Caldwell 39
Caldwell 39, also known as NGC2392, is a bipolar double-shell planetary nebula (PN). It was discovered by astronomer William Herschel in 1787. The formation resembles a person's head surrounded by a parka hood. It is surrounded by gas that composed the outer layers of a Sun-like star. The visible inner filaments are ejected by a strong wind of particles from the central star. The outer disk contains unusual, light-year-long filaments.NGC 2392 lies about 6500 light-years away, and is visible with a small telescope in the constellation of Gemini. At the center of NGC 2392, there is an O-type star (designated HD 59088) with a spectral type of O(H)6f. On 11 August 2020, the IAU Working Group on Star Names (WGSN), NASA/IPAC Extragalactic Database (NED), and SIMBAD Astronomical Database (CDS) discontinued use of three nicknames that were perceived as offensive – "Eskimo Nebula", "Clown Face Nebula", and "Clownface Nebula" – and strongly recommended the nebula be referred to by its NGC designation in further publications.
source: Wikipedia
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NGC2392
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Planetary Nebula
Gemini
07h 29m 12s
+20° 55.0′
6,500 ly
9.7
30 January
73º S
Conditions
Caldwell 39 is a great winter target. A maximum altitude of 73° is reached in late January, when observed from the remote observatory at IC Astronomy in Oria , Spain. C39 was photographed from the observatory in Spain over 8 nights, across various phases of the moon cycle from mid January until mid February 2025.
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 and SII (all 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 1.9.3
Imaging
Caldwell 39 as a planetary nebula is a typical narrowband object, with main visibility in Ha and OIII and a little bit of signal in SII. So the image was shot as a typical narrow-band image, using 10 min. exposures for each of the narrowband filters. For star colours, RGB images were collected as 10x3 minute subs. The total exposure was 25.5h.
Resolution (original)
Focal length
Pixel size
Resolution
Field of View (original)
Rotation
Image center
1800 × 1200 px (2.2 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
9' x 6'
180.0°
RA: 07h 29m 10.783s
Dec: +20° 54’ 41.86”
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 the process flow below.
StarXTerminator was not able to distinguish between the nebula and the stars, and extracted most of the nebula as a star out of the image. To prevent that from happening, a mask was created the size of the nebula, using the GAME script. Applying SXT with the mask in place extracted all the stars nicely and left the nebula (with central star) in the starless image.
The individual narrow-band channels were carefully stretched using HistogramTransformation. The image was stretched until the brightness of the central part of the nebula reached values around 0.8. At this point, all of the nebula was very well visible. Under regular stretching algorithms, the nebula typically ends up as one big giant star, but using this manual method, it was remarkably easy to pull out detail in the nebula without blowing out any highlight detail. The SII signal was very low, so this had to be stretched significantly more.
This manual stretching method did not result in a very representative colour balance. This was corrected using the NarrowbandNormalization script. This is where art takes over from science, as there is no real answer here. I pushed down the strong OIII signal a bit, to create more of a teal colour than a bright blue colour. And since SII was such a weak signal, it was also boosted significantly. To balance it all out, a significant amount of SCNR had to be applied.
The size of the nebula is really small. As mentioned above, with normal stretching it looks more like a big star than a nebula. To focus more on the nebula, while maintaining some of the local context, the original image was heavily cropped in from a 60MP image to a 2MP image. This created an image with a field of view of 9’ x 6’. Still the nebula comes out rather small. The nebula definitely deserves it to be admired from close-up. So here is a crop from the center of the image. Also, you may want to have a look at the full resolution version of the image.
Close-up of the planetary nebula Caldwell 39
According to regular conventions, the image here is upside down, with north pointing to the bottom of the image. However, I like the very bright star HD59087 much better in the bottom part of the image than the top part.
The rest of the processing followed a very standard processing workflow.
Processing workflow (click to enlarge)
This image has been published on Astrobin
