The Fishhead Nebula in Cassiopeia
IC 1795 is the “blood drop” at the bottom of the Heart Nebula.
Exposure: Total exposure time about 25.7 hours, 30:24:23 SII:Ha:OIII. All bin 1x1. Captured in October and November 2017.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of subs about 2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% reduction)
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
Image Integration
Mure Denoise
3. Linear Processing
Dynamic Crop
Dynamic Background Extraction
Deconvolution with Dynamic PSF and star mask (on Ha only)
4. Stretching
Histo Trans
Curves Trans
Noise Reduction following David Ault’s Astro Imaging Channel tutorial:
TGV Denoise
Aggressive Multiscale Median Transform to remove lumpiness in background, using an inverted and blurred range mask to protect highlights
5. Color merge
Merged narrowband channels in PixelMath, mapping SII:Ha:OIII to R:G:B
SCNR to shift green to yellow
Curves Trans to increase color saturation
Local Histogram Equalization in bright areas to enhance contrast
6. Final
Remove magenta star halos and casts in corners:
Invert image
SCNR on green
Re-invert image
Mild Curves Transformation
Multiscale Median Transform to sharpen the dark lane, using a layer mask in Photoshop
Downsize 50% and save as JPG
