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The Cygnus Loop - A 41 Hour, 4 Panel HOO Mosaic | by PR^photography
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The Cygnus Loop - A 41 Hour, 4 Panel HOO Mosaic

Well, my single most ambitious astrophotography project to date is finally complete and I am VERY pleased with how it came out. It's my first time doing a mosaic as well as my first time shooting over multiple nights as well as my first time getting over 10 hours of exposure time on a single target. Whew. This is gonna be a long write up.


The Cygnus Loop is a supernova remnant - left behind long after a star exploded. It leaves behind a beautiful burst of colors and shapes, including the Eastern and Western Veil Nebulae and Pickering's Triangle.


This image is a 4-panel HOO bicolor mosaic totaling 41 hours of exposure between all 4 panels.





- Scope: AstroTech AT65EDQ Quad APO

- Mount: Rowan Belt Modded Orion Sirius EQ-G

- Imaging Camera: ZWO ASI1600mm-Pro

- Guidescope: QHY MiniGuideScope

- Guide Cam: QHY5L-II Mono



- EQMOD/StellariumScope for mount control

- APT for image capture and platesolving

- PHD2 for guiding

- PixInsight for Processing





- Location: Sugar Land, TX

- Gain: 200; Offset: 50; Camera @ -10C

- 248 x 300" Ha - Chroma 5nm

- 244 x 300" OIII - Chroma 3nm

- Nights: 6/5/20, 6/6/20, 6/10/20, 6/11/20, 6/12/20, 6/13/20, 6/15/20, 6/16/20, 6/17/20


Total Integration Time: 41 Hours




Pre-Processing of each Panel (Apply to each Ha and OIII master panel)

- Integration of all panels

- DynamicCrop to get rid of edges

- DBE on masters




Initial Processing (done on each Ha and OIII Master Panel)

- Deconvolution via Oke's tutorial

- TGV+MMT via Jon Rista's Tutorial




Mosaic Process for Each set of 4 masters per Filter

- Run each master through ImageSolver to assign coordinates w/ distortion model generated

- MosaicByCoordinates script to register masters for mosaic

- dnaLinearFit to match background values of registered images

- GradientMergeMosaic to make mosaic for each filter




Processing of each Mosaic

- HistogramTransformation to Stretch

- HistogramTransformation to bring down background levels

- LocalHistogramEqualization to bring out nebula details w/ rangemask




Combining the Ha and OIII Mosaics

- DynamicCrop to remove mosaic edges from each filter mosaic

- StarAlignment to align both mosaics

- Combine mosaics into color via PixelMath with the following combination:


- R: iif(ha>.15,ha,(ha*.8)+(oiii*.2))

- G: iif(ha>0.5,1-(1-oiii)*(1-(ha-0.5)),oiii*(ha+0.5))

- B: iif(oiii>.1,oiii,(ha*.3)+(oiii*.2))




Further Processing

- DynamicCrop to remove edges

- invert -> SCNR -> invert to remove magenta tint

- ABE to remove background gradient

- CurvesTransformation on hue to change red hue

- CurvesTransformation on red to bring down red tone

- HistogramTransformation to reduce background further

- CurvesTransformation for contrast

- ColorSaturation w/ rangemask to deepen blues

- CurvesTransformation w/ RangeMask to darken background further

- StarNet++ for StarMask

- Binarize and Convolution on StarMask

- MorphologicalTransformation to reduce stars

- Export to Photoshop to manually take care of seams

- Save and Export

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Taken on August 1, 2020