MISAO Project

Home Page       Tue Sep 2 14:15:44 JST 1997

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Next: Experimental result Up: PIXY system Previous: Overview

Current progress

The prototype system has been already completed and now experiments and improvements are repeated. This prototype detects stars from the image, compares them to those in a catalog and obtains the map function to convert the position in the image to R.A., Decl. We can see the finding chart of the same area as the original image. The chart is created with data in a catalog and we can compare the two on a display.

Here describes the method of star detection and matching in detail.

  • Star detection
    The process to detect stars from an image is as follows.
    1. Calculates a flatfield function tex2html_wrap_inline288 .
    2. Obtains a standard deviation tex2html_wrap_inline290 of the difference between each pixel value and the flatfield.
    3. On each pixel, if the value p is more than tex2html_wrap_inline294 , the pixel is regarded as part of a star and the value is set as tex2html_wrap_inline296 . If not, the value is set as 0.
    4. The pixels with non-zero value are seperated into some small areas. Each area is regarded as a star and the system obtains the coordinates of the center of gravity and total value of pixels in the area as a brightness of the star.

    Because the center of an image is brighter than the rims generally, the flatfield function is expressed as a quadratic function of x,y:


    The parameters tex2html_wrap_inline302 are obtained in the method of least squares.

    The current software cannot seperate close double stars or clusters. It cannot see if the object is widely diffused or is a linear noise, either.

  • Matching
    Before matching, we have to determine the magnitude of detected stars as properly as possible. The way of the current system is as follows. First of all it picks up stars data from a catalog in an approximately same area as the image. The data are sorted in magnitude. The detected stars are also sorted in brightness. Then it allots the magnitude to the brightness in order. That is, the magnitudes of detected stars have some errors in this step.

    The purpose of matching is to obtain a map function. A map function is a coordinates converter to determine the position x,y in the image is where in the chart created from a catalog. Definitely, after rotating with an angle tex2html_wrap_inline306 and magnifying with rate k, the image should correspond to anywhere in the chart. The difference between the two coordinates is tex2html_wrap_inline310 . The map function consists of the four parameters.





    The map function is calculated with the following steps. First of all, the system picks up three stars from the image and three stars from a catalog. If the two trianles, which the selected three stars form, are similar and the magnitudes of each corresponding pair of stars are also similar, these three stars in the image may match to the three stars in a catalog. So a map function between the two triangles is calculated. These processes are tried to all combinations of trianles. By the way, a true map function produces many similar triangles. It means, the true value of map function is most often calculated in this method. On the contrary, the function calculated with accidental similar triangles is diffused and not calculated many times. Therefore the system adopts the most overlapping parameters as a true map function. Definitely, the parameter space is divided into some pieces with proper intervals and the system counts how many times the parameters in each pieces are calculated. Then it finds out the most overlapping piece. The map function is obtained as a average of parameters in the piece.

    This method strongly depends on the additional information input at first. It assumes the approximate direction is absolutely in the image. The true width of the image should be between the 0.67 times and 1.5 times of the approximate value. Even with the proper values, the parameters of map function do not concentrate at one point and some candidates of true function are often obtained. The large errors of magnitudes alloted to the detected stars at the first step influence much. However, when the approximate angle to rotate, for example when the images are always taken as the north is up, only true map function is obtained.

next up previous
Next: Experimental result Up: PIXY system Previous: Overview

Copyright(C) Seiichi Yoshida (comet@aerith.net). All rights reserved.