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Photographic astrometry is a branch of astrometry.The main task is to usephotographMethods to determine and study the relative position and movement of celestial bodies, including: ① the determination of the spatial position of celestial bodies (including artificial objects); ②fixed starvoluntarilyDetermination of; ③KonductraandAggregate starMeasurement of system movement; ④parallaxDetermination of; ⑤Compilation of photographic catalogue; ⑥Verification of the relativistic effect during total solar eclipse.Over the past hundred years, with the continuous innovation of photographic technology, photographic astrometry has been greatly developed.The trend is: due to the gradual development of observation towards dark stars, more and moreReflecting telescopeIt is applied to astrometry and fully automatic photoelectricCoordinate measuring instrumentTo measure the negative film to improve accuracy and efficiency.In addition, we are experimenting with photoelectric technology to directly measure the position of stars on telescopes, and then use fast electronic computers for processing, so as to gradually realize the automation of instruments and equipment.
Chinese name
Photographic astrometry
Foreign name
photographic astrometry
Purpose
Determine and study the relative position and movement of celestial bodies
As early as the early 20th century,Sheshan ObservatoryThe 40cm equatorial "binocular refractor" is used for photographic astrometry.During the 19~20 years of the Republic of China, after the 433 asteroid Aishen rushed to the front of the sun, the telescope was used to make photographic positioning measurements of Aishen, and 243 photographic positioning data were obtained.
Since 1955, the then Sheshan Observatory has participatedSoviet UnionThe compilation of the MSI Catalogue.From 1955 to 1963, 10 selected asteroids were photographed for positioning, and more than 700 accurate positions of asteroids were obtained.
In the same year, Yushan Observatory cooperated with Pulkovo Observatory of the Soviet Union toExtragalactic galaxyPhotographic observation.From 1956 to 1965, a total of 51 sky regions and 122 first phase negatives were taken, with the equatorial range of+42 ° to - 23 °.In 1965, he published the Catalogue of Extragalactic Nebulas Selected for Determining the Absolute Motions of Stars in 50 Regions of Declination - 5 ° - 25 °.In this work, 262 extragalactic nebulae were identified, 60 of which were not recorded in other catalogs, and more than 100 pairs of binary stars were identified on a large number of negative films, 9 of which were newly discovered.
Since 1962,Shanghai ObservatorydevelopRR Lyrae variable Self determination.In 1980, he published the "RR type Lyra variable star catalog".It had the most stars and the highest precision in the same kind of star catalog at that time, and was well received by peers at home and abroad.Using the data in this catalog and abroad, a series of studies have been carried out on the absolute magnitude of RR type Lyra variable stars, the relationship between the kissing orbit parameters and the metal content index Δ S, and valuable results have been obtained.
From September 1963 to March 1964, the Shanghai Astronomical Observatory made 29 photographic positioning of the moon, with the purpose of determining the calendar time.The difference between the almanac time and Universal Time obtained from the photographic positioning result is 1963.85:470=32.32S ± 0.32S.This work requires specialTwo speed lunar camera, the moon and stars can be photographed on the same negative at the same time, and special tables for calculating the moon's speed and charts for correcting the moon's edge are required. At that time, only Shanghai Astronomical Observatory, Pulkovo Astronomical Observatory of the Soviet Union andUS Naval ObservatoryEtc.
Since 1979, Shanghai Astronomical Observatory has carried out the second phase of negative photography and data processing in a planned way, and calculated eight sky regions, and measured the absolute self motions of 1311 stars, which is better than the results of Purkov Observatory in the Soviet Union.
In November 1979, the Shanghai Astronomical Observatory began to useRefracting telescopeconductfixed starThe experimental observation of trigonometric parallax has measured the trigonometric parallax of 15 stars by 1989, and the measurement accuracy has reached the level of similar instruments in the world.This observation result was included in the new version of the Master Table of Star Triangular Parallax compiled by the Yale University Observatory.
During the return of Haley's comet from 1985 to 1986, the Shanghai Observatory used photographic astrometry to accurately determine the position of Haley's comet.This work provides data for improving orbits, calculating accurate calendars, conducting research on the non gravitational effects of orbital evolution, and sending the observation results directly to theInternational Haley Comet SurveyThe relevant units of the organization are responsible for the navigation of spacecraft flying to Halley's comet.
