, it is mainly equipped with CCD detectors in H α Ca Ⅱ 8542 and He Ⅰ 10830 A wave bands, which can obtain the two-dimensional spectrum of solar activity with high temporal and spectral resolution. In addition, it is equipped with H α filters for monochromatic light tracking and observation of the solar tower, which won the second prize of the first National Award for Science and Technology Progress in 1985
Through the observation of the solar tower, a large number of spectral data of solar activity flares, prominence sunspots, etc. have been obtained. Using these data, a series of important achievements have been made. The 22 week observation and research of solar activity won the first prize of the Ministry of Education for scientific and technological progress in 1995 and the third prize of the National Natural Science Award in 1997. The spectral diagnosis of solar flares and research on the physical mechanism of coronal mass ejection won the first prize of the Ministry of Education for natural science 200four[3]
Academician Fang Cheng, astrophysicist, born in Jiangyin, Jiangsu Province on August 10, 1938, graduated from the Astronomy Department of Nanjing University in 1959 and was elected as an academician of the Chinese Academy of Sciences in 1995Academician of the Third World Academy of SciencesProfessor of Nanjing University, Chief Editor of Chinese Astronomy and Astrophysics English Edition, former Chairman of the Chinese Astronomical Society, Chief Scientist of the National Climbing Program, first systematically mastered and applied the theory of nonlocal thermal dynamic balance, and developed a complete set of practical calculation methods and programs in the structure of solar activity and atmospheric model, flare spectral line asymmetry, and velocity field, flare dynamic model and spectrumHe won important achievements in diagnosis and other research, presided over the design and development of China's first solar tower, created the solar tower laboratory, won the first prize of the National Science and Technology Progress Award of the National Education Commission in 1995, and won the first prize of the National Science and Technology Award nominated by the Ministry of Education in 2004
He and his collaborators have successively published more than 220 papers in academic journals at home and abroad, presided over the design and development of China's first 21 meter high heliostat with a diameter of 60 cm, the tower solar telescope, which is at the forefront of the world in terms of comprehensive indicators of time resolution and spectral resolution. Academician Fang Cheng and his collaborators diagnosed non thermal flares in the semi empirical model spectrum of solar activityHigh-energy particleThe method of using chromosphere compression region to explain the long-term puzzling asymmetry of flare spectral lines and the numerical simulation of magnetic fluid have obtained many innovative achievementsNational Natural Science AwardFirst and second prizes of national ministries and commissions and other awards[3]
