A quick project, before sky conditions started to deteriorate due to clouds, rain, jetstream and the full moon. I captured some LRGB data of M 27 through gaps in the clouds with the C14 Hyperstar & ZWO ASI 183 MM Pro and combined it with Ha and OIII data from 2021. Processing the core and fainter stuff in the surrounding area was quite challenging. Especially the features in the bright core can be easily overprocessed, so I kept the application of contrast enhancement methods to a minimum. First time using SPCC and GHS, which are quite amazing tools btw.

Equipment

ZWO ASI 183 MM Pro @ -15°C
+ C14 & Starizona Hyperstar v2 @ 675 mm (f/1.9)
+ Baader 2" LRGB filters
+ Baader 2" Ha/OIII Highspeed filters (old versions)
+ Celestron EF
+ Tempest Cooler
+ Omegon Dew Heater
+ Astrozap Dew Shield for C 14 Hyperstar
on CGE-Pro mount

Cable managment to mitigate Hyperstar diffraction spikes.

Autoguiding:
Artesky 70 mm Guidscope (f=400 mm)
ZWO ASI 120 MM mini
PhD2 (Multistarguiding)
Dither every 4 subs

light frames:
LRGB (gain=75, offset=15)
Ha/OIII (gain=139, offset=21)

acquired with N.I.N.A.

Average FWHM: 1.6" - 2.7"

Remaining Issues
- requires more HaOIIILRGB data to enhance the fainter Ha/OIII features in the vicinity of the nebula
- add more high resolution (LI) data < 2"

Post-Processing (PixInsight)

For further details regarding the combination of narrowband with LRGB data, I refer to theAdvanced Narrowband Combination tutorial by Charles Hagen

HaOIIILRGB (linear stage)

- automatic rejection of subs with the subframe selector based on number of stars, FWHM and eccentricity
- manual rejection of bad subs
- image calibration (darks only) & image registration with WBPP script
- NSG & image integration for each channel
- dynamic crop
- create undistorted, synthetic star reference frame with Script -> Render -> CatalogStarGenerator
- Star Alignment using Thin Plate Splines and Distortion Correction
- DBE of each channel
- BXT
- linear fit of L, Ha, OIII, R & G channel wrt B channel

Ha & OIII Continuum Subtraction with the Synthetic Color Flow Method (linear & non-linear stage)

- Create HaRR image with ChannelCombination, Background Neutralization, Color Calibration, remove stars with SXT, subtract continuum using PixelMath:
RGB/k $T[0] - ($T[1] - med( $T[1])), create new grayscale image Ha_clean
- Create OIIIGG image with ChannelCombination, Background Neutralization, Color Calibration, remove stars with SXT, subtract continuum using PixelMath:
RGB/k $T[0] - ($T[1] - med( $T[1])), create new grayscale image OIII_clean
- GHS Ha_clean & OIII_clean

RGB (linear & non-linear stage)

- RGB channel combination
- DBE
- Script -> Image Analysis -> Image Solver
- SPCC (White reference: Average Spiral Galaxy)
- GHS
- remove red tint using histogram transformation
- extract RGB stars with SXT
- Apply SCNR (Color to remove: Green, Protection Method: Average Neutral, Amount: 1.0) to RGB stars

LHaOIII (linear & non-linear stage)

- remove stars from L image using SXT
- add continuum substracted Ha data using PixelMath: RGB/k HaL = L + B*(Ha_Clean-med(Ha_Clean)) with B=1
- add continuum substracted OIII data using PixelMath: RGB/k HaOIIIL = HaL + B*(OIII_Clean-med(OIII_Clean)) with B=0.5
- GHS

LRGB (non-linear stage)

- color saturation enhancement using ColorSaturation
- LRGB combination of L and RGB data

HaOIIILRGB (linear & non-linear stage)

- apply Histogram Transformation to Ha_clean and OIII_clean images to clip sky background
- add Ha to LRGB image using PixelMath

// R:
$T * ~R + R * mtf(~m, (mtf(m, $T) + mtf(m, Ha_clean)))

// G:
$T * ~G + G * mtf(~m, (mtf(m, $T) + mtf(m, Ha_clean)))

// B:
$T * ~B + B * mtf(~m, (mtf(m, $T) + mtf(m, Ha_clean)))

// Symbols:
R = 0.6,
G = 0.0,
B = 0.05,
m = 0.999,

- add OIII to HaLRGB image using the same PixelMath as for Ha but with

// R:
$T * ~R + R * mtf(~m, (mtf(m, $T) + mtf(m, OIII_clean)))

// G:
$T * ~G + G * mtf(~m, (mtf(m, $T) + mtf(m, OIII_clean)))

// B:
$T * ~B + B * mtf(~m, (mtf(m, $T) + mtf(m, OIII_clean)))

// Symbols:
R = 0.0,
G = 0.4,
B = 1.0,
m = 0.999,

Enhancement of faint Ha features in M 27

To increase the pink color of the faint Ha features within the core of M27 ...

- Create a lum mask based on the Ha_clean image with strongly clipped black point, so only the Ha features within M27 remain visible.
- add Ha mask to HaOIIILRGB image
- add Ha data to HaOIIILRGB image with channel weights:

// Symbols:
R = 1.0,
G = 0.0,
B = 0.05,
m = 0.999,

Further contrast enhancements within  the M 27 core ...

- HDRMT (number of layers:7, scaling function: Gaussian(7), To lightness, Lightness mask)
- color saturation enhancement using ColorSaturation
- remove green tint using histogram transformation
- create range mask
- invert range mask
- desaturation & darkening of the background using curves transformations
- remove range mask
- NXT (Denoise: 0.8)
- MMT - Bias = 0.05 on layer 3 & 0.025 on layer 4
- LocalHistogramEqualization (Kernel Radius: 126, Amount: 0.2, Histogram Resolution 12-bit)
- Blend script: create new image Soft_light = Softlight of HaOIIILRGB with itself, AutoSTF = off, Opacity = 50%
- apply luminance mask to HaOIIILRGB
- create new image HaOIIILRGB_softlight = combination of Soft_light with luminance masked HaOIIILRGB image using PixelMath
- remove lum mask
- NXT (Denoise: 0.5)

Adding stars to starless image

- add RGB stars with pixel math combine(HaOIIILRGB,stars,op_screen())
- create star mask by exctracting L channel from RGB stars
- add star mask
- enhance star color using Color Saturation

I applied this workflow twice, using two different GHS for the core and surrounding fainter parts, which were later combined into one image using a range mask and PixelMath.

Location: Schwentinental (Germany)

- CS Daniel