At the end of 1980s, the newly developedReflecting telescopeThe triangulation parallax of dark stars at close range, especially stars with astrophysical significance, is measured.
advantage
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Determine by photographic methodcelestial bodiesLocation.Compared with visual observation, this method has the following advantages: ① photographic film has a cumulative effect on starlight, so darker objects can be observed by properly extending the exposure time; ②The position of multiple stars can be measured simultaneously on one negative; ③The negative film can be stored for a long time, and can be measured and reduced at any time when necessary, so it is documentary.There are three basic processes in photographic astrometry.
Photograph negative
in order toshotThe exposure time of faint stars often takes tens of minutes, so telescopes are required to track the daily movement of starsEquatorial device。If the telescope cannot accurately track the diurnal motion of the star during the whole exposure process, a clear star image cannot be obtained on the negative film, and thus the position cannot be accurately determined.Therefore, it is required that the intersection of the star image and the moving wire should be kept coincident in the guide mirror.If there is a slight deviation, the telescope position should be adjusted immediately, usually by the observer through visual observation with a inching screw.This visual star guiding method has gradually been replaced by photoconductive star technology.In addition, the optical system of the telescope also requires the elimination of field curvature, astigmatism, coma and other aberrations as far as possible.Before observation, the focal length should be adjusted according to the temperature, and the exposure time should be reasonably selected, so that high-quality negatives can be shot.
Measuring film
After taking the negative, first useCoordinate measuring instrumentMeasure the measurement coordinates of all star images on the negative in a rectangular coordinate system.When measuring, the negative frame should be adjusted so that the X and Y axes of this rectangular coordinate system are parallel to the declination circle and the right ascension circle as far as possible (see the celestial coordinate system).In order to improve the measurement accuracy, it is necessary to rotate the negative by 180 ° and measure again.Generally, a prism in the eyepiece of the rotating coordinate measuring instrument is used to achieve this purpose.In recent years, automatic photoelectric coordinate measuring instrument has been used to meet the requirements of heavy workload and high precision.
The reduced measurement coordinates can only give information about the relative positions of these celestial bodies.The relationship between the measurement coordinates (x, y) and the equatorial coordinates (α, δ) takes the ideal coordinates (ξ, η) as the transition.The ideal coordinate is also a rectangular coordinate system, its origin is at the optical center of the negative, and the coordinate axis is parallel to the declination circle and the right ascension circle respectively. The relationship between the ideal coordinate and the equatorial coordinate can be expressed by the following strict mathematical formula:
`\xi=\frac{cos\delta sin(\alpha-A)}{sin\delta sin D+cos\delta cos D cos(\alpha-A)}`
$\eta=\frac{sin\delta cos D-cos\delta sin D cos(\alpha-A)}{sin\delta sin D+cos\delta cos D cos(\alpha-A)}$
Or simplified into the following form:
ξ=tg(α-A)cos m sec(m-D)，
η=tg(m-D)；
tg m=tgδsec(α-A)。
Where A and D are the equatorial coordinates of the optical center of the negative, and m is the auxiliary quantity for calculation.
The ideal coordinates of a star image are not the same as the measurement coordinates.This is because: ① the origin of the measuring coordinate does not coincide with the origin of the ideal coordinate; ②X axis and Y axis are not exactly parallel to ξ axis and η axis; ③X axis and Y axis are not strictly orthogonal; ④Coordinate measuring instrumentThe scales of x and y are different; ⑤It is affected by poor atmospheric refraction and poor aberration.According to the above reasons, the relationship between ideal coordinates and measuring coordinates can be expressed as:
ξ=ax+by+c，
η=dx+ey+f，
In the formula, a, b, c, d, e, f are called film constants.
