Monday, July 4, 2016

Starry night from suburbia

Captured June 25, 2016. Canon 1000D, Rokinon 16mm f/2.0 lens @f/2.0, 30 second exposure @ ISO 1600. Custom white balance.
Here is an image of the night sky facing east from my neighborhood, taken for fun while my other camera was occupied capturing another object. The brightest star near top-center is Deneb in Cygnus. The North American Nebula is evident just beneath Deneb and nebulosity near Cepheus is also visible on the left.

The sky was actually pretty washed out due to light pollution in the original 30 second exposure so I used a mask to select only the sky and reduce the brightness. This is a tricky task because it is easy for the transition from foreground to sky to look unnatural with a mask. I am new to landscape astrophotography processing so I may or may not have pulled this trick off very well - I am certainly dancing on the line between believable and novice. The foreground lighting is really interesting with the high contrast, multiple shadows and variety of color temperatures. Believe it or not I get pretty decent image from here! I only cropped the bottom of the image, so lens aberrations are readily visible at the top. Despite the aberrations, the Rokinon 16mm f/2.0 is a really fun lens. Wide angle shots like this remind me of images from the film days that inspired me to pursue astrophotography. I appreciate the simplicity of the technique and connection between Earth and sky it emphasizes.

Processing Workflow (ACR for first step, then PixInsight)
1. Open .CR2 in Adobe Camera Raw, save as .TIFF with “as-shot” white balance applied and no other modifications.
2. Open .TIFF in PixInsight for remainder of processing steps.
3. FastRotation 90 degrees counter-clockwise
4. Create mask to select sky/foreground by extracting blue channel (greatest contrast between foreground & background) then using range mask (smoothness 2.0) to select most of foreground. Clone stamp remaining foreground.
5. Neutralize sky (BackgroundNeutralization on preview near top with range mask selecting sky).
6. Reduce sky brightness (GammaStretch, R/K 1.66 with range mask selecting sky).
7. Increase contrast in sky gently with subtle “S” curve (CurvesTransformation to RGB/K with range mask selecting sky).
8. Increase color saturation of H-alpha in sky (CurvesSaturation, increasing all colors but orange which was mostly light pollution, with range mask selecting sky)
9. Reduce contrast in foreground by increasing brightness in shadows and decreasing brightness in highlights (CurvesTransformation to RGB/K with range mask selecting foreground).
10. Reduce noise (ACDNR to luminance and chrominance with lightness mask).
11. Repair vignetting in upper-left hand corner where lens hood was accidentally rotated and darkened a small bit of that corner. Increased brightness of that area very selectively (CurvesTransformation to RGB/K with range mask selecting sky).
12. Increase color saturation of foreground (CurvesTransformation to RGB/K with range mask selecting foreground).
13. Increase color saturation of sky a little bit more (CurvesTransformation to S with range mask selecting sky).
14. Repair a few hot pixels with CloneStamp.
15. Make stars sharper (MultiscaleMedianTransform +0.1 bias to layer 2 with mask selecting stars generated by extracting B channel (greatest contrast between stars and sky) and using range mask, smoothness 2.0).
16. Sharpen (UnsharpMask, amount 0.39 with luminance mask).
17. Blur words on street signs by duplicating image, convolving the duplicate twice and then using CloneStamp to copy the blurred areas to the original image.
18. Final crop to 5x7 aspect ratio (DynamicCrop)

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