Resolution Camera - "Mind the Gap"
|The Advanced Camera for Surveys
on Hubble Space Telescope includes two cameras, the Wide Field Channel
(WFC) and the High Resolution Channel (HRC). Images from the WFC are
roughly 4,000 pixels square with a scale of roughly 0.05 arcseconds
per pixel. Images from the HRC are smaller in pixel size, 1,000 pixels
square, but have a finer resolution, 0.025 arcseconds per pixel. The
HRC is preferred for images of planets, or objects appearing smaller
on the sky, where higher resolution outweighs larger field of view.
The HRC also has two occulting masks that can block starlight from
bright sources, and allow fainter objects nearby that are otherwise
lost in the glare of the brighter object to be visible. One of the
masks can be removed, but the other mask, seen in this Mars image
at the lower left corner, can not be moved out of the light path.
This "occulting finger" blocked a small portion of the light
from Mars. The small amount of missing data in the final color composite
of Mars was corrected by filling with nearby sections of the image
of similar color and texture. This is a method commonly used in digital
photography to remove blemishes, but is not used with data published
in scientific papers.