For myself and many city-bound photographers, Cygnus is one of the most popular Summer constellations due to the presence of many bright emission nebulae. It's ideal for narrow band imaging from our light-polluted homes.

My earlier collaboration on the Tendrils of Cygnus photo laid the foundation for a very ambitious follow-up project: a huge 12-panel narrowband survey of the entire Cygnus constellation. I planned this mosaic with the tools on telescopius.com 
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About the image
The final image is presented at 50% resolution and cropped to 39 megapixels — Highly recommended to zoom in and explore!!

There is so much interesting stuff in this shot that we rarely see in detailed telescopic observations. I’ve included an annotated version that can be used as a reference, or just to find objects that may be hidden in the background. 

Revision D was processed without any tone mapping and an alternate color mapping that makes it easier to see certain hues. Planetary Nebula are labeled in green, Sharpless objects (HII regions) in purple, NGC objects in light red, and Messier objects in blue.

5 years of astrophotography
I have been photographing objects in this field with both my APO refractor and big Newtonian telescope since I bought my ZWO ASI1600MM-C camera and first filter set in back in June 2016. A compilation photo of my adventures really puts things in perspective. 



Narrowband astrophotos in Cygnus (2016 to 2021) - 88 hours of combined exposure time

Technical
Work on the 12 panel project started in early June 2021 and continued until mid-August 2021. I shot 300 sec exposures and got 36 subs in H-alpha (15 minutes per panel) plus 129 subs OIII (about 50 minutes per panel, I spent more time on the first and last panels since they were partially shot during twilight hours). The total exposure time was about 13 hours, spread over 5 Summer nights and mostly without astronomical darkness. I calibrated, registered and stacked the subs of each panel in PixInsight.

The initial alignment of the 12 panels showed a big issue though: the edges of my panels were just a bit too bright compared to the center and you could see the 3×4 grid of the mosaic panels very clearly. Even after ‘calibrating my calibration’ with night flats, the glows did not go away completely. Was this whole project a mistake? Honestly, the results I got were almost unusable for imaging faint structures that fill the frame and the poor flat correction almost completely ruined my ambitious plans for this lens setup. In the end I resorted to running DBE on each panel before the assembly to deal with the edge glows, which is definitely not ideal for conservation of large-scale structures and Milky Way brightness but it was the only way to get decent panel borders.

PixInsight is easily ranked as the best purchase in my entire astrophotography journey, and I absolutely love working with this piece of software. Star registration of the mosaic panels drove me up the wall though. I researched and tried various methods to assemble the mosaic from my 12 stacks. But there were always either small registration errors on the border of certain panels, some panels would not platesolve for the MosaicByCoordinates script, or they would just not register at all.

Then I got help from my friend Axel for the mosaic assembly. He used his knowledge of AstroPixelProcessor (APP) to find the optimal settings for 1) perfect star registration of all panels with distortion correction and 2) very smooth blending of the DBE panels with local normalization. This process took many hours of his computing time.The assembled image was over 200 Megapixels at native resolution and spans 27 x 27 degrees of sky. For reference, a full Moon is only 0.5 degrees in diameter!!

Processing this into a color image took me several weeks; I would make something, reflect on it for a couple of days, then start from scratch again with a different approach. In the end, I mixed a HOO tonemap version with a synthetic green to get deep-orange shadows and bright blue OIII highlights, almost as if I used a SII filter and classic Hubble palette.