NGC 7293 The Helix Nebula

Since my retirement, I have devoted most of my time to what I enjoy the most: imaging faint nebulae. From time to time, however, I revisit some of the earliest images in my portfolio (those first attempts taken with a Canon DSLR) and I cannot resist the temptation to create an improved version.

The Helix Nebula, designated NGC 7293 and also catalogued as Caldwell 63 (*), is a remarkable planetary nebula in the constellation Aquarius. It was discovered by Karl Ludwig Harding, probably before 1824 (**). Situated at a distance of about 700 light-years according to Gaia measurements, it is one of the nearest and most visually striking nebulae known.

Its large apparent size (spanning nearly 25 arcminutes, almost half the apparent diameter of the full Moon) corresponds to a true extent of around 2.5 light-years. This combination of proximity and scale provides an unusually detailed view of its intricate structure, making it an excellent object for studying fine-scale features.

One of the Helix Nebula’s most fascinating characteristics is the presence of thousands of cometary knots. These dense clumps of gas resemble miniature comets with glowing heads and elongated tails, arranged like spokes along its inner rim. Their fibrous structures point back towards the nebula’s central star.

At the heart of NGC 7293 lies a hot DAO-type white dwarf, designated WD 2226-210, with a surface temperature of about 120,000 K. Recent observations have revealed a complex, multi-ring structure surrounding this stellar remnant, including inner discs and outer toroidal shells.

(*) Often referred to in popular culture as the “Eye of God” or the “Eye of Sauron” due to its spectral resemblance to a human eye.

(**) The discovery was first reported in 1824 in “Astronomische Nachrichten, Beobachtungen des diesjährigen Kometen [...], neue Nebelflecke.”

Image Acquisition

Although NGC 7293 is by no means a faint object, one of my aims in this project was to bring out as much detail as possible in its cometary knots. For that reason, I devoted a substantial amount of exposure time to recording its fine structure. Between July and September 2025, I collected data with Hα and O III narrowband filters, complemented by additional RGB exposures for the stars. The final image was composed using the HOO palette.

All data were acquired remotely with my Takahashi TOA-150 refractor paired with an SBIG STL-1000M monochrome CCD camera for the narrowband data, and with my Takahashi FSQ-106N for the RGB star data. I employed Astrodon filter for Hα and a Baader filters for O III and RGB. The total integration time amounts to 40 hours.

Image Processing

The overall processing strategy on the first image was to remove the stars from the linear images using StarXTerminator, allowing the nebular structures and the stars to be processed independently.

The starless nebula image was then processed with the following PixInsight tools:

NarrowbandNormalization to balance the Hα and O III channels.

HDRMultiscaleTransform to enhance local contrast and reveal internal structures.

LocalHistogramEqualisation for improved detail and depth.

NoiseXTerminator for noise reduction.

Curves for colour enhancement.

Once the nebular processing was completed, the RGB stars (processed independently in order to preserve their natural colours) were seamlessly re-inserted using the excellent script by Mike Cranfield and Bill Blanshan.

For the crop centred on the cometary knots, the workflow followed essentially the same steps but with clearly different parameters on the HDRMultiscaleTransform. Also because of the nebula’s smaller angular scale in this region, I first applied a drizzle ×2 integration, achieving an image scale of 0.845″/pixel. Finally, for better showing the knots, this image is starless.

The third image included here is simply a reproduction of my earlier RGB work on the Helix, taken back in 2006!!, offering an interesting comparison with the new data😊.

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

Image processing: Pixinsight.

Observatory automation and remote operation with  Talon6.