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Making Color Images of NGC 4650A

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.

The Cleaned Data

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.

A 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.