Back in May 2021, Axel and I thought it would be fun to try a very deep ultra wide field integration of a rarely imaged part of the sky. We started searching the DSS survey on Telescopius.com and ended up with a particular field in the Northern section of the Cygnus constellation. Most photographers use either long focal length setups to photograph specific nebula in the field, or they completely ignore this part of the sky in favor of bright nebulae such as the North America Nebula (NGC 7000), Sadr Region (IC 1318) or Crescent Nebula (NGC 6888) that are just outside the field we selected. 

I had really hoped there would be faint stuff in the top right corner, but unfortunately we could not find any significant structures in our data. That said, there’s a TON of objects in this frame and I want to highlight several lesser known ones such as W63 in the description below and in subsequent posts, since that was the focus of this project.

Green is quite dominant in the SHO image (original revision), so I processed the data further with SCNR and made adjustments to both contrast and color saturation for the final version. The frame was cropped to improve the perception and framing of the S-shaped backbone of red nebulosity. I nicknamed this field the Tendrils of Cygnus; faint curly structures that crawl to the frame with bright patches that act as stems or resting spots for your eyes.

You can find Axel’s version of the project on his AstroBin page.

Technical
I collected OIII and SII with my Sigma 105 mm lens for the color mapping, while Axel focused his efforts on the Ha only with his razor-sharp Canon 200 mm lens. To obtain the same field of view, Axel had to shoot a 2×2 mosaic with minimal overlaps. My Sigma 105 mm data was 2X drizzle stacked to match the resolution of the Canon 200 mm lens, and to improve the star images as they were very blocky at 14 arcsec/px. Axel prepared his Ha mosaic in AstroPixelProcessor, I did the final registration and clean-up in PixInsight. STF stretched stacks of the 3 channels are shown as revisions. 

For post-processing I prepared a SHO color map from starless images and used Ha as a base luminance layer. The OIII tone map was also added to the luminance layer to improve visibility and color of the bright OIII structures. Halos and spikes from the lens aperture blades were cosmetically corrected in GIMP. I decided to go for white stars as real star color do not make sense in a narrowband SHO palette (at least for me).

My data:
Optics: Sigma 105mm f/2.8 EX DG @f/4
Camera: ZWO ASI1600MM-C
Location: Herent, Belgium (Bortle 7)
OIII 3nm: 103 x 300s
SII 3nm: 92 x 300s

Axel data:
Optics: Canon EF 200mm f/2.8 L II USM @f/4.5
Camera: ZWO ASI1600MM-C
Location: Paris, France (Bortle 8)
Ha 7nm : 80/80/73/81 x 180s

Night flats
It soon became clear that my flat correction was not good enough, especially in OIII. The usual t-shirt flat method produced an overly bright center on the Sigma lens. Sky Flats were better in the center, but the correction around the edges was far from perfect and it is almost impossible to separate real signal from the background. I think this is due to internal reflections  The images below show highly stretched versions of my normal t-shirt masterflat and a sky masterflat, with their corresponding starless OIII stacks to show the background.



One way of correcting for this residual glow, is to use so-called night flats. The process is described by Fabian Neyer in a presentation that can be found here. Below are stretched versions of the night flat from 3 random 300 sec exposures in empty sections of the sky, extracted flat residual, and corrected masterflat. The new method has greatly improved the glow around the edges of the OIII stack, but it was unfortunately not completely removed. This is especially noticeable in mosaics, and I will talk more about that in one of my next posts (hint: it’s a BIG mosaic).




Planetary Nebula
The H-alpha data from Axel shows a remarkable amount of detail at native resolution, despite the small focal length of this Canon 200 mm lens. Planetary nebulae are typically very small but some were within the reach of his system. I’ve processed several small crops to maximize visibility of these tiny structures. They are shown at 200% resolution (2 arcsec/pixel). 

Minkowski 4-17 (M 4-17, G79.6+5.8, PK79+5.1)


Weinberger 1-10 (We1-10, PK 086+05.1, PN G086.1+5.4)


Planetary Nebula candidate PM 1-320