IC 2118 (Witch Head Nebula)

The Witch Head Nebula is another example of a reflection nebula, where light is scattered by tiny particles. This so-called Rayleigh scattering is most effective for blue light, hence the color. The same mechanism is responsible for the blue daytime sky on Earth. In this example, the illuminating star is Rigel in the constellation of Orion, located 2.5° to the east. Note that IC 2118 itself is located in Eridanus, which has the greatest North-South extension of all 88 constellations.

The image has been rotated so that South is nearly at the top. This makes it easier to see the distinctive shape of the witch's face.

The Witch Head Nebula has a very low surface brightness, and substantial contrast stretching was necessary to see the nebula at all. To suppress grain, 3 exposures had to be stacked. I had two images of the southern (top) part, and one of the northern part (partly covering the witch's "neck"). To make full use of all images, a combination of "stacking" (i.e., averaging) and "stitching" was required. This task was done by a home-written program which averages whenever there is more than one image available at a given location in the final image, and otherwise copies the pixel values of the single image, where multiple exposures are not available. Soft blending of adjacent images is also possible. Initial registration was done using the same technique that helped assembling the Milky Way Panorama. Note the increased grain near the bottom of the image, where no averaging was done, compared to the central part covered by all 3 exposures.



Instrument 8" f/4 Newtonian with coma corrector
Film Kodak PJ-400 hypered
Date 29 Dec. 2000
06:32 - 07:14 UTC, 07:17 - 07:57 UTC and 08:05 - 08:45 UTC
Site Natural Bridges National Monument, Utah
Air temperature was -10 °C (14 °F)!


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