Although it’s still invisible to the naked eye, Comet ISON (C/2012 S1) is brightening as expected. Amateur telescopes can reveal its tiny head, its growing outer atmosphere that astronomers call the coma, and its lengthening tail. And, as we see here, long-exposure images also show the comet’s greenish color. Adam Block captured this image through a 32-inch Schulman Telescope and an SBIG STX-16803 CCD camera. It's an RGB image with exposures of 18, 18, and 16 minutes, respectively, taken October 8, 2013, from the University of Arizona’s Mount Lemmon SkyCenter