Sh2-200
Sh2-200 - Click here for full resolution
Sh2-200, also known as the Bearclaw Nebula, is a planetary nebula in the constellation Cassiopeia. It was included in the Sharpless catalog by American astronomer Stewart Sharpless in 1959. However, Sh2-200 is actually a planetary nebula, which is distinct from the typical H II regions cataloged by Sharpless. Over time, astronomers observed that its morphology, central star, and spectral characteristics (such as emission lines of ionized gas) were more consistent with the properties of planetary nebulae. It was not until 1983 that its nature as a planetary nebula was recognised, in a study of some nebulae of unusual appearance.
source: ChatGPT
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n.a.
Bearclaw Nebula
Planetary Nebula
Cassiopeia
03h 11m 01s
-62° 47.7′
11 November
65º N
Conditions
Sh2-200 is a circumpolar object, so visible all year round. But best time to image is in autumn/winter time. Maximum altitude reached is 65° in mid November. Sh2-200 was photographed over 8 nights during October, November and December 2024 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 1.8.9-3
Imaging
Sh2-200 consists of a fairly dim center which emits mostly in OIII and is surrounded by an even fainter cloud of Ha gas. It is therefore a good target to image as a dual-band object. Imaging this faint nebula is therefore quite a challenge and requires long exposure times. Narrowband images (Ha and OIII) were shot using 10 min exposures. For the stars, quick 3m RGB images were shot. The total exposure was 23.7h..
Resolution (original)
Focal length
Pixel size
Resolution
Field of View (original)
Image center
9168 × 6072 px (55.7 MP)
2585 mm @ f/7.3
3.8 µm
0.30 arcsec/px
47' x 31'
RA: 03h 10m 56.660s
Dec: +62° 47’ 46.17”
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.
Stretching this very faint nebula was quite a challenge, and multiple combinations of HT, GHS, automated scripts and CurvesTransformation were tried. And even then, the red Ha nebula was still fairly faint, and required a selective (colormask) red and saturation increase. Another by-product of the complex stretching was that somehow a lot of single pixel zero’s had appeared in each channel, resulting in distracting blue or red pixels in the final image. These were eliminated using a technique originally developed by Adam Block. A PixelMath equation will select individual pixels that have a significantly higher value in one of the colour channels than their neighbouring pixels. Each if these pixels is given the value white and the resulting image of a bunch of white pixels is used as a mask. MMT is then used to give these pixels the average value of their background. For those interested, the PixelMath formula is iif($T[1]>a*($T[0]+$T[2]),1,0), where the variable ‘a’ can be used to change sensitivity (0.7 is often a good starting point). In my case ‘a’ was lowered to 0.53.
A dual-band image is not necessarily false colour. In fact, the red Ha and blue-ish OIII represent the actual colours. The NBColourMapper is an ideal tool to apply the exact colour that the wavelengths of Ha and OIII represent. However, when I translated 500.7nm of OIII to an actual colour, by using a tool such as this, the outcome was completely green. Not blue-ish, teal or turquoise, just plain green, with a hue of 152. I started a thread on Astrobin asking why we present OIII often as a blue-ish colour, while it is in fact green. As expected, many people think that the actual colour of OIII is blue-ish/teal/turquoise. Others knew that the colour is actually green. But regardless, the general consensus was that blue images are aesthetically more pleasing than green. So most would choose for a dual-band image still a blue-ish tint for OIII. Perhaps the Hubble palette is to blame here. In that palette OIII is allocated to the pure blue channel. So we’re used to red/yellow/gold images with bright blue parts and this has become some sort of reference for narrowband imaging. But the real colour of many planetary nebulae should be green. In the current image, I stayed with the more conventional opinion and coloured it blue-ish (hue 185, saturation 0.7). But based on this findings, I’m still contemplating what colour I’d like to use for these nebulae going forward.
Processing workflow (click to enlarge)
This image has been published on Astrobin
