NGC7380 - Wizard Nebula
NGC7380 - Wizard Nebula - Click here for full resolution image
The open star cluster NGC7380 in the constellation of Cepheus is probably most known for its surrounding emission nebulosity, known as the Wizard Nebula. In the Sharpless catalogue, the nebula is known as Sh2-142. The distance is around 8,500 lightyears from the Earth. At the heart of the nebula is a variable binary star system called DH Cephei, consisting of two massive O-type stars. This pair are the primary ionising source for the surrounding H II region, and are driving out the surrounding gas and dust while triggering star formation in the neighbouring region.
Planning
Object
Visual Magnitude: 7.19
Apparent size: 25 x 30 arcmin
R.A.: 22h 47m 48.62s
Dec: +58º 12’ 25.0”
Conditions
Astr. night: 21:45
Astr. dawn: 05:11
Moon: 90%
Moon set: 11:21
Humidity: 67-83%
Pressure: 1021-1031 hPa
NGC7380 is a circumpolar object, but best observation period is in the fall and early winter, where it reaches really high altitudes. Imaging sessions were held on three consecutive days, 17, 18 and 19 September 2020. September 17 was the first session of the season after the summer-stop. And it turned out the weather was very stable, so the equipment stayed out the whole week and multiple targets were pursued. There was quite a bit of moon interference, but since the images were made in narrow-band it did not affect their quality too much.
Capturing
Getting started in the ‘new season’ is always a bit of a challenge. Usually various pieces of software have seen updates and it’s not a guarantee that everything works together immediately. This year was no exception. The ZWO-INDI driver had to be updated manually, the focuser had to be reset to his 0-position and Mountwizard had seen some updates that required a modification to the configuration file. Luckily after a while it was all sorted and imaging could begin.
The image is captured in 5nm narrow-band, using Ha, SII and OIII wavelengths. Exposures of 180 at unity-gain typically lead to proper exposures with little to no blown out highlights and a decent amount of detail. As for framing, the goal was to have the nebula pointing upwards to get the impression of the gas clouds escaping from a cave. This was achieved by proper rotation of the imaging setup, while the automatic pointing from SkySafari got the image right in the middle. In subsequent nights, the same rotation angle was kept.
Because of the series of clear nights, it was possible to collect a lot of images. In the end the final images consists of 386 images, making a total of 19.3h of exposure.
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
131 x 180s @ Gain 139/21
127 x 180s @ Gain 139/21
128 x 180s @ Gain 139/21
19.3h
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the WeightedBatchPreprocessing script. The OIII image was with a mean ADU of 67 a tiny bit brighter than H-alpha at 65. But especially the SII image was significantly dimmer at a mean ADU of 37. Both OIII and SII were therefore adjusted to H-alpha level. Especially SII and OIII were suffering from some gradients in the frame that led to pretty bright corners. An Automatic Background Extraction corrected for this.
From here it was decided rather than to treat the whole image as one, to separately work on the colours (SHO palette) and the noise and sharpness using luminance.
As synthetic luminance, a copy of the H-alpha channel was used. This was stretched using the MaskedStretch tool with a target background of 0.125 and 100 iterations. Stretching can be done in several ways, and this was one of the first times MaskedStretch was used. It worked well and seemed quite easy to use, so probably will be tried more often. Noise reduction was applied using the MMT method as described here and a standard deconvolution was applied. Because the target is a nebula, the emphasis on the stars was a bit reduced by shrinking them using MorphologicalTransformation (amount 0.35, size 5). Some remaining high frequency noise was removed using TGVDenoise (Strength 2.2, Edge protection 0.00205, Smoothness 2.0). Using CurvesTransformation, a slight S-cuve was applied to enhance contrast. This completed the Luminance part of the image.
The colours were created by channel combination, mapping SII to Red, H-alpha to Green and OIII to Blue. After stretching the image with HistogramTransformation, a colour calibration was performed. Previews of a pretty big star and a small piece of background were used as white and background references. This seemed to have reduced the magenta halo’s enough and helped the actual star colours, that are always a bit difficult to get right in a Hubble palette. Using the ColorMask tool, Cyaan, Magenta, Yellow and Green masks were created (blur 3) and the typical pulling and stretching of colours was done to create the typical Hubble palette colours. For a detailed description, see here. To remove chromatic noise, a convolution (StDev 5.2, Shape 2.0) was applied, which essentially smears out the colour over neighbouring pixels. The sharpness was then brought back in using the separately created Luminance image. This appeared to be a rather effective way to remove chromatic noise and create these soft and even areas in the nebula. The pictures below show a zoomed in portion of the image during each of the steps.
The remainder of the processing included some finishing touches. The DarkEnhance script was used to accentuate the dark bands in the nebula. Using CurveTransformation the red colours were accentuated and overall contrast and saturation were enhanced. The image was then rotated 180º to bring the curved based to the bottom of the frame, making it look like the clouds are being released from a cave. A final crop was applied, to get the nebula straight in the middle of the frame.
This image is published on Astrobin.
