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Photometric problem using USNO-A2.0     This page introduces the photometric problem of CCD images based on the USNO-A2.0 catalog, showing one example. The CCD observers sometimes estimate the magnitude of their target objects comparing to the magnitude recorded in the USNO-A2.0. The catalog contains the R-band magnitude and B-band magnitude. In general, a CCD camera has a high sensitivity in red band, so the observers estimate the magnitude comparing to the USNO-A2.0 R-band magnitude. However, the magnitude recorded in the USNO-A2.0 has large error and it is hard to determine the accurate magnitude. Sometimes the result becomes very wrong, 2 mag or more shifted from the true magnitude. This page introduces one example of such cases. This is the unfiltered CCD image taken by KenIchi Kadota, which covers the field around R.A. 21 hour, Decl. +44 deg. The star chart of the same field where the USNO-A2.0 stars are plotted in R magnitude is as follows. At a first glance, we can see that many stars are lacked in the USNO-A2.0 in this field. In addition, it is hard to make match between the stars on the image and stars on the chart. We can make match easily between only a few very bright stars. Now let's measure the magnitude of stars in this sample image. We measure it in two ways, comparing to the USNO-A2.0 R magnitude, and measuring the proper magnitude for the CCD chip using the Tycho Catalogue as described in the following page. Photometry http://www.aerith.net/misao/pixy/tutorial/photometry.html First, I examined the image based on the USNO-A2.0 and identified with the Tycho Catalogue using the PIXY System 2. Then selected the "Photometry" menu, measured the magnitude in two ways and showed the tables of residuals. The result in case of simply comparing to the USNO-A2.0 R magnitude is as follows. It is 22.63 mag per 1 pixel count. A 100-count star becomes 17.63 mag. The photometric error is large as 1.54 mag, which implies that the magnitude accuracy in the USNO-A2.0 is not good. On the other hand, the result in case of measuring the proper magnitude for the CCD chip (KAF-1600) using the Tycho Catalogue is as follows. It is 20.67 mag per 1 pixel count. A 100-count star becomes 15.67 mag. Very large difference as 2 mag appeared between the two results. That is, the magnitude becomes 2 mag fainter than the true value in this field when we use the USNO-A2.0 R magnitude as reference.