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    Celestial regions where matching between image and chart is hard    

This page introduces the celestial regions where matching between an image and a star chart using the USNO-A2.0 is hard.

As the USNO-A2.0 almost covers down to about 20 mag, it is convenient for astrometry and photometry even in the case of narrow field CCD images.

We can measure the position of a comet or an asteroid precisely enough using the USNO-A2.0. However, sometimes we meet a difficulty to find the same pattern of stars between an image and a chart using the USNO-A2.0, depending on the celestial area of the image. Especially in the midst of the Milky Way in Sagittarius, it is very hard to find the same star between the image and the chart.

The PIXY System 2 automatically finds matching between an image and a chart. Then it plots stars in green color which exist both on the image and the chart. It marks up detected stars not recorded in the star catalog, and catalog data not detected from the image, in red color.

The general result of the image examination using the PIXY System 2 becomes as follows. The pattern of stars on the image almost corresponds to that on the star chart. The PIXY System 2 plots most stars in green color. There are only a few red stars, detected stars not recorded in the star catalog, or catalog data not detected from the image. In this case, it is easy to find the same star between the image and the chart.

USNO-A2.0 data around R.A. 18 hour, Decl. -10 deg

However, the pattern of stars between an image and a star chart becomes very different in the region where R.A. at 18 hour, Decl. between -28 deg and -30 deg. After the image examination of this region using the PIXY System 2, most stars come to be plotted in red color. That means the data in the USNO-A2.0 do not corresponds to real images.

Here introduces the examination results of images at Decl. of -28 deg, -29 deg and -30 deg taken by KenIchi Kadota. We can see unique features on each region, depending on the slight difference of areas, though all images are in the midst of the Milky Way in Sagittarius.

At R.A. 18 hour, Decl. -28 deg, some "open clusters", condensations of bright stars, are conspicuous on the USNO-A2.0 chart. However, the real image shows no evident condensations of stars as on the USNO-A2.0 chart. On the other hand, many stars are distributed all over the image. Most stars of those "open clusters" are plotted in red color on the USNO-A2.0 chart, which means they are not detected from the image.

USNO-A2.0 data around R.A. 18 hour, Decl. -28 deg

At R.A. 18 hour, Decl. -29 deg, only a few stars are found on the USNO-A2.0 chart, as if a large dark nebula covered this area. However, the real image shows that many stars are distributed all over the image. That is, most of the stars in this region are missing in the USNO-A2.0.

USNO-A2.0 data around R.A. 18 hour, Decl. -29 deg

Drawing a star chart at R.A. 18 hour, Decl. -30 deg using the USNO-A2.0, it looks no problem at first glance. However, the examination result using the PIXY System 2 reveals that the pattern of stars does not correspond between the image and the chart, as many bright stars are plotted in red color. In this case, it is very hard to find the same star between the image and the chart. Mis-identifications can happen easily in such a region.

USNO-A2.0 data around R.A. 18 hour, Decl. -30 deg

In those three cases, many detected stars are plotted in red color. We can see there are many bright stars not recorded in the USNO-A2.0.

It is necessary to select correct stars carefully for astrometry of a comet or an asteroid using the USNO-A2.0 in those regions.

The PIXY System 2 can automatically find the same stars between detected stars from an image and data in the USNO-A2.0, even in those regions.

By the way, there are some other regions outside of the midst of the Milky Way in Sagittarius where strange data are recorded in the USNO-A2.0. A region at R.A. 17 hour, Decl. -20 deg, is one of them. We can find a belt lying north and south of false stars on a star chart using the USNO-A2.0.

USNO-A2.0 data around R.A. 17 hour, Decl. -20 deg

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