Caldwell 2
C2 - Click here for full resolution
Caldwell 2, also known as NGC 40 or the Bow-Tie Nebula, is a planetary nebula located in the constellation Cepheus. It was discovered by William Herschel in 1788 and is included in the Caldwell catalog compiled by Sir Patrick Moore. The nebula is approximately 3,500 light-years away from Earth. Caldwell 2 is formed from the outer layers of a dying star that have been expelled into space, creating a glowing shell of ionized gas. The central star, a white dwarf, is very hot and emits ultraviolet radiation that causes the surrounding gas to fluoresce. The nebula's distinctive shape, resembling a bow-tie or an hourglass, is a result of the complex interactions between the ejected material and the radiation from the central star. The nebula spans about one light-year in diameter and is relatively young in astronomical terms, estimated to be around 4,500 years old. It is a very small object, with an apparent diameter of only 0.9 arcmin.
source: DeepSeek
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Tr. Alt (Oria, Spain):
NGC40
Bow-Tie Nebula
Planetary Nebula
Cepheus
00h 13m 00s
+72° 32.0′
3,500 ly
11.4
11 October
55º N
Conditions
C2 is circumpolar, but best visible during the winter months. Observed from the remote observatory at IC Astronomy in Oria , Spain, it reaches a maximum altitude of 55 degrees in mid October. C2 was photographed from the observatory in Spain over 11 nights from mid January, until early February 2025, across a wide range of moon phases and moon distances
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) as well as 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
C2 is a typical narrowband object, with relatively strong signal in all three narrowband channels. The default exposure times were used. This includes 10 min subs for each of the three narrowband filters, and 3 min subs for the Red, Green and Blue filters. The total exposure was over 25h. Despite the small field of view of this long focal length telescope, the final image was heavily cropped to highlight the small planetary nebula structure.
Resolution (original)
Focal length
Pixel size
Resolution
Field of View
Rotation
Image center
2336 × 1740 px (4 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
11' x 8'
179.853°
RA: 00h 13m 0.432s
Dec: +72° 31’ 20.84”
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.
In the wider captured image, some faint nebulosity was visible. It was almost impossible to pull this nebulosity forward without blowing out all the detail in the object itself. Therefore the focus was on getting a balanced stretch of the nebula and crop the image in to highlight C2.
The narrowband color channels for C2 show a clear circle-shaped signal in OIII. SII and H-alpha, however, have similar signal distributions. This could be why it was hard to find a good color mapping.
Several colour palettes were tried. The problem with all these false colour palettes is that it is hard to determine what looks better and what looks worse. So I ended eventually up with an SHO like palette. The colours were created through the NBColourmapper script though. The hues that were selected were 222 for OIII, 49 for H-alpha and 0 for SII.
In the final image, the central star in the planetary nebula had a strong magenta cast. This was removed by applying SCNR to the inverted image.
Processing workflow (click to enlarge)
This image has been published on Astrobin
