Caldwell 56
Caldwell 56 - Click here for full resolution
Caldwell 56, also known as NGC 246 or the Skull Nebula, is a planetary nebula located in the constellation Cetus, approximately 1,600 light-years from Earth. It was discovered by William Herschel in 1785 and later included in the Caldwell catalog. The nebula spans about 4 light-years in diameter and has a striking appearance, with a bright central region surrounded by fainter outer structures that give it the nickname "Skull Nebula." Its distinctive shape arises from the interaction of the ejected stellar material with the surrounding interstellar medium. At the heart of Caldwell 56 is a white dwarf (HIP 3678 A), the remnant of a Sun-like star that reached the end of its life. The star’s mass-loss process created the nebula, leaving behind the dense, hot core. The white dwarf's intense ultraviolet radiation causes the nebula to emit its characteristic glow. The central star has a companion star, HIP 3678 B. In 2014, astronomers discovered a second companion to NGC 246's central star, a red dwarf known as HIP 3678 C. The discovery was done using the European Southern Observatory's Very Large Telescope.
source: ChatGPT and Wikipedia
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NGC246
Skull Nebula
Planetary Nebula
Cetus
00h 47m 00s
-11° 53.0′
1.6 kly
8
20 October
41º S
Conditions
Caldwell 56 is an Autumn/Winter target, but never gets really high in the sky. A maximum altitude of 41° is reached in late October. Caldwell 56 was photographed over 13 nights during December 2024 and early January 2025, from the remote observatory at IC Astronomy in Oria, Spain.
Equipment
The default rig at the observatory was used. The core of this rig is a Planewave CDK-14 telescope on a 10Micron GM2000 mount, coupled to a Moravian C3-61000 Pro full-frame camera. The RoboTarget module in Voyager Advanced automated the process to find optimal time-slots during astronomical night.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14, Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro, cooled to -10 ºC
Chroma 2” H-alpha, OIII (both 3nm) and Red, Green and Blue unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Windows 11, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.2
Imaging
Caldwell 56 as a planetary nebula is a typical narrowband object, with main visibility in Ha and OIII. So the image was shot as a typical dual-band image, using 10 min. exposures for each of the narrowband filters. For star colours, RGB images were collected as 10x3 minute subs. During processing, a rather high percentage of frames could not be registered for some reason, resulting in quite a number of rejected frames. The total final exposure was 15h.
Resolution (original)
Focal length
Pixel size
Resolution
Field of View (original)
Rotation
Image center
7120 × 4488 px (31.9 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
35' x 22'
0.1°
RA: 00h 47m 03.906s
Dec: -11° 52’ 17.94”
Processing
All images were calibrated using Darks (50), Flats (50) and Flat-Darks (50), registered and integrated using the WeightedBatchPreProcessing (WBPP) in PixInsight. All further processing was done in PixInsight, including the use of scripts and tools developed by RC-Astro, SetiAstro, GraXpert, and others. For a step-by-step description of the processing techniques applied, see process flow below.
Unfortunately a high percentage of images could not be registered during the automated WBPP process. Therefore a manual registration was done with somewhat more forgiving parameters, still leaving a number of images out as rejected. The final set of registered images was manually stacked using ImageIntegration.
There are various ways to get to an HOO image like this. For this image several of those options were tested. The best results were obtained by a manual stretching of both the Ha and OIII channel, combine them in an HOO image and then adjust the final colour palette using NarrowbandNormalization. During the GHS stretching it was easy to overdo the contrast, resulting in a pretty harsh image. So gentle stretching with GHS was applied, followed by adjusting the black point using HistogramTransformation to get at a background level of around 0,1.
During star processing with the Red, Green and Blue channel, SPCC did not complete its operation. There were too few stars for it to work on. So I reverted back to PCC as the alternative. After stretching there was a slight Magenta-cast around some of the brighter stars. This was removed by inverting the image, apply SCNR to remove the green, and inverting it back again into the normal colours.
The central white dwarf star is long known to have a companion star, which is resolved in the image above.
Processing workflow (click to enlarge)
This image has been published on Astrobin
