Due to the lack of clear nights, I reprocessed some old C14 Hyperstar HaLRGB data of galaxy M 33 captured in October 2021 with some modern processing tools. Beside the usual suspects, BXT and SXT, I also tried GraXpert to remove gradients in my data. Quite impressive how the KI is able to remove gradients with one click of a button. Because I typically don't take flats in C14 Hyperstar mode, the most obvious gradient in the stacked data is due to vignetting. Revision 1 shows an autostrech of the stacked L data before and Revision 2 the autostrech after the application of GraXpert - quite impressive result. Due to the short integration time of ~ 15 min / RGB channel, 42 min L and 72 min Ha data, I did not apply NXT or any other noise suppression due the unnatural look of the resulting galaxy background. For some reason, I also lost a Ha region during processing, which appears as artifical blue cloud at the right edge of the galaxy.

The stacked master files can be downloaded here

Equipment

ZWO ASI 1600 MM Pro @ -20°C
+ C14 & Starizona Hyperstar v2 @ 675 mm (f/1.9)
+ Baader 2" LRGB filters
+ Baader 2" Ha 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 (gain=139, offset=21)

acquired with N.I.N.A.

Average FWHM: 2.9" - 3.3"

Remaining Issues
- requires more HaLRGB data to enhance the fainter Ha regions and resolve more stars in the galaxy
- add more high resolution (LI) data using the ZWO ASI 183 MM with FWHM <= 2"

Post-Processing (PixInsight)

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

HaLRGB (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
- image integration for each channel
- dynamic crop
- Star Alignment using Thin Plate Splines and Distortion Correction
- DBE of each channel using GraXpert (Subtraction of backgroudn)
- BXT (AI version 4, sharpen stars= 0.33, Adjust star halos = 0.0, Automatic PSF, Sharpen Nonstellar = 0.53, LuminanceOnly)
- linear fit of L, Ha, R, G & B channels wrt Ha channel

Ha 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
- GHS Ha_clean

RGB (linear & non-linear stage)

- RGB channel combination
- Script -> Image Analysis -> Image Solver
- SPCC (White reference: Average Spiral Galaxy)
- GHS
- extract RGB stars with SXT

HaRGB (non-linear stage)

- apply Histogram Transformation to Ha_clean image to clip sky background
- add Ha_clean to RGB image using the "Combine Ha with RGB" script by Jürgen Terpe with Amount = 0.3

LHa (linear & non-linear stage)

- remove stars from L image using SXT
- add continuum substracted Ha data using PixelMath: RGB/k LHa = L + B*(Ha_Clean-med(Ha_Clean)) with B=0.25
- Masked stretch of LHa using default parameters

HaLRGB (non-linear stage)

- color saturation enhancement of HaRGB using ColorSaturation
- HaLRGB combination of LHa and HaRGB data
- add RGB stars back into HaLRGB image with pixel math combine(HaLRGB,stars,op_screen())

Further contrast enhancements ...

- curves transformation
- create lum mask
- color saturation enhancement of the galaxy using ColorSaturation
- invert lum mask
- desaturation & darkening of the background using curves transformations
- invert lum mask
- color saturation enhancement of the galaxy using ColorSaturation
- remove lum mask
- slight color corrections using histogram transformation
- definition of final black point with histogram transformation
- Dark Structure Enhance script (Amount = 0.2)

Location: Schwentinental (Germany)

- CS Daniel