August: Planetary and Emission Nebulae

Abstract

The brilliant teal glow in the center of the Cat’s Eye Nebula impresses visual astronomers with large telescopes. This central bright region, 20 arcsec in diameter, is just a small portion of the much larger (386 arcsec) outer halo that is a 1,000-fold dimmer, and only appears in long-exposure images such as this one. Many fainter intermediate layers each reflect episodic rigors of a dying star. The Cat’s Eye Nebula resides at a distance of 3,000 light-years. Imaging. As with other small planetary nebulae, image the Cat’s Eye with the highest resolution that your skies, mount, and tracking will allow. The inner nebula is bright and easy to capture even with a single-shot color camera or routine RGB imaging. Be careful not to overexpose the central region. On the other hand, the outer faint layers are better revealed with a clear filter using long exposures that intentionally overexpose the center. The overexposed center can later be subtracted during processing. Furthermore, because the outer layers are lower in both contrast and detail, you can acquire the clear exposures binned 2 × 2 to improve your signal-to-noise ratio in the dim areas. Processing. Calibrate and align your images routinely. If you want to try deconvolution to get a head start on sharpening the central area, then only select your very sharpest unbinned RGB individual exposures to combine for a synthetic luminance channel. Combine your RGB channels routinely. If you have an overexposed clear luminance for the outer nebula, create several copies each with slightly more aggressive digital development adjustments. Apply these as multiple luminance layers, each with 10–20% opacity; for each, select the central core, feather generously, and cut out the area that was burnt out by digital development (Fig. 8.1).