Making Color Images of NGC 4650A
Stepping through the processing sequence
The Raw Data
This WFPC2 image of NGC 4650A was taken on
April 9th, 1999. This image is an example
of the raw data that comes directly from the
telescope. No processing has been done to
the image other than a format change so that
it may be viewed via the web and a mosaicing
so that all four chips appear as one image.
Other background galaxies are visible in
the field. The speckle pattern that appears
as faint dots and dashes is actually produced
by cosmic rays (high energy particles, usually
protons or electrons) that hit our detector
during the time that the telescope was collecting
light from our source object. The galaxy was
purposely placed lengthwise along chips 3
and 4 and just above the chip seam.
Further processing of the data has include
removal of cosmic rays and combining several
pointings of the same filter into one single
image of that filter.
The background galaxies and foreground stars
now stand out clearly. Other features in the
spiral arms are also clearly visible.
Differences Among Filters
Placing each filter next to each other and
at approximately the same display level shows
differences among the filters. Notice intensity
changes in the stars, the center of the galaxy
and fine structures in the spiral arms. The
faint line appearing in each image is the
chip seam between the two adjacent chips.
Sample Color Image
A sample color image is created using all
three filters. The filters are arranged chromatically
such that the reddest filter F814 is assigned
the color red, and likewise for the filters
F606 (green) and F450 (blue). This draft color
composite begins to tell the story of the
object- this galaxy has two prominent features-
a smooth reddish disk with older stars and
a thin narrow polar ring that lies perpendicular
to the disk. Where the two cross each other,
the thick dust lane from the polar ring becomes
reddened, and as well, it obscures star light
from that portion of the disk behind it.
These are a few of the color renditions explored
before selecting the color image of
NGC 4650A for the release on May 6th.
The HST Exposures
of NGC 4650A page provides links to general
overviews of making color
images, the challenges
of observing with HST (including the ``chevron''
shape of the detector), information about the polar-ring
galaxy, and more.