The Rosette Nebula (NGC 2237) is one of the most recognisable emission nebulae in the night sky. Located in the constellation Monoceros, its large circular structure surrounds the young open cluster NGC 2244, with intricate clouds of ionised gas and dark dust extending throughout the region.
I captured this Rosette Nebula image from Cambrai, South Australia, using an Askar 107PHQ and ToupTek 2600MM monochrome camera. The final image combines 13 hours and 40 minutes of SII, Hα and OIII narrowband data, collected across three nights in January 2025.
For the final SHO processing, I chose a predominantly gold-and-blue colour palette. SHO offers considerable freedom in colour interpretation, and while many Rosette images use stronger reds and deeper blues, I preferred the contrast between the warm golden outer structures and the lighter blue tones across the interior of the nebula.
Acquisition Details
The narrowband data was collected over three nights on 3, 6 and 7 January 2025.
Telescope: Askar 107PHQ
Camera: ToupTek 2600MM
Location: Cambrai, South Australia
Hα: 60 × 300s — 5 hours
OIII: 44 × 300s — 3 hours 40 minutes
SII: 60 × 300s — 5 hours
Total Integration: 13 hours 40 minutes
Pixel Scale: 1.469″/pixel
Orientation: 340.165°
Field Radius: 1.039°
The image was captured entirely in narrowband, with separate SII, Hα and OIII datasets later combined into the final SHO image.
Imaging the Rosette Nebula in SHO
The Rosette Nebula is a large region of ionised hydrogen surrounding the open star cluster NGC 2244. Radiation and stellar winds from the young stars within the cluster have helped shape the large central cavity, while the surrounding nebula contains dense ridges, dark structures and complex layers of emission.
These structures make the Rosette particularly well suited to narrowband astrophotography.
Rather than recording a conventional RGB image, narrowband filters isolate specific emission lines from the nebula. In an SHO image, ionised sulfur (SII), hydrogen-alpha (Hα) and doubly ionised oxygen (OIII) provide three separate datasets that can be mapped into colour during processing.
Each channel reveals somewhat different structures within the Rosette. Hα provides a strong foundation across much of the nebula, while SII and OIII add their own distributions of emission and help separate structures that can become less distinct in a conventional broadband image.
For me, this is one of the most interesting aspects of SHO astrophotography: the captured signal is determined by the physical emission from the nebula, but there is considerable creative freedom in how those channels are translated into the final colour image.
Processing the Rosette Nebula in SHO
I did not approach the processing with the intention of reproducing a natural-colour view of the Rosette Nebula.
SHO is a false-colour technique, and there is no single colour interpretation that every SHO image needs to follow. The same SII, Hα and OIII data can produce very different results depending on how the channels are combined and how the colours are developed later in the processing workflow.
For this image, my main consideration was therefore not finding a supposedly “correct” SHO colour, but finding a palette that worked with the structure of the Rosette and with the way I wanted to present the data.
Building the Gold-and-Blue Palette
For the final image, I settled on a predominantly gold-and-blue palette.
The outer structures of the Rosette carry warmer gold and orange tones, while much of the interior shifts towards cooler blue and cyan-blue hues. I preferred this treatment to the stronger red and deep-blue combination often seen in other SHO interpretations of the Rosette.
Rather than reducing the nebula to two sharply separated colours, I also wanted to preserve the transitions between them. Across the image, the warmer outer regions gradually move through more muted yellow, green and cyan tones before reaching the stronger blues within the interior.
Those intermediate colours are an important part of the final image. They help maintain separation between different structures in the narrowband data while avoiding an overly rigid two-colour appearance.
The resulting palette is ultimately an aesthetic choice. Another astrophotographer could process the same three narrowband channels and arrive at a very different interpretation without either version being inherently more correct.
That creative flexibility is one of the reasons I enjoy working with SHO data. The SII, Hα and OIII signal provides the structure of the image; the final colour palette becomes part of the photographer’s interpretation of that data.
Beyond Amateur Narrowband Imaging
While amateur narrowband imaging reveals the Rosette Nebula through emission lines such as SII, Hα and OIII, professional observatories allow the same region to be studied at very different wavelengths and levels of detail.
Observations from the Hubble Space Telescope have revealed fine structures within the Rosette’s gas and dust, while infrared observations from the James Webb Space Telescope can probe deeper into obscured regions and the processes associated with star formation.
These observations serve a very different purpose from my SHO image, but they also provide another perspective on the structures recorded in amateur narrowband data. What appears as colour and texture in an astrophotograph ultimately traces a much more complex physical environment within the nebula.