Sharpless 2-88

Sharpless 2-88Click image for full size version

September 17, 2019

This image shows Sharpless 2-88, an emission nebula in Vulpecula (the Little Fox).  This nebula lies about 6,500 light years away. Some of my friends call these “shapeless” objects, rather than Sharpless objects. I like them because they are a bit off the beaten track. This object is about 1.5 million years old and a mass about 1300 times that of the Sun. This nebula glows from the energy emitted by Vulpecula OB1 association, a group of hot, young stars in the region. This nebula is very close to Sh2-88. In fact, both of these objects’ images came from the same single wide field master image.

Tekkies: 

Takahashi FSQ-106 ED IV @ f/3.6, QHY367C one-shot colour camera, and Optolong L-eNhance narrowband filter, Paramount MX, unguided. Acquisition, and focusing with TheSkyX. Focus with Optec DirectSync focus motor and controller. Automation with CCDCommander. Equipment control with PrimaLuce Labs Eagle 3 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Waning crescent Moon, average to above average transparency and fair to average seeing. Data acquired August 5-10, 2019.

118x10m with Optolong L-eNhance filter (Total = 19hr40m)
Image scale 1.3 arcsec per pixel

Data Reduction and Cleanup
Preprocessing: The BatchPreProcessing script was used to perform calibration, debayering, and registration of all frames. ImageIntegration followed by DrizzleIntegration with the CFA Drizzle option was used to make the master, which was then cropped.

Gradient Removal: Gradient removal was not necessary.

Channel Registration:  To improve channel registration, the RGB colour channels were extracted and aligned with ImageIntegration, using 2D Surface Splines with Distortion Correction and the Green channel as the reference frame. The registered colour channels were recombined with RGBCombination.

Luminance Extraction:  RGBWorkingSpace was applied to normalize the channels, and the Luminance was extracted for separate processing as described below.

Colour
Colour Balancing:  The colour image was colour balanced with ColorCalibration (note; this did not correct star colours, since this image used a narrowband filter)

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold and strength: Layer 1: 5.0 0.75   Layer 2: 3.0, 0.70  Layer 3: 2.0, 0.6  Layer 4: 1.0, 0.2.

Stretching: HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Luminance
Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold and strength: Layer 1: 3.0 0.9   Layer 2: 2.0, 0.75  Layer 3: 1.0, 0.6  Layer 4: 0.5, 0.2.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Recombining Luminance and RGB
LRGB Combination: The luminance image was applied to the RGB image using LRGBCombine with default settings.

Additional Processing
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to target the background areas and protect the stars (max. 1000 iterations and convergence selected for both lightness and chrominance). 

Contrast Enhancement: LocalHistogramEqualization was applied twice (scales 50 with strength 0.42, followed by scale 150 with strength 0.2) using a mask to protect stars and low-signal regions of the image.

Final Steps: Background, nebula and star brightness, contrast, hue and colour saturation were adjusted in several iterations using CurvesTransformation with masks as required. The DarkStructureEnhance script was applied with a strength of 0.2 and 8 layers.