Now that the 10″-Newtonian is set up and operates largely without issues, it was time for a challenge…
My fascination with planetary nebulae (PN) comes in handy here, as many of these beautiful stellar shells are simply very small and often very faint. The “older” ones, in particular, no longer shine very brightly. A prime example of this is Abell 28, which, due to its extremely low surface-brightness, is rarely captured by astrophotographers…

Visibility, Discovery an the POSS-plates

Located in the Constellation Ursa Major (UMa) the object is marked in the DeepSkyAtlas as being not visible with 12″ aperture telescopes. Ronald STOYAN describes it as being barely visible with a 20″ Dobsonian. This note comes from a highly experienced visual observer … Ultimately, however, it is precisely this extremely low surface brightness that drives me here.

With regard to its discovery history, there is no classical narrative for Abell 28. Rather, its first documented appearance to be its cataloguing by George Ogden Abell himself. He initially listed the object—then not yet designated as Abell 28 but as Abell 18—among other (more than 70) faint planetary nebulae identified during the analysis of the POSS-plates in 1959. POSS stands for the Palomar Observatory Sky Survey.

The Palomar Observatory Sky Survey (POSS) was a major photographic survey of the northern sky conducted at Palomar Observatory (near San Diego, California) using ia the 48-inch (122 cm) Samuel Oschin Schmidt-Telescope:

It produced deep, large-scale sky images on photographic plates, enabling the systematic identification of faint astronomical objects such as galaxies, nebulae and planetary nebulae. The survey, carried out in multiple phases starting in the late 1940s, became a foundational dataset for modern observational astronomy and many subsequent catalogs.

Abell identified objects (such as Abell 28) through a systematic visual inspection of these photo-plates. He searched specifically for large, extremely faint, diffuse nebulae with roughly circular morphology that appeared consistently on both blue and red plates. Objects lacking stellar or galactic structure were classified as candidate planetary nebulae and subsequently catalogued. Reiner VOGEL published the Observing Guide to the Abell Planetaries in 05/2008 where the original POSS-plates are shown – also for Abell 28:

Astrophysical Background – …only a PN mimic?

I was wondering, why Abell 28 was listed in the HASH-database only as „Likely PN“ (Link within the Image):

A major open question concerns the physical nature of Abell 28 and whether it is consistent with standard PN evolution. According to REINDL et al. (Spectroscopic survey of faint planetary-nebula nuclei – Seventeen hydrogen-rich central stars, 2024), the object is indeed classified only as a “likely PN” reflecting existing uncertainties. A key issue is a timescale discrepancy: using the observed angular diameter (~330″) and a typical expansion velocity of ~20 km/s implies a kinematic age of about 15,000 years. However, the cooling age of the central white dwarf is estimated to be on the order of ~600,000 years – more than an order of magnitude larger. This inconsistency suggests that Abell 28 may not represent a standard, freely expanding planetary nebula shell. Instead, it could be a PN mimic (e.g., ionized interstellar medium) or an object with non-standard kinematics (Reindl et al. 2024).

Image Aquisition

The acquisition sessions for an object of such extreme faintness can, in essence, be described quite simply: one is imaging for hours into what appears to be “nothing.” The signal level of the nebular structures is so low that even in a deliberately stretched real-time preview (N.I.N.A.`s Preview Window), no specific features are visible.

This has important practical implications for the imaging strategy. Data acquisition must be executed with a high degree of procedural discipline and trust in the underlying methodology rather than immediate visual feedback. Framing, focus, guiding performance, and exposure parameters must be validated independently, as the target itself provides no usable real-time confirmation.