When determining the position of celestial bodies by photoastrometry, there should be a certain number of stars called calibration stars on a negative film, and their precise equatorial coordinates are known.The purpose of the calibration star is to determine the film constant.First, convert the equatorial coordinates of the calibration stars into ideal coordinates with an electronic computer or an existing data table, and then measure the measurement coordinates of these calibration stars.Theoretically, only three calibration stars can be used to calculate the film constant, but in order to improve the accuracy, about ten calibration stars with uniform distribution are generally selectedleast square methodSolve the film constant.Once the film constant is obtained, the measurement coordinates of other stars to be located can be converted into ideal coordinates, and then their equatorial coordinates can be calculated.
measuring method
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Photoastrometry is a relative measurement method.Usually, first select a coordinate system on the negative, measure the relative position of the star image in this coordinate system, and thenCatalogueSelect some stars with known equatorial coordinates as calibration stars, and use these calibration stars to reduce the measured relative coordinates to equatorial coordinates (see photographic astrometry method).The accuracy of photographic astrometry mainly depends on the measurement error of the negative.Increasing the number of calibration stars can reduce the accidental errors of the calibration star measurement and the catalog, but the accidental errors of the measurement of the undetermined objects, the systematic errors of the catalog and the systematic errors of the measurement will still be all reflected in the final equatorial coordinates.The accuracy of photographic astrometry also depends on the self motion of the calibration star.Generally speaking, the measurement accuracy of the negative is about 1~2 μ m. For the telescope with a focal length of about 2 meters, the average photographic positioning accuracy is 0 prize 15.The following are some of the most active topics in modern photographic astrometry.
Establish reference coordinate system
The work of establishing a reference system based on the position and self motion of stars is mainly to expand the catalog to a darker range.The representative isGerman Astronomical SocietyThe third star table AGK3, which publishes the positions and proper motions of stars brighter than 12, is the best star table for selecting calibration stars in the photographic positioning of asteroids, comets and other celestial bodies.In 1932, Soviet astronomers put forward the plan of compiling the "Dark Star Catalogue".One of its characteristics is toExtragalactic galaxyTo determine the self motion of stars for the background.If the lateral speed of the extragalactic system is 1000 kilometers per second, the position change of the nearest galaxy is only 0 prize 0001 every year, which is much smaller than the measurement error of its own motion. Therefore, it can be considered unchanged within 100 years, which can be used as a stationary reference coordinate system to measure the star's own motion.The United States has similar plans.Recently, using the self motion data of the Soviet Union and the United States relative to galaxy determination, we have obtainedPrecession constantThe corrected value of and the Oort constantAandB(See Galaxy Rotation).The corrected value of precession constant is not much different from the value calculated according to the basic catalogBThe values also agree well, butAThe values differ greatly.These results show that, compared with the system of galaxy seeking star's own motion, the system is better in declination, but has a larger system difference in right ascension, and the reason remains to be studied.
Determination of the self motion of dark stars
In order to study the mechanical characteristics of the Milky Way, it is necessary to measure the self motion of dark stars up to the 21st grade, includingOpen cluster、Planetary nebulaThe self motion of nova.According to the self motion data, we can identify the members of star clusters and study the internal motion, diffusion motion and absolute self motion of star clusters.The discovery of dark stars with large self motions and the determination of their self motions are of great significance to the study of the mechanical characteristics of the Milky Way near the sun.In the past one or two decades, several observatories have been engaged in this work, and published tens of thousands of constellations with their own performances greater than 0 prize and 2 every year.Dark stars with large self motions may be near dwarfs, and their parallax needs to be measured.
Determination of Triangular Parallax of Dark Stars
From 1837 to 1839BesselIt has been more than one hundred years since people first measured the parallax of stars accurately, and its significance is gradually recognized by people.The US Naval Observatory in Washington has specially developed an astrometric telescope with an aperture of 1.55 meters to measure the parallax of dark stars.Of the thousands of stars whose parallax has been measuredVisual magnitudeOnly more than 100 were waiting for 14.
To study the motion of binary and convergent star systems
The mass of stars can be accurately measured by photographic observation of binary stars, especially those within a distance of 20 seconds.In order to determine the binary orbit and its mass, it needs decades or even hundreds of years of observation data and hundreds or even thousands of negative films.The position of each sub star of the binary star relative to the calibration star can also be obtained by using photographic observation, so that the orbit relative to the center of mass of this system can be calculated.A detailed analysis of orbital periodic changes can also find invisible companion stars with small mass, and even find possible planetary like companion stars.
