AAO image reference AAT 15c. « Previous || Next »
Top left is NE. Image width is about 31 arc min
Image and text © 2001-2010, Australian Astronomical Observatory, photograph by David Malin.
This faint object is the nearest planetary nebula to the Sun and on deep photographs has a diameter of about half a degree -- the same apparent size as the Sun in the sky. The AAT colour picture shows the brighter parts of the nebula, revealing various ionisation levels within the shell of matter ejected from the central star. The greenish middle portion is evidence of excited oxygen atoms, while the outer red is predominantly light from nitrogen and hydrogen.
To reveal the detail and structure in the original plates a process known as unsharp masking has to be used. There is more information about this in my technical notes, but basically unsharp masking allows complete control of contrast AND detail rendition in photographic images. The process also now appears in many image processing packages. The picture above was made without unsharp masking, while image AAT 15 was made with the aid of unsharp masks applied to the same plates that produced AAT 15 and AAT 15a and AAT 15b. The Helix is about 700 light years away, almost 200 times more distant than the nearest stars.
AAT 15. The Helix nebula
AAT 15a. The Helix nebula, wide field, with unsharp mask
AAT 15b. The Helix nebula, wide field, deep image
Constellation of Aquarius (external site)
For details of photographic exposure, search technical table by AAT reference number.
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