Hartl-Dengel-Weinberger 3 (HDW 3)

Hartl-Dengel-Weinberger 3 (HDW 3) is an extremely faint, highly evolved planetary nebula in the constellation of Perseus, also catalogued as PK 149−09.1 and PN G149.4−09.2. It was discovered in 1983 by Herbert Hartl, Johann Dengel and Ronald Weinberger. According to astrometric measurements from the Gaia DR2 catalogue, it lies at a distance of approximately 2,760 light-years from Earth.

With an angular size of roughly 10 arcminutes, HDW 3 is a large, diffuse object with exceptionally low surface brightness. Its most striking feature is its unusual, asymmetrical morphology, with delicate filaments forming a distinctive braided or twisted appearance. This structure is thought to result from the interaction between the expanding nebular shell and the surrounding interstellar medium as the nebula moves through the Galaxy.

Like other planetary nebulae, HDW 3 formed when a dying star expelled its outer layers into space, leaving behind a hot stellar remnant. However, its advanced evolutionary state makes it particularly intriguing: much of the expanding gas has become extremely faint, and only portions of the shell remain readily detectable. The central star, designated WD 0322+452, is a faint blue-white star located close to the centre of the visible nebular arc. Its ultraviolet radiation contributes to the ionisation of the surrounding gas.

The contrast between the faint, diffuse shell and its intricate emission filaments makes HDW 3 a challenging target for deep-sky astrophotography. Long exposures, particularly through narrowband filters such as Hα and [O III], can reveal structures that are almost invisible in conventional images.

Image Acquisition

Given the extremely low surface brightness of HDW 3 and the surrounding nebulosity, capturing this region required a substantial amount of exposure time. The project was therefore spread over two imaging seasons, from February to March and again from September to October 2026.

The narrowband data were acquired using a Takahashi TOA-150, with Hα and OIII filters, accumulating a total integration time of 61 hours. In addition, RGB exposures were collected with a Takahashi FSQ-106N, specifically for the stars, allowing them to retain their natural colours in the final image.

The version presented here is an HOO composition, combining Hα and OIII data for the nebular structures with the RGB data for the stars. This approach provides a strong contrast between the red emission regions and the delicate blue OIII structures, bringing out the intricate morphology of HDW 3 and its surrounding nebulosity.

However, this is not yet the final chapter of the project. I am still accumulating additional L and RGB exposures with the aim of producing a second, hybrid version using an L-RHa-GO-BO combination. This alternative processing should offer a different interpretation of the region, combining the luminance and broadband data with the narrowband information to explore the nebula's structures in greater detail. 

Image Processing

The processing workflow followed a similar approach to that used for my Sh2-115 image. The stars were removed from the linear datasets using StarXTerminator, making it possible to work independently on the nebular structures and the stellar component.

The starless narrowband image was then processed in PixInsight using an HOO palette. The main tools and processing steps included:

- BlurXTerminator to refine the definition of the nebular structures.

- NarrowbandNormalization to balance the Hα and O III channels and achieve the desired colour rendition.

- MultiscaleAdaptiveStretch (MAS) to bring the image into the nonlinear domain.

- HDRMultiscaleTransform to enhance local contrast and control the dynamic range.

- LocalHistogramEqualization to bring out finer details and improve the perception of depth.

- New MLDenoise for noise reduction.

- CurvesTransformation for colour enhancement and final tonal adjustments.

Once the nebular processing was complete, the RGB stars, acquired separately with the FSQ-106N, were processed to preserve their natural colours and then seamlessly integrated back into the image using the ScreenStars script, developed by Mike Cranfield and Bill Blanshan.

This HOO version is the first result of the project. A second processing approach is still in progress, with additional L and RGB data being collected for a hybrid L-RHα-GO-BO version, which will combine broadband and narrowband information to produce an alternative rendition of this region.

In this image, north is up.

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

Image processing: Pixinsight.

Observatory automation and remote operation:  Talon6.