M76, The Little Dumbbell Nebula

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September 25, 2023 

This is M76, also known as the Little Dumbbell nebula, and it is the faintest object in the Messier catalogue (but still visible in binoculars in a dark sky). It’s nickname is a nod to the bigger, brighter showpiece object, the Dumbbell Nebula (M27). Both of these planetary nebulae glow because gas thrown off by a dying central star is now absorbing the dying star’s energy and re-emitting it as visible light.  Hydrogen accounts for the red; oxygen for the blue-green. M76 is located in the constellation Perseus around 2,500 light years distant. Nebulae of this type dissipate over a period of thousands of years. The term “planetary nebula” has nothing to do with planets. Early telescope observers saw these objects as being disc-like, sort of like planets, and gave them this name. I have imaged M76 previously, but always thought I could do better with the addition of narrowband data. That, and a longer focal length, are the reason the new image shows much more detail in the core, and faint extensions on both ends.

Tekkies:
Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX and PHD2. Primalucelab low-profile 2″ Esatto focuser and ARCO rotator. Equipment control with PrimaLuce Labs Eagle 4 Pro. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Below-average transparency and above average seeing. Data acquired September 23-24, 2023 under a nearly moonless sky.

Celestron 14″ F/11 EDGE HD telescope at f/11 (3,912 mm focal length)  and QHY600M camera binned 2×2 with Optolong filters.

10x5m Red = 0hr50m
10x5m Green = 0hr50m
12x5m Blue = 1hr00m
29x5m Ha = 2hr25m
36x5m O3 =3hr00m

Total: 8hr15m

Image scale 0.4 arcsec per pixel
 
Finder Chart: 
  
 
 
Data Reduction and Linear Processing

Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization and integration of all frames.

Colour master: A colour master was made from the Red, Green and Blue masters using ChannelCombination in RGB mode.

Gradient Removal: DynamicBackgroundExtraction was applied to the RGB, Ha and O3 masters.

Colour Calibration:  SpectrophotometricColorCalibration was used to calibrate the RGB master.

Deconvolution: BlurXterminator was used on the Ha, O3 and RGB masters with Automatic psf , and star sharpening set to 0.10 with “Correct First” selected. The same settings were used with Automatic PSF on the Color image.

Addition of Ha and O3: The NBRGBCombination script was used to combine the Ha and O3 with the RGB using default settings except for NB bandpass (3nm for both Ha and O3).

Linear Noise Reduction: NoiseXterminator was applied to the NBRGB image with settings Amount=0.9 and Detail=0.15

Star Removal:  StarXterminator was used to remove the stars, with default settings.

Stretching:  HistogramTransformation was applied to the NBRGB master to make a pleasing image. Approximate background level after stretch was 0.10.

Nonlinear Processing
Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the image with settings Amount=0.9 and Detail=0.2

Re-stretch: HistogramTransformation was used to boost contrast by moving the dark point to the toe of the histogram and slightly decreasing the mid-point slider.

Contrast Enhancement: HDRMultiscaleTransform was applied twice (scale 6 and 4) using a mask to select just the core of the nebula. LocalHistogramEqualization was applied twice using a mask to select the entire nebula.  A Contrast Limit of 1.5 and 1 iteration was used for each LHE application (scale 40, strength 0.5; scale 90, strength 0.5; scale 150, strength 0.5).

Sharpening: The mask used for LHE was used for sharpening with MultiscaleMedianTransform (Layers 1 – 5 with strengths of 0.01, 0.03, 0.05, 0.05, and 0.01, respectively).

Stars-only steps: MorphologicalTransformation was used in Morphological Selection mode to shrink the stars through a mask made by extracting the Luminance from the stars-only image. CurvesTransformation was used to add saturation to the stars. 

Star Restoration: PixelMath expression combine(starless, stars, op_screen())  was used to combine the starless and stars-only images created with StarXterminator.

Final Steps: Background, nebula and star brightness, contrast and saturation were adjusted in several iterations using CurvesTransformation with masks as required. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.  The finder chart was made using the FindingChart process.