[vsnet-chat 5548] Celestial regions where matching between image and chart is hard

[Seiichi Yoshida <comet@aerith.net>]

MISAO Project Announce Mail (September 30, 2002)

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

The web page which introduces the celestial regions where matching
between an image and a star chart using the USNO-A2.0 is hard, is
published in the MISAO Project Home Page.

  Celestial regions where matching between image and chart is hard
  http://vsnet.aerith.net/misao/report/general/usno-matching.html

The USNO-A2.0 is a star catalog compiled in the U.S. Naval
Observatory. As it 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.

In general, the pattern of stars on an image almost corresponds to
that on a star chart. So it is easy to find the same star between the
image and the chart. After the image examination using the PIXY System
2, most stars are plotted 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.

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.

The web page introduces the examination results of images at Decl. of
-28 deg, -29 deg and -30 deg taken by KenIchi Kadota. It is very
interesting that 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
  http://vsnet.aerith.net/misao/report/general/usno-matching/1800-2800.html

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
  http://vsnet.aerith.net/misao/report/general/usno-matching/1800-2900.html

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
  http://vsnet.aerith.net/misao/report/general/usno-matching/1800-3000.html

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
  http://vsnet.aerith.net/misao/report/general/usno-matching/1700-2000.html

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/

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