Many of you have probably already seen this photo I posted over the weekend of the Tsuchinshan–ATLAS Comet, taken last Monday (21st) when it had moved far enough south to be included with the Milky Way. Like many of you, I had been waiting for a later moonrise that was at least an hour past the Astronomical Dusk — a nice window of opportunity for optimum sky clarity and contrast, under a beautiful Bortle Class 3 sky.
Now, I'm going to show you, my Patreon supporters, how I did the post processing for this image...
Backstory on the Barn: This old barn was built many years ago by Ukrainian emigrants, in the Skull Valley ranching area, about 50 miles west of Salt Lake City, Utah. My paternal grandfather herded sheep in this valley back in the 1930's.
TECHNIQUE & EXIF: Stacked • Canon R6 (astro-modified to H-alpha by Spencers Camera) + 24mm Rokinon f/1.4 • f/2.8, 10 sec, ISO 3200 - 15 "lights" and 9 "darks" stacked in Starry Landscape Stacker.
Applying an "S" curve to the sky (before and after): After I synced and did preliminary processing of all 24 stacked in Adobe Camera Raw or Lightroom (see pages 191-193 of my eBook), i exported the images as TiFF's and stacked them in Starry Landscape Stacker (you can use Sequator if you're on a PC). This greatly reduced the image noise.
My next step is to use Photoshop to select the sky in this stacked image (Select > Sky) and save it as an alpha channel that can be used as a mask (Select > Save Selection).
Using that sky selection, I create a Curves adjustment layer (Layers > Adjustment Layer > Curves) and use two anchor points to adjust the contrast of the night sky via an "S" curve. The above image is a before and after of that "S" curves adjustment. This one, simple adjustment often provides 80 to 90% of the contrast and color for my final image! You can learn more about the power of Curves adjustments on my blog.
The secret power of "S" curves lies in its ability to build contrast while protecting the image's shadow and highlight details. The yellow circled area show how the anchor points have been moved to allow the top part of the curve (highlights) as much contrast as possible, without flattening ("pancaking") into the top of the graph, which would "blow-out" the highlight detail of the stars (and the core of the Milky Way) — which is not recoverable later by other methods, i.e. by using the burn tool to darken light areas. More information on this technique is covered on page 195 of my eBook (which most of my paid Patreon supporters already have received via their membership). NOTE: The blue and green lines are slight color adjustments to the blue and green channels.
Darkening the barn with Levels: I wanted to show off the beautiful architecture and workmanship of this old Ukrainian barn, rather than just letting it be a black silhouette, so I added some Low Level lighting to the foreground. Even though I used the lowest setting on my Lume Cube LED Panel light (at 300 feet or about 100 m. from the barn), I thought it was still a little too bright.
By taking the "sky" (alpha channel) selection, I had saved earlier, and inverse-ing it (Select > Inverse), I could now use this foreground selection as a mask for a Levels adjustment layer to darken that foreground. I chose Levels rather than Curves because the bottom slider in Levels allows you to easily add highlight density to the image (see the yellow circle). This not only darkens the image but does NOT add contrast like using the middle gamma slider (above the "1.00") would have done (I wanted to tone-down the highlights without losing more shadow detail in the barn's wood texture). NOTE: The middle image shows the after effect of darkening just the foreground.
Adding vibrance to the sky: The right image has had +24 units of Vibrance added to the sky via a Vibrance adjustment layer (Layer > Adjustment Layer > Vibrance), using the previously saved sky selection. The Vibrance adjustment also has a Saturation slider to add saturation to the image, but I advise you to use it very lightly (never more than about +12 units). Over applying saturation to your image can be very destructive, as it removes black and lowers image contrast. I usually add only a little saturation to intensify the yellow-orange-red areas of the image. Vibrance is less destructive to your image and has more of an effect on the blue-magenta areas of your image. Even though adding vibrance is less destructive (and less garish) than adding saturation, I suggest you don't add much more that +32 units.
Distortion correction: Many of you might not notice the "keystone" distortion in the left image, but because the barn is an architectural feature, its distortion is more noticeable to some people. When we photograph objects, like buildings, that have parallel lines, those lines converge at some infinity point much faster when we use wide angle lenses, especially when we lean or point the camera upward to include more of our sky. We can correct that distortion with Photoshop.
The barn image on the right has had about 80% of that keystone distortion removed using "Edit > Transform > Distort." Doing this distortion correction correctly can cause some cropping to your image (notice the cropping out the the bright star a the top left). To prevent even more cropping of your image, you can employ "Content-Aware" to fill in areas that would normally have to be cropped out (see image below). More help on this can be found on my blog.
