Research On The Method For Digitization Of Astronomical Plates And Its Applications

Before CCD detector widely used in astronomical observations, astronomical plate was the main detector of this research field. Astronomical plates as the main data carrier record more than one hundred years astronomical information. Every plate keeps the original data at that time, and these data can’t be reproduced. Combining the astronomical data recorded by astronomical plate and CCD will have a significant role for astronomy research work. But photographic astronomical plates are coated with light-sensitive materials, which are emulsions. With the passing of time and the change of environment, the emulsion will deteriorate gradually. In order to preserve these precious historical data, astronomical plates should be digitized as soon as possible. So it is very important to research the fast effective method for digitization of astronomical plates.In recent years the quickly digital technology have been developed in many countries, including developing the special scanner and using the conventional scanner. Special scanner is expensive and scarce, and it can only be fixed in a particular astronomical plate library, which brings limitations when using it. The advantage of conventional scanner is low cost, and easy to move, and the disadvantage is that the scanning accuracy is not good as special scanner. If there’s some ways could improve the precision of digital image, it may be a simple and effective method to digitize astronomical plates by using conventional scanner. Moreover, the accuracy of history reduction of astronomical plates is not good enough. Now with the high-precision star catalogue, digitizing astronomical plates and processing the digitized images maybe solve this problem effectively. Based on the measurement of the performance of the conventional scanner, this paper is the first study on the method of astronomical plate digitization and data reduction. And the study results are applied to the precise positioning of natural satellites and the accuracy evaluation of GEO satellite optical positioning system. The main research work and innovation points in this paper are as follows:1.The study on the method for digitization of astronomical platesThe measurement of the performance of the conventional scanner show that this scanner has a satisfy accuracy in direction of CCD. Because of the mechanism movement in the scanning direction, the scanner has poor accuracy in this direction(Y direction). In this paper, the measured coordinates are comprehensively computated by the measured coordinates in two scanned images of 0° and 90° directions, which can effectively improve the accuracy in the scanning direction.The digitization of astronomical plate is composed of three parts.(1) File the information recorded on the plate jackets and observation logs.(2) Digitize astronomical plates. First, quite preview plate at low resolution in JPEG format for saving the marks. Then, use absorbent cotton and distilled water to clean the marks, which generally were written on the plate of non-emulsion coated side. At last, scan plate at high resolution in 0° and 90°directions with TIFF format.(3) Standardize the digitized images. Converte TIFF images to FITS images and add the information of FITS header.2.Develop the specific astrometric software which can be applied to the reduction of the digital image of astronomical plate.Depend on the comparison of the feature between digital images of astronomical plate and CCD images, we can know that the common astrometric software does not apply to the digital image of astronomical plate. So the specific astrometric software, which can applied to the reduction of the digital image of astronomical plate,is developed in this paper.The software is composed of three modules.(1) Preprocess the digitized images. This module includes converting TIFF images to FITS images and adding the information of FITS header.(2)Process the digitized images. This module has four parts as following: dividing the image into several blocks, extracting stars, centring stars, determine the measure coordinates which are computated by two images scanned in 0° and 90°directions.(3) Astrometric reduction. This module consists of extracting stars information form reference catalog, matching the reference stars, computing the constant model and calculating the position of objective stars. The software applies to the reduction not only of the digital images of astronomical plate, but of CCD images.3.Digitization of natural satellite plate and its application.With the astronomical plates of National Time Service Center(Shaanxi Observatory), this paper finished the study of digitalizing natural satellite plates and data processing. The results show that the digitalization of astronomical plate and data processing is not only beneficial to keep the history data of astronomical plate, and also to improve the accuracy of historical data. Moreover, it helps to find the missed observation information in the historical data processing, which is first-hand observation data. The long-time and accurate observation positions of natural satellite are advantageous to the computation of the mass and shape of the ruler, so the positions of Saturn satellite are applied to the computation of Saturn’s mass and shape.4. The accuracy evaluation of GEO satellite optical positioning system.A new 50 cm GEO satellite optical positioning system is established by National Time Service Ceneter, Chinese Academy of Science. The accuracy of this system is evaluated by the specific astrometric software which developes in this paper. The results show that this system has high astrometry accuracy. So it can satisfy the needs of the observations of GEO satellites and natural satellites. The results also show that the specific astrometric software also can be applied to the reduction of CCD images.