My God, it’s full of stars.
Ok, apologies to the ghost of Arthur C. Clarke for appropriating Dave’s line in 2001: A Space Odyssey, but I couldn’t resist.
This is an image of NGC 7000, better known as the North America Nebula, one of the objects that really does look a lot like its namesake. I took this on September 19th, about five days before the full moon. This is far from ideal but since ti’s the first clear night we’ve seen in weeks I had to take advantage of the clear night even if the moon wouldn’t set until after 1:30am.
The Astronomy Stuff
NGC 7000 is an emission nebula. It’s basically a big cloud of hydrogen gas that gets excited by the nearby stars and emits that distinctive reddish light that characterizes emission nebula. There is also light there from other components of the cloud like oxygen and sulfur but without special filters that isolate each band of light it all gets swamped by the hydrogen light. Effectively the light is caused by the same thing that causes neon lights to make their light. Electrons get excited, jump to a higher energy state and eventually drop back down to a lower energy state. When they do this they emit light. The color of that light depends on what the element is and what the energy states are. In this case it is that reddish color.
The nebula is about 1,600 light years away but it is large. it covers an area of the sky that is roughly 2 degrees by 1 ⅔ degrees. By comparison, the moon is ½ degree across. This is four times larger in it’s biggest dimension and three times larger in the smaller dimension. You might wonder why we can’t see it with the naked eye since it’s so large. The reason is that it is also very faint. It’s so faint that it’s even hard to see visually in a telescope unless the light pollution is minimal.
The nebula is situated near the star Deneb in the constellation Cygnus. Since the Milky Way lies behind Cygnus there are a lot of stars in that direction and the image shows that very clearly. Visually the stars threaten to overwhelm the image.
Getting the Image
Since the ground was so soft from all the recent rain I opted to set up on the back deck. I expected this to be problematic since there wouldn’t be a solid base under the tripod but figured it would be useful to see how bad it was. And, it was bad. It cost me roughly 20% of my frames so I don’t think I’ll go that route again if I can avoid it.
My guiding graph was pretty awful. I wasn’t expecting to get anything useful, but as I watched the frames come in just enough of them looked useable to keep going. I had the software set to image all night and by 11pm I was in bed worrying that all the effort wasn’t going to produce any data worth processing.
The sky stayed clear until perhaps around 3AM when the sub exposures started to show fewer and fewer stars. I’m not sure if it was high thin clouds or the sky glow from Leesburg. By 4AM the neighbor’s tree was starting to enter the frame. Since I had woken up around 3:30 I decided to go shut things down since nothing useful could be done now.
I ended up with 201 usable sub exposures, each at 60 seconds. The telescope was the Stellarvue SV80-3SV and the camera was the ASI294MC Pro at gain 200 and 0 degrees celsius. All that was riding on a Losmandy GM811G mount.
Earlier I said the nebula was faint. To show just how faint, here is one of the original sub exposures. The nebula is barely visible. The image contains 60 seconds of light and the nebula is just beginning to show up. The human eye can integrate about a tenth of a second of light at night and doesn’t stand a chance of seeing it unaided.
After stacking the images in pixinsight I had to figure out how to process them. This starts with the Dynamic Crop tool to trim off the edges of the image that don’t have as much signal thanks to dithering. Then normally I’d run Dynamic Background Extraction to remove gradients. That was going to be problematic here since there was virtually no place in the image that wasn’t covered by nebulosity or stars. Without clear spots to place sample points the tool was going to end up doing some harm to the nebulosity.
I opted to skip it the first time around. I got to the end and was unhappy with the results. The puffs of nebulosity on the right side of the frame were orange rather than red. I was fairly sure that this was caused the orangish light pollution swamping the color from the nebula. It was also affecting the entire right side of the image in more subtle but still problematic ways.
I redid the processing this time attempting to run DBE. I placed half a dozen sample points manually where it seemed like they would do the least harm and hoped for the best. It helped quite a bit! The right side became much clearer and the bulk of the nebula appeared more distinct. While it may have done some harm it did more good than harm.
Next I ran Photometric Color Calibration to color balance the image and then two passes of Multiscale Linear Transform to remove first luminance noise and then chrominance noise. The ASI294MC Pro seems to produce fairly clean images at 0 degrees Celsius but a light touch of noise reduction helps.
Next it was time to stretch the images. I used this technique to combine an arcsinh stretch which preserves color with a histogram stretch which produces better contrast. I followed that up by Local Histogram Equalization to help bring out some detail in the nebula and then did some final histogram and curve tweaks to work on contrast and saturation. Finally, I used Morphological Transformation to shrink the stars some so they were less visually intrusive. Then I exported a TIF to bring into photoshop where I did some final contrast tweaks and sharpening.
Overall I was pretty happy with the results. I’ve clipped some of the brighter stars which perhaps could have been prevented with some masking when doing the stretching. Perhaps I’ll go back and try that. I think it’s only egregious on that pink star in the middle but many of the bigger stars are also clipping.
I’m still a novice at astrophotography. This is one of the most challenging things I’ve ever tried and it’s been challenging, both in getting the images and then processing them. Each image seems to bring new challenges and obstacles. However, it let’s me see the night sky in a way that my own feeble eyes can never hope to do on their own. I can’t wait to try again. I only hope that I don’t have to wait another month for a clear night!
You can find the image on astrobin.