The Astro Imaging Channel is a Google+ group and YouTube channel that conducts weekly tutorials, presentations, and discussions on various aspects of astro-photography (https://plus.google.com/+TheAstroImagingChannel).  Each week, they select an “Image of the Week” from member submissions and make it the banner image for their home pages on Google and Facebook.  This week they selected my image of the California Nebula!

You have probably seen this image as the banner on MY Facebook page.  Here’s a little more information about the object and the image:  The California Nebula (NGC1499) is an emission nebula in the constellation Perseus.  It is about 1,000 light years from Earth and it’s apparent length is about 3 degrees.  As is often the case with emission nebulae, it consists primarily of hydrogen.  There is a fair amount of sulfur as well, but very little oxygen (these 3 elements being the ones whose spectral lines are typically captured in narrowband astro-photos such as this).  In this case, the oxygen is represented by the blue region, but that’s where this image gets interesting, and tricky.

Because the oxygen signal is very weak, my first attempt at processing this image showed very little blue.  That’s often the case with photos of this target, but I wanted to have more color variation, so started experimenting with ways to bring it out.  In studying the 3 channels separately, I saw that most of the nebula has both hydrogen and sulfur (both of these elements appear as red), but there is also a region that has plenty of hydrogen, but not sulfur, and that region corresponds fairly well to the region that has the strongest oxygen component.  So I created a new image in which the sulfur channel is subtracted from the hydrogen channel.  This new image was then “colorized” with the blue from the original oxygen channel, and that forms the blue part of this image.  In other words, the color comes from the true oxygen signal, but the brightness of the blue region really represents “H-S”, or places where there is hydrogen, but no sulfur.

This technique would not on all such targets, it just happens in this case that H-S is a pretty good proxy for O.

If you want to learn more about capturing and processing astro-photos, TAIC is a great place to be!  Thanks to everyone at TAIC for selecting my image this week.