[vsnet-chat 4536] Photometric problem using USNO-A2.0

[Seiichi Yoshida <comet@aerith.net>]

MISAO Project Announce Mail (June 19, 2001)

Hello. I am Seiichi Yoshida working on the MISAO project.

The USNO-A2.0 is a huge star catalog compiled in the U.S. Naval
Observatory, which contains stars down to about 20 mag over the whole
sky. It consists of 11 CD-ROMs. The data files are available at:

  http://adc.gsfc.nasa.gov/pub/adc/archives/catalogs/1/1252/usnoa/

Of course, the PIXY System 2 can read this catalog. The USNO-A2.0
editors kindly presented us the CD-ROMs for the MISAO Project
activity. We appreciate it very much. Most of the images offered to
the MISAO Project recently are examined based on this catalog, the
detected stars are matched with the USNO-A2.0 data, then the R.A. and
Decl. of the image center, the field of view, etc., are calculated.

Because the USNO-A2.0 contains magnitude of faint stars, 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. 

One example of such cases is introduced in the following MISAO Project
web page.

  Photometric problem using USNO-A2.0
  http://vsnet.aerith.net/misao/pixy/result/usno-photometry.html

The sample image in this page is taken by KenIchi Kadota, which covers
the field around R.A. 21 hour, Decl. +44 deg. The page shows the
unfiltered CCD image and the star chart of the same field where the
USNO-A2.0 stars are plotted in R magnitude.

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. 

As this example shows, it is hard to make match between a CCD image
and a star chart based on the USNO-A2.0 in some field, especially in
the summer Milky Way. Of course, the PIXY System 2 can find the
matching automatically. 

Using the PIXY System 2, I made the matching of the sample image and
measured the magnitude of all stars based on the USNO-A2.0 R
magnitude. The web page shows the result and the table of residuals. 
The result 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.

Then I measured the magnitude of all stars based on the Tycho
Catalogue using the PIXY System 2. The result became very different
from that based on the USNO-A2.0. The result is 20.67 mag per 1 pixel
count. A 100-count star becomes 15.67 mag. That is, the magnitude of
a star became 2 mag brighter than that based on the USNO-A2.0 R
magnitude.

Because the Tycho Catalogue contains the standard V and B magnitude,
we can measure the proper magnitude for our own CCD chips, as
described in the following web page.

  Photometry
  http://vsnet.aerith.net/misao/pixy/tutorial/photometry.html

In conclusion, the magnitude becomes 2 mag fainter than the true value
in this field when we use the USNO-A2.0 R magnitude as reference.

The large difference between the USNO-A2.0 R magnitude and unfiltered
CCD magnitude also appears on some variable stars.

MisV1111, one of the new variable stars discovered in the MISAO
Project, R.A. 21h00m08s.21, Decl. +44o21'22".3, is a red variable star
between 12.4 mag and 14.0 mag on the unfiltered CCD images offered to
the MISAO Project.

  http://vsnet.aerith.net/misao/data/MisV/MisV1111.gif

This star is also recorded in the USNO-A2.0. But the cataloged
magnitude is 17.9 mag in R-band and 20.2 mag in B-band, much fainter
than the measured magnitude in the MISAO Project.

We can see the DSS (Digitized Sky Survey) image of this star at:

  Astronomical Image On-line Access Interface
  http://dss.mtk.nao.ac.jp/

On the DSS R-band and B-band plates, MisV1111 is really faint. 
However, it is bright on the I-band plate. So the main light of this
star is infrared. 

P.S.
The past MISAO project announce mails are available at:
  http://vsnet.aerith.net/misao/

--
Seiichi Yoshida
comet@aerith.net
http://vsnet.aerith.net/

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp