The Hourglass Nebula in Cygnus

Sharpless 2-106 is a bipolar emission nebula located 2,000 light years away in the hydrogen-rich clouds of the Cygnus Milky Way. Hidden in the band of dust crossing the center of the nebula is S106 IRS 4, a 15 solar mass star formed 100,000 years ago. Twin lobes of superheated gas stretch outward from the central star. The ring of dust and gas orbiting the star acts like a belt, cinching the expanding nebula into the hourglass shape. Hundreds of low mass brown dwarfs have also been detected in the nebula. This is a bicolor narrowband image, with RGB added for the brighter stars. Sh2-106 was mapped as H:O:O for R:G:B. For the surrounding nebulosity Ha was used for luminance, and HOO for color.

Exposure: Total exposure time about 12.7 hours, 16:19 x 20 mins Ha:OIII, 14:9:9 x 2 mins R:G:B. All bin 1x1. Captured from June to October 2018.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of best subs about 1.9 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 0.75 arcsecs/pixel (reduced to 80% of original scale)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon 5nm Ha/SII, 3nm OIII, Type IIe LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.0.0.8
Guide SW: PHD2.6.3, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2
Processing Workflow by Workspace in PixInsight 1.85

1. Calibration
BatchPreProcessing with flats, darks and bias, using Cosmetic Correction with master dark

2. Stack and Mure Denoise
Blink to preview and reject a few frames
Subframe Selector to confirm selections and weight by FWHM and SNR (with this data, I found it essential to weight before registration to get accurate SNR and FWHM estimates)
Local Normalization for Ha and OIII frames
Image Integration
Mure Denoise
RGB Combination for RGB frames
Dynamic Crop

3. Narrowband Linear Processing
Dynamic Background Extraction
Deconvolution with Dynamic PSF and star mask (on Ha core only)

4. Narrowband Stretching
Histo Trans
Masked Stretch
TGV Denoise

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration
Deconvolution (parametric) of R and B channels to match FWHM of G image

6. RGB Stretching
Histo Trans
Curves Trans

7. Color Combination
Merged narrowband channels in PixelMath, mapping Ha:OIII:OIII to R:G:B
Multiscale Linear Transform with bias to sharpen core
Curve Trans to bring up background, masking off core
Aggressive Multiscale Median Transform to remove lumpiness in background, using an inverted and blurred range mask to protect highlights

Create second (HaHOO) image for surrounding nebulosity:
LRGB Combination using Ha as luminance on the color narrowband image
Curves Transformation (masking off the core) to brighten nebulosity
Mild (radius 128, contrast 1.5) Local Histogram Equalization (masking off the core) to enhance contrast

8. Star Removal
Stars were reduced in the HaHOO image using a single iteration of Morphological Transformation and a star mask
Stars were then replaced with the RGB image using PixelMath’s max command and the star mask