Sh2-131 - Elephant’s Trunk
Nebula IC1396 is a large region of ionised gas, located in the constellation Cepheus, about 2,400 lightyears from Earth. A small portion of that is a concentration of interstellar gas and dust with a typical form, often referred to as the Elephant’s Trunk and classified as IC1396a
Just east of IC1396a, there is a very big bright star (HD206267) that illuminates and ionises the dense cloud, creating the bright rim of the trunk. Two relatively young stars are present in a small, circular cavity in the head of the globule. Winds from these young stars may have emptied the cavity.
The Elephant’s Trunk is a site of active star formation, with many new stars being discovered in infrared images as recent as 2003.
Planning
Object
Visual Magnitude: 5.6
Apparent size: 14 x 2 arcmin
R.A.: 21h 36m 09.59s
Dec: +57º 28’ 57.0”
Conditions
Astr. night: 21:34
Astr. dawn: 05:17
Moon: 12.8%
Moon set: 21:04
Humidity: 86-92%
Pressure: 1008-1020 hPa
IC1396a is a circumpolar object, so above the horizon all year around. Best observation opportunities are in Autumn, when it reaches it’s highest altitudes. Imaging sessions were held on three consecutive days, 20, 21 and 22 September 2020. During these days, the moon had set. Culmination was just after 23:00h reaching a highest altitude of 85.7º. At those altitudes, the camera/filterwheel touch the tripod. So imaging was stopped just before and resumed a little after. On September 21, fog had come in during the night, so images from the second half of the night had to be rejected.
Capturing
The image is captured in 5nm narrow-band, using Ha, SII and OIII wavelengths. Exposures of 300s at unity-gain typically lead to proper exposures with little to no blown out highlights and a decent amount of detail.
Framing the image was critical, as the total length of the trunk only just fitted in the field of view. When slewing the telescope to IC1396a, the image centered around the ‘head’ of the nebula, so had to be manually adjusted to get the whole trunk in. Furthermore, camera rotation was adjusted so that the trunk was pointed horizontal. In processing the image was then rotated 90 degrees to straighten up the trunk. Because the positioning was so critical, in subsequent nights, a reference image from the first night was used to point the telescope correctly to the target.
Technical details
Telescope
Mount
Camera
Sensor Temp.
Takahashi TOA-130 + FL67 flattener
10Micron GM1000HPS
ZWO ASI1600MM Pro
-25 ºC
Exposures
Ha (5nm)
OIII (5nm)
SII (5nm)
Total Exposure
61 x 300s @ Gain 139/21
60 x 300s @ Gain 139/21
58 x 300s @ Gain 139/21
14.9h
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the WeightedBatchPreprocessing script. Due to the nebulosity across the image, it was impossible to determine any gradients, so no background corrections were conducted for Ha and OIII. Only for SII a slight gradient was removed with background extraction. Each of the channels was then subjected to a serious noise reduction treatment, using the MMT method. For details, see the blog-post on this technique.
The three narrow-band channels were then combined into a Hubble palette, with SII mapped to Red, Ha mapped to Green and OIII mapped to Blue. With Colormask, four color masks were created (Cyan, Green, Magenta and Yellow, all blur 3), and using CurvesTransformation, the colours in the image were adjusted to taste. While there was not too much noise in the image at this stage, a modest convolution was applied to create very smooth tone-curves across the image. Obviously this convolution creates a very soft image, but that will be corrected when the Luminance is added.
The Ha-channel is doubling as a Luminance layer as well. Regular deconvolution was applied to retrieve maximum amount of detail. The MMT-based noise reduction had been very effective, so the only thing left to do was to apply a little bit of contrast using HistogramTransformation.
The Luminance was then added back to the RGB image and final touches were made using CurvesTransformation, in combination with some of the existing colormasks.
Because no background corrections had been made in the beginning, the final image suffered from a slight magenta-cast at the top and bottom. This was corrected with some subtle HSL adjustments in Affinity Photo. Finally, a little bit more contrast and vibrance was dialed into the image.
This image is published on Astrobin, has received ‘Top Pick’ status and was ‘Image of the Day’ on October 15, 2020.
