Walton, OR and Gold Canyon, AZ
October 2 and 3, October 29 – November 4, 2021
Reprocessed 01/30/2023

Original image (2021):

I started this project in Oregon at the Linslaw Point dark site and finished it in Gold Canyon Arizona.  This was the first test of my Losmandy G11 mount using a Periodic Error Correction curve generated with the PEMPro application.  Guiding was improved with an average RMS (standard deviation) error in PHD2 between 0.4 and 0.6 arcseconds.  RMS error was highly dependent on environmental conditions, especially seeing.  SH2-131 was the first of two or three targets imaged each night.  This effort was also an experiment in collecting a large number of subframes and selecting the best of the Hydrogen-, Oxygen-III and Sulfur-II narrowband filter data in order to improve image quality.  The best 50 to 60% of the Hydrogen-, Oxygen-III and Sulfur-II 7nm narrowband filter data were combined using the Hubble pallet by assigning S-II to the red, H (and about 20% O-III) to the green, and O-III to the blue channel.  I also used the new GradientScaleNormalization script, which worked very well as the scope tracked from reasonably dark skies in the east to light polluted skies in the west toward Mesa.  I struggled with the Starnet tool in PixInsight to create starless images for processing.  Instead, I tried an evaluation copy of Russ Croman’s StarXTerminator tool in PixInsight to remove the stars so that the nebula could be processed separately from the star field.  StarXTerminator worked exceedingly well on the combined and stretched color image.  Starless and stars images were processed separately in PixInsight, and then were combined to produce the final image.

Reprocessing (2023):

Since I had good results with the Needle Galaxy using my current workflow and new PixInsight tools, I decided to reprocess an emission nebula.  Also, I wanted to create a more colorful and detailed rendition of SH2-131, the Elephant Trunk.  After running the WBPP script, I created an RGB false color image using the standard Hubble pallet (SHO).  Next, I tried Spectrophotometric color calibration (SPCC) for nebulae, concentrating on the stars.  Some of the green was removed with the SCNR tool.  BlurXTerminator was used to deconvolve both color and extracted luminance images.  StarXTerminator was applied to remove the stars in the linear state.  RGB and luminance starless images were denoised with NoiseXTerminator.  One issue arose in the stars image.  There were halos and diffraction spikes around the brighter stars.  They adversely affected the appearance in the combined image, so I tried the ReduceStarHalos script for each bright star and then used the CloneStamp tool to smooth things out.  Stars and starless images were stretched with the GeneralizedHyperbolicStrecth tool.  The stars were lightly saturated.  The nebula luminance image was sharpened with the MultiscaleLinearTransform tool, and the contrast was enhanced with the HistogramEqualization tool run with three circular kernel sizes (356 with 12 bit resolution, 128 with 10 bits, 64 with 8 bits).  The RGB starless nebula image was blurred, and then the sharpened luminance was combined with the RGB color image.

Next, the fun started with LRGB nebula image color and saturation adjustments.  Previously, I created a set of color masks using the information presented by Adam Block that described an error in the ColorMask script.  I centered color masks on different colors, red, orange, yellow, violet, etc., and varied the color ranges.  Convolution was applied to the masks to blur them.  For each mask applied to the RGB image, the Curves and HistogramTransformation tools were used to adjust the colors and saturation in the unprotected regions.  I learned to make subtle changes so that mask boundaries did not appear.  PixelMath was used to combine the corrected stars image with the nebula.  Photoshop was used to fine tune and crop the combined image.

Description:

The Elephant Trunk nebula is in a region rich in gas and dust in Cepheus and is a small part of a very large emission nebula spanning three degrees of the sky.  The elephant’s trunk is many hundreds of light years across and is about 3,000 light-years distant.  The edge of the dark trunk is illuminated by ionized gas receiving radiation from the luminous blue star HD 206267.  This star forming region has several newborn stars discovered in 2003 that are less than 10,000 years old (Skysafari Pro).

Imaging details:

Celestron 9.25" Edge HD SCT
Celestron 0.7x Focal Reducer (FL = 1645mm, f/7)
Celestron off-axis guider with a ZWO ASI 174MM mini guide camera
Losmandy G11 mount with Gemini 2
ZWO ASI 2600MM Pro cooled monochrome camera (-10C)
ZWO 36mm Hydrogen-, Oxygen-III and Sulfur-II filters
Equatorial camera rotation: 340 deg

Software:    Sequence Generator Pro, ASTAP plate solving, PHD2 guiding, 
    Losmandy Gemini ASCOM mount control and web client interface,
    SharpCap Pro for polar alignment with the Polemaster camera,
    PixInsight 1.8.8-10, 1.8.9-1 with StarXTerminator, NoiseXTerminator and BlurXTerminator
    Photoshop CC 2022/2023

Hydrogen-a    10 min x 36 subframes (360 min), Gain 100, Offset 68, 1x1 binning
Oxygen-III    10 min x 33 subframes (330 min), Gain 100, Offset 68, 1x1 binning
Sulfur-II      10 min x 39 subframes (390 min), Gain 100, Offset 68, 1x1 binning