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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008
3. STAR IMAGE SELECTION
As related above, this on-calibration method is based on star
images. But not all star images are identified successfully by
the reasons of the asymmetry distribution of stars in the
celestial sphere, or the insufficiency of the number of the
detected stars, or the limitation of the star identification
algorithm etc. (Liebe, 1995). Only the identified star images can
reach the basic requirement for calibration, in other words, the
number of star image points of each calibrated image is more
than three at least.
In this foundation, the distribution of star image points of each
calibrated images should be considered, because the calibration
accuracy has correlation with the distribution of control points
(Feng, 2002). Ten sequential images in one calibration period
are shown in the figure 3. It is seen from it that different image
has different distribution, and the calibration accuracy based on
different image may be different.
In this paper, the convex area method is used to evaluate
whether this image is eligible for calibration. Take the eighth
star image as an example, which is shown in figure 4. The steps
of this method are as follow:
(1) The convex is constructed by the star image points on the
edge of the image by the convex algorithm (Jin et al., 1999;
Zhou et al., 2003; Rourke, 1998).
(2) As the coordinates of connection point is known, so then the
area of the convex is computed.
(3) The area percent of the convex in the whole image is got. If
the percent is more than fifty percent, which indicates the
distribution is relatively even, the distribution of the image is
considered good and the image is selected.
From figure 3, the result evaluated by the convex area method
is the distribution of figures (e), (g), (h), (j) are good and figures
(b), (c), (d) are bad.
(e) The fifth image (f) The sixth image
(i) The ninth image (j)The tenth image
Figure 3. The distribution of part of images
Figure 4. Convex constructed by star image points
4. EXPERIMENT
The star catalogue Tycho-2 (J2000)(Li, 2006)was downloaded
for experiment, whose data types include star index, magnitude,
the right ascension (hour, minute, second) and the declination
(degree, arcminute, arcsecond) etc.. Average positions of the
stars in standard epoch are transformed to intraday visual
position in real attitude determination. The FOV is 8°x8°, and
the size of array CCD is 512*512 pixels, and each pixel size is
13 urn. The upper and lower limit of magnitude is 0 and 6.0
(mv), and then there are 5001 stars selected in star catalogue for
these experiments. Supposed one stellar image is got by the
stellar camera each 0.1 second, and the calibration period is 2
second in this experiment in the experiments.