2017-01-30
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Mt. Kent Observatory
CDK700

Observer: Rhodes Hart
Processing: John Kielkopf

Caldwell 74 or NGC 3132
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C74 is also known as the Eight-Burst Nebula or the Southern Ring Nebula.
It is a planetary nebula in the constellation Vela, about 550 parsecs or
2000 light years from the Sun.

Faint star is energy source because the star at the center is actually too old
and not hot enough to provide the ultraviolet light that excites the gas.
The two stars make a physical binary system.

The color image is a composite of red, green and blue layers that are actually
the red (r), infrared (i), and blue-green (g) filtered images.  The red
components of the nebular emission are from hydrogen, the blue components from
hydrogen ionized oxygen.  There is little emission from the nebula by
comparison in the infrared.

Each of the images contributing to the color composite is a 300 second (5
minute) total exposure that is the sum of 3 individual 100 second exposures.

Thus the color image is a combination of 9 separate images totaling 900 seconds
of exposure time.

The color image is shown on a "square root" stretch to enhance the fainter parts
compared to the bright stars.  Background skylight has been cancelled in each
layer to make the sky appear black.  The brightest regions of the nebula were
recorded with approximately 10000 photons per pixel in all colors over the total
duration  of the exposure, or about 10 photons per second captured by the
telescope and camera.

c74_g.fits   Sum of 3 100 second exposures in the Sloan "g" band (blue-green)
c74_r.fits   Sum of 3 100 second exposures in the Sloan "r" band (orange-red) 
c74_i.fits   Sum of 3 100 second exposures in the Sloan "i" band (near-infrared)

c74_rig_rgb_full_frame.png  Color image with red (r), green (i), and blue (g)
c74_rig_rgb.png             Same, but full frame, north up, east to the left
c74_r_log.png               Red image only on a log scale to show the nebula