Last week I set out to revisit NGC 7000 the North American Nebula. It was a hot day with temperatures around 100 degrees Fahrenheit, but it cooled pretty quickly into the upper 70s by the evening. I wanted to take longer subs for this shot to see if I could really capture the nebulosity of this beautiful target.
I removed my Sky-Watcher AZ-GTi from its stock tripod and moved it over to a wood and fiberglass surveying tripod I had from when I used a Sky-Watcher Star Adventurer as my main mount. This tripod is built like a rock, so I was hopeful it would provide some added stability while taking longer exposures.
I setup during the day so I could take my time and ensure everything was set just right and double check every screw or nob was camped down tight. My plan was to shoot two hours of 120 mins sub-frames with the camera cooled to 0 degrees Celsius. I wish I could bring that temperature down further, but I think it puts too much strain on the cooler during these hot evenings.
Once the sun had set, I adjusted the focus and polar aligned the scope. I set NGC 7000 as my target in my ASIAIR Pro and let the telescope slew to the nebula. I took a preview shot and noticed yet again the most interesting part of the North American Nebula was out of frame. So I manually adjusted the positioning to make sure the Cygnus Wall was included in the shot.
I set everything to autorun and told the ASIAIR to return to home position when complete and to shut down. This allowed me to go inside and get some much-needed sleep. I went outside the next morning and took flat frames and brought everything inside. I had already created a master bias frame and had the appropriate master dark frame in my library, so I did not need to reproduce this data.
I think the results were outstanding!
Post-Processing:
I stacked the 60 lights, 30 flats, master dark, and master bias in PixInsight using the WBPP script. Once completed, I completed a dynamic background extraction and color calibration on the image. I auto stretched the image using the screen transfer function and the histogram transformation tool and saved the current image as a tif file.
This is where my processing diverged from it usual path. I opened the tif file in Photoshop and tried a new plug-in filter called StarXTerminator. This does the same process as StarNet++ of extracting a starless layer from the image. However, unlike StarNet++ this is done in Photoshop and in my opinion does a much better job in a fraction of the time. There were none of the typical color artifacts which needs cleaning with a healing brush that I encounter with StarNet++. I’ll post a full write-up and review of StarXTerminator after I do some more testing.
I created a starless layer and star layer and saved each as a tif file and reopened them in PixInsight. With the starless layer selected, I split the RGB channels and deleted the blue channel. I used the histogram transformation tool to match the green channel as best as possible to the red channel then combined them in PixelMath using the formula r*.6+g*.4 to create a new blue channel. These were then merged in the LRGB Combination tool with L=r, R=r, G=b, and B=g. I set the saturation to .200 and check chrominance noise reduction.
From there I adjusted the blue and red channels of the new RGB image in the curves transformation tool. For both channels I increased the mid-tones and lowered the shadows. I applied the correct magenta stars script to fix the magenta hue in the blue parts of the image. I would usually run SCNR again at this point, but it seemed to overwhelm this image, so I hit undo and left the color balance where it was. Next, I created a range mask between the highlights and shadows and lowered the luminance & saturation in the shadows and boosted both them in the highlights.
I created a new range mask with only the absolute brightest parts of the image and applied an unsharp mask at 2.0 standard deviations to add structure. I ran the dark structure enhance script to add some more punch to the structure of the image. Lastly on this layer, I ran ACDNR with a light mask at .180 mid-tones at 1.5 standard deviations for some slight smoothing of noise.
Moving to the star layer I applied SCNR at 1.0 and upped the saturation with a curves transformation adjustment. I reduced the stars intensity with a morphological transformation of .5. Finally, I combined the starless and star layers with PixelMath. I still am not satisfied with the lack of color in the stars, but I think that has more to do with the length of subs with my ASI533MC. Next imaging session I plan to take a second set of light frames with a greatly decreased exposure time so I can use the star layer from that stack to combine in PixelMath with the starless layer.
I saved the image as a tif and opened it n Lightroom where I applied sharpening at 25 with masking at 80, and noise reduction at 15 with 30 contrast and 30 color contrast.
Acquisition Details:
•Imaging Scope: Apertura 60EDR
•Imaging Camera: ZWO ASI533MC
•Filter: 2” Optolong L-eNhance
•Mount: Sky-Watcher AZ-GTi (EQMod)
•Guide Scope: ZWO 30F4 Mini Scope
•Guide Camera: ZWO ASI120MM-Mini
•Lights: 60x120s, Gain 100, Temp 0C
•Flats: 30 x Auto
•Darks & Biases from master file
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