Thursday, July 7, 2016

Jupiter rising over the Chesapeake Bay, January 11, 2016

Canon 550D, Rokinon 16mm f/2.0 lens @ f/2.0. Exposure: 20 sec @ ISO3200. January 11, 2016, 11:54 p.m.
Here is an image of Jupiter rising over the Chesapeake Bay taken this past January at Bethel Beach Natural Area Preserve, Virginia. My, how things have changed since then! Jupiter was rising around 10 p.m. in January and now is setting in the evening after giving amateur astronomers a marvelous show this spring. During this apparition I witnessed a Jovian moon transit, two shadow transits and had enjoyed my best views of the Great Red Spot. Now we can look forward to Juno’s discoveries over the coming months!

This was my first night out with my Rokinon 16mm f/2.0 lens and mainly I wanted to test how long I could expose on a stationary tripod without excessive star trailing. The 20 second exposure roughly follows the “rule of 200” for a 16mm lens on a crop sensor. Star trailing is apparent when mildly zooming in on this image, but it is not quite objectionable without zooming. Still, I am anxious to try this out with my Vixen Polarie Star Tracker at 1/2 sidereal rate.

For processing, I retained a custom white balance and made no effort to correct for the orangish-yellow cast due to Hampton Roads light pollution which illuminates the scene. I used a mask to select the sky and foreground separately, though this mask did not select these two elements exclusively (selecting the sky still selected about 7% of the foreground). I think this helped make the transition from foreground to sky less artificial due to the differences in processing the two. Most of the heavy lifting for noise reduction was completed by TGVDenoise and MultiscaleMedianTransform to chrominance, but I really liked how Manfred Schwarz’s RemoveDarkPixel and RemoveHotPixel PixelMath expressions cleaned up the salt and pepper noise due to the high ISO I used.

Processing Workflow (PixInsight)
1. Correct hot pixels, save .CR2 as .xisf (CosmeticCorrection, CFA, Auto Detect, Hot Sigma 1.3)
2. Apply in-camera custom white balance
2a. Using exifftool, extract Red_Balance, Blue_Balance and Average_Black_Level from .CR2 metadata. Since PixInsight reads the .xisf file as 16-bit, divide each of these values by 65536.
2b. Open .xisf image from step 1 as Pure Raw.
2c. Scale the image to account for the black level by applying PixelMath expression: ($T - Black_Level)/(Saturation - Black_Level). Here, I assume Saturation is the maximum 14-bit value (0.25 after dividing by 65536).
2d. SplitCFA
2e. Apply PixelMath expression to CFA0: CFA0*Red_Balance
2f.  Apply PixelMath expression to CFA3: CFA3*Blue_Balance
2g. MergeCFA
3. Non-linear stretch (MaskedStretch).
4. Reduce noise (TGVDenoise in CIE L*a*b* mode, lightness edge protection 1e-4, chrominance strength 3 and edge protection 2e-3, iterations 200, local support extracted L image; applied w/ inverted lightness mask).
5. Increase image brightness (CurvesTransformation to RGB/K, lower midtones slider).
6. Create mask to select sky by extracting blue channel, convolving, clone-stamping out stars and then convolving again.
7. Contrast curve in the sky (CurvesTransformation w/mask selecting sky).
8. Neutralize sky at the very top of the image (BackgroundNeutralization using preview at top; w/mask selecting sky)
9. Darken shadows in sky (CurvesTransformation to RGB/K w/mask selecting sky).
10. Increase saturation (CurvesTransformation, C channel).
11. Large scale chrominance noise reduction (MultiscaleMedianTransform to Chrominance using David Ault’s recipe [reference below] w/inverted luminance mask).
12. Increase contrast in foreground shadows between the plants (CurvesTransformation w/mask selecting foreground).
13. Reduce dark speckle noise (PixelMath with RemoveDarkPixel expression by Manfred Schwarz [reference below]).
14. Reduce hot speckle noise (PixelMath with RemoveHotPixel expression by Manfred Schwarz [reference below]).
15. Sharpen image (MultiscaleMedianTransform, bias +0.03 to layer 4 of 5).
16. Crop to 5x7 aspect ratio (DynamicCrop).
17. Resample by 50% for web (Resample).

“M42 PixInsight Tutorial” by David Ault for linear noise reduction

Manfred Schwarz’ galaxy tutorial:

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