The Photopic Sky Survey is a 5,000 megapixel photograph of the entire night sky stitched together from 37,440 exposures. Large in size and scope, it portrays a world far beyond the one beneath our feet and reveals our familiar Milky Way with unfamiliar clarity. When we look upon this image, we are in fact peering back in time, as much of the light—having traveled such vast distances—predates civilization itself.
Seen at a depth thousands of times more faint than the dimmest visible star, tens of millions of other suns appear, still perhaps only a hundredth of one percent thought to exist in our galaxy alone. Our Milky Way galaxy is the dominant feature, its dusty arms sweeping through the frame, punctuated by red clouds of glowing hydrogen. To the lower right are our nearest neighbors, each small galaxies themselves with their own hundreds of millions of stars.
Making The Survey
On a journey that covered 45,000 miles by air and 15,000 by land—the equivalent of nearly 2½ passes around the equator—I toured much of the American west and twice visited the Northern Cape of South Africa. Through it I was able to enjoy not only the wonders of the natural world but also the company of my retired father whom I was able to convince had nothing better to do.
Travel was necessary as capturing the full sphere of the night sky brought with it certain limitations. What might be seen in the northern hemisphere isn't always visible from the south and, likewise with the seasons, what may be overhead in the summer is below the horizon in the winter. Complicated by weather and moon cycles, this made for some narrow windows of opportunity which we chased through the remote areas of Arizona, Texas, Nevada, Colorado, California and Oregon.
As light pollution continues to spread in America, these western states contain some of the last havens of true darkness, a shade few of us are any longer familiar with in which the Milky Way alone can cast a soft shadow. This darkness was crucial because even the slightest amount of light pollution could overpower faint starlight and haze the exposures.
As anyone who has tried to capture night scenes with their camera would know, stars present a formidable challenge. They don't stay still and their light is very weak, requiring long exposures and a tripod mount that rotates in sync with Earth's spin. I also wanted to capture the most amount of detail, one that pushed the limits of practicality without testing my sanity, so I decided each piece of the puzzle would be just 12 degrees wide. This was about the width of a palm at arm's length captured by not just one, but six air-cooled cameras each fitted with their own lenses and filters.
It was clear that such a survey would be quite difficult visually hopping from one area of the sky to the next—not to mention possible lapses in coverage—so this called for a more systematic approach. I divided the sky into 624 uniformly spaced areas and entered their coordinates into the computer which gave me assurance that I was on target and would finish without any gaps. Each frame received a total of 60 exposures: 4 short, 4 medium, and 4 long shots for each camera which would help to reduce the amount of noise, overhead satellite trails and other unwanted artifacts.
And so it was with this blueprint that I worked my way through the sky, frame by frame, night after night. The click-clack of the shutters opening and closing became a staccato soundtrack for the many nights spent under the stars. Occasionally, the routine would be pierced by a bright meteor or the cry of a jackal, each compelling a feeling of eerie beauty that seemed to hang in the air. It was an experience that will stay with me a lifetime.
Prepping the cameras for a night of shooting in Colorado. This particular night dropped to -6°F (-21°C).
For those interested in the specifics and tools used to make the survey, a list follows.
Cameras—Finger Lakes ML-8300 monochrome
Lenses—Zeiss Sonnar 85mm f2.8
Filters—Astronomik LRGB, Astrodon Ha
Mount—Takahashi EM-11 Temma 2 w/ custom armature
Generator—Yamaha EF1000iS for USA trips
Laptop—Intel Core i7-820QM running Linux Fedora, 8GB RAM w/ 4TB external storage
MaximDL—mount control, image capture, and creation of calibration frames (dark, bias, flat)
IRAF—many scripted tasks such as up-scaling, registration, saturated pixel replacements with scaled exposures, stacking, and median background modeling
SExtractor—building each frames list of objects
SCAMP—cross referencing to create position and distortion headers
Swarp—reprojection and stitching of frames
PixInsight—generate final LRGBHa color composite, midtones transfer function, noise reduction
GIMP—final assembly, curve & saturation tweaks
Your support of this project and its ongoing hosting costs is greatly appreciated.
Comments or questions about the survey? Please feel free to contact me atnrisinger [at] skysurvey [dot] org