SNR G206.9+2.3

The supernova remnant SNR G206.9+2.3, a rarely imaged object, is located in the constellation Monoceros, close to the border with Gemini. Its designation follows the standard galactic coordinate system used for cataloguing supernova remnants within the Milky Way. SNR G206.9+2.3 was first identified and characterised in the early 1970s, primarily through radio observations conducted by R. D. Davies in 1971, and has since been detected in optical emission-line surveys, where its extended and fragmented structure distinguishes it from other types of nebulae.

SNR G206.9+2.3 is the remnant of a supernova explosion that occurred thousands of years ago, marking the final evolutionary stage of a massive star. The violent event expelled stellar material at high velocity, generating a shock wave that continues to expand through the surrounding interstellar medium. As this shock interacts with ambient gas, it heats and ionises it, producing the faint optical emission observed today. The remnant appears as a diffuse and incomplete shell, spanning approximately one degree in the sky, with irregular filaments and arcs shaped by variations in the density of the interstellar environment.

The age of SNR G206.9+2.3 is not precisely known, but its morphology suggests a relatively evolved remnant rather than a very young object. Distance estimates remain uncertain; however, it is generally placed at several thousand light-years from Earth. This uncertainty is common for isolated supernova remnants that lack clear associations with pulsars, stellar clusters, or molecular clouds that could provide more reliable distance constraints.

 At optical wavelengths, SNR G206.9+2.3 is extremely faint, with emission dominated by narrow spectral lines such as hydrogen alpha and ionised oxygen. These emissions trace regions where the shock front encounters denser material. The low surface brightness of the remnant makes it a challenging target for astrophotography.

Image Acquisition

This image was captured using both of my telescopes: the Takahashi TOA-150 and the Takahashi FSQ-106N.

Taking into account the very low surface brightness of the nebula, as indicated earlier, the project was planned primarily as a narrowband acquisition, with RGB data used only for the stars.

Using the TOA-150, I acquired 36 sub-exposures of 30 minutes each in Hα and 36 sub-exposures of 30 minutes each in O III, resulting in a total narrowband integration time of 36 hours. For the TOA-150, an Astrodon filter was used for Hα and a Baader filter for O III.

The RGB data were acquired with the FSQ-106N, using Baader RGB filters. These data were intended exclusively for star colour, while the nebular structures are entirely based on the narrowband datasets.

Image Processing

The processing strategy for this image was based on removing the stars from the linear images using StarXTerminator, allowing the nebular structures and the stellar component to be processed independently.

The starless nebula image, mapped using an HOO palette, was then processed in PixInsight with the following tools and steps:

MARS for gradient correction.

NarrowbandNormalization to balance the Hα and O III channels.

HDRMultiscaleTransform to enhance local contrast and manage dynamic range.

LocalHistogramEqualisation to improve fine detail and perceived depth.

NoiseXTerminator for noise reduction.

CurvesTransformation for colour enhancement and fine tuning.

Once the nebular processing was completed, the RGB stars—acquired with the FSQ-106N and processed separately to preserve their natural colours—were seamlessly reinserted using the ScreenStars script by Mike Cranfield and Bill Blanshan.

Click on the image for full resolution version, or go to the Gallery section for complete exposure details.

Image processing: Pixinsight.

Observatory automation and remote operation with  Talon6.