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

The pattern of stars between an image and a star chart becomes very different in the midst of the Milky Way in Sagittarius.

Here introduces the example of an image at R.A. 18 hour, Decl. -28 deg. We can find some false open clusters on the star chart of this region using the USNO-A2.0.

Here is the unfiltered CCD image of this region taken by KenIchi Kadota.

Examining this image using the PIXY System 2, it shows a chart of detected stars, and another chart of the same area plotting the USNO-A2.0 data. Here is the result.

A green star is a normal one, detected from the image, and also recorded in the USNO-A2.0. A red star on the "Detected Stars" chart is one detected from the image, but not recorded in the USNO-A2.0. A red star on the "USNO-A2.0" chart is one recorded in the USNO-A2.0, but not detected from the image.

On the "USNO-A2.0" chart, some "open clusters", condensations of bright stars, are conspicuous. 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.

In addition, there are many red stars on the "Detected Stars" chart. We can see there are many bright stars not recorded in the USNO-A2.0.

The number of stars in this region is as follows.

Stars on both image and chart2756
Stars only detected from image1303
Stars only recorded in USNO-A2.02463

The astrometric error of the examination using the PIXY System 2 is 1.7 arcsec.

NOTE: The stars on the USNO-A2.0 chart are plotted based on the magnitude converted by the Taichi Kato's formula:

V = R + 0.375 * (B - R)
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