The accompanying screenshot of an imaging session (H-alpha) illustrates this situation clearly:

Despite ongoing integration, the individual frames (and even their stretched previews) show no visible trace of the nebula. Only through subsequent integration and careful post-processing does the accumulated signal emerge from the noisy background…

After the first H-alpha Session I could not resist to integrate the first 3.5 hours of exposure time – just to get sure, that „at least something“ was captured:

Admittedly, this process also requires a certain level of mental resilience 🙂 … the ability to continue a data acquisition campaign over several nights without any immediate visual reward, relying instead on experience and confidence in the photon count …

Quick & Dirty to reveal the distinct features

In my image processing workflow, I occasionally begin with quite an aggressive stretch at an early stage – somehow a quick & dirty approach to serve as a diagnostic step, allowing me to assess which processing techniques will be required to effectively reveal and emphasize the scientifically or aesthetically relevant structures of the object. What first attracted my attention when looking at the quick & dirty processing results of the Nebula were the southern regions of the shell structure. They seemed neither closed nor homogeneous, nor is the overall shape perfectly circular around the central white dwarf. This is especially visible in H-alpha. The inner O-III part seems more centric.

Based on this rapid processing step, the focus of the processing becomes evident: the primary objective will be to prominently extract and enhance the more compact regions of the H-alpha shell. But now, let us start with the first steps of processing.

Standard Processing (but carefully)

In general, I apply a dual-workflow approach for nebulae, processing the nebular structures and the stellar components separately. The stretching behaviour of extended emission regions fundamentally differs from that of point sources; therefore, isolating both components allows for a more targeted and physically meaningful enhancement.

A schematic sequence of standard processing steps is as follows: AutoDBE for gradient removal (Particular caution is required when dealing with such extremely faint objects, as even moderate processing steps can easily introduce artefacts or distort the underlying signal), followed by deconvolution using BlurXTerminator, noise reduction via NoiseXTerminator, and finally star separation using StarXTerminator to enable the described dual-workflow.

Prior to star removal, the two channels present as follows: rather striking, given that even after more than 17 hours of total integration time, only these extremely faint nebular structures emerge:

In particular, the O-III emission is extraordinarily faint. Ronald Stoyan characterizes it in his Atlas of Planetary Nebulae as only about 15% of the already extremely weak H-alpha intensity. This makes it all the more remarkable that he reports having visually detected “a diffuse, round brightening with an O-III filter” using a 20-inch aperture telescope. His corresponding sketch, however, documents a sky brightness of 21.1 mag/arcsec² (SQM).

After the images were astrometrically solved, the Process Console of PixInsight reveals that the frame have a rotation of 92.644°. The final image will later be rotated accordingly to ensure a north-up orientation. At this stage, the two channels must first be aligned to allow for a subsequent DynamicCrop, ensuring identical image geometries.

The initial stretch with Histogramm Transformation reveals the nebulae and is followed by a Generalized Hyperbolic Stretch. This is of course the tool of choice when it comes to stretch specific regions of the shell structure. The symmetry point of the tool is placed around the interesting regions and the contrast is enhanced targeted at these. The desired southern compacted zones (actually on the left side) are visible now quite good:

After all the stretches have been applied, the images now are no longer in linear state and ready for Channel Combination. Sometimes astrophotographers choose to use the H-alpha Channel as a Luminance Channel too in order to process it a bit more aggressive to reveal structural details. At Luminance I applied therefore again a bit of Deconvolution (BXT) and the process HDR-MultiscaleTransformation in order to point at the compacted zones. With Foraxx-Palette Utility the two Channels are combined to a HOO-image (H-alpha representing Red and O-III representing Green and Blue). The H-alpha Luminance is added via PixelMath. The Screenshot below is showing the Luminance details in the greyscale-image:

As Deconvoltion usually decreases the Color Saturation the Blue and Red parts are enhanced via the ColorSaturation process and the image is brought to its final stages of processing: a bit of CurvesTransformation, Rotation counterclockwise at 92.644° to face North up, DynamicCrop and some minor Photoshop-color adjustments close the image processing.

Final Thoughts

Imaging Abell 28 is less about immediate visual reward and more about disciplined signal accumulation at the very limits of detectability. What initially appears as “nothing” gradually reveals a physically structurally complex object, whose fragmented shell and extreme faintness challenge also the processing methodology. Ultimately, this project highlights that, with sufficient integration time and a carefully controlled workflow, even the most elusive planetary nebulae can be brought out of the noise—while still leaving open questions about their true astrophysical nature.