Fighting Light Pollution
After solar imaging under absolutely clear sky on the morning of August 19th I left my telescope mount and equipment outside anticipating a cloud free night to follow. The night did continue without clouds or bright Moon, but when I stepped outside to begin setting up my Stellarvue refractor, I was immediately blinded by a neighbor's glaring backyard spotlight. I was so discouraged and disgusted I packed up the mount and began hauling equipment inside. Just as I moved the last piece inside, the neighbor turned off the light! The resulting sky was now as dark as it gets above my less than ideal neighborhood location. The Milky Way and Sagittarius were visible in the south above nearby rooftops. This night of new Moon was probably my last opportunity to photograph the Milky Way, so I hastily attached my Nikon Z62 to a tripod and tried getting some short exposure unguided photos.
After initial trial and error I started accumulating photos to eventually stack together into a single equivalent longer exposure. My work was interrupted several times by various neighbor spotlights turning on and off while dogs were let in and out. I ran out of energy after recording 145 individual exposures at various settings. Only three exposure groups gave decent results. The first picture below, a stack of twenty-one, 6-second exposures, is a wide field image obtained with the lens set at 35 mm, f/2.8, and ISO 3200. Lights and rooftops originally visible along the bottom were cropped out. (Click on images for a larger view.)
Unfortunate background noise appears in the lower left where the sky should be black. Nevertheless, bright star clouds in the Milky Way's center are visible!
The next slightly more magnified image was made from twenty-five, 5-second exposures with the lens set at 50 mm, f/2.8, and ISO 3200. The sky in lower left is darker here, but the right half of the Milky Way star clouds is missing.
Finally, I set the lens at 70 mm, f/2.8, and ISO 6400, and took twenty-six, 5-second exposures to get the following nice view of Sagittarius. This image is the best of the three, I think.
The famous "teapot" shape of Sagittarius is outlined in the next picture along with labels for some obvious celestial objects. I was surprised to see the Lagoon Nebula and Trifid Nebula showing up!
The images above aren't awful, but they could be better. While processing images I'm always losing faint details when I make the background sky reasonably black. Total exposure times are always too short, and light pollution brightens the background sky. I did take dark frames to eliminate camera noise, but I didn't take flat field frames. Travel to a darker location might help the bright background problem, but it's extremely hard to give up the convenience of my back yard!
There's another puzzling thing I don't understand. Software I use to stack individual images into a combined single image runs on my newer laptop. When stacking is complete the software produces a final image that looks great. I tweak this final image with Photoshop (on my newer laptop) and save the image. Photoshop on the newer laptop tells me dark areas have a zero pixel brightness value, and the background looks nice and dark. When I examine this final image on my older laptop, however, the background looks bright with pixel values Photoshop (on the older laptop) now says are about 18, significantly higher than zero! Unfortunately, the unsatisfactory view on my older laptop is the same view I get on a phone, or other devices. It seems like the original zero pixel levels from the new laptop are brought up to a higher level by some mysterious process. Consequently, in order to display a dark background sky on different devices, I have to lower dark pixel values from their unexpected value of 18, down to zero. This creates an overall dimming effect on the entire image and causes dim features to disappear.
