Sh2-129 - Flying Bat and Squid Nebula

Sh2-129 - Flying Bat and Squid Nebula - Click here for full resolution

Sh2-129 (R.A.: 21h 12m 20.24s, Dec: +60º 02’ 15.1”) is a Bright Nebula of Visual Magnitude 10 in the constellation Cepheus at a distance of 9318 lightyears from Earth. It is one of the many beautiful H-alpha emitting objects that can be found in the Sharpless catalogue. The nebula is also known as Flying Bat Nebula. Interestingly, at the heart of the nebula, there is another nebula located, often referred to as Flying Squid Nebula (Ou4). This OIII emitting nebula was only recently discovered by the French amateur astrophotographer Nicolas Outters in 2011. Recent research seems to indicate that Ou4 is indeed located within Sh2-129. It is assumed that the nebula is getting it’s energy from a strong outflow from HR8119, a triple star system right in the middle of the Flying Squid Nebula.

Planning

Sh2-129 is a circumpolar object, so visible throughout the year. The best time for observing would be in autumn time. But images were shot here in February and March 2021, at which it still reaches altitudes in the range of 30 degrees.

*Visibility charts showing altitude at 22:00h throughout the year (left) and throughout the session on March 02,2021 (right).*

Conditions

All images were taken from the backyard in Groningen, The Netherlands (53.18, 6.54), divided over 4 nights from mid February to mid March 2021. During some nights, the moon was very present, but did not disturb the narrow-band images too much.

Capturing

Sh2-129 is a large object. Often people refer to mosaics to capture the full dimensions. The combination of the FSQ-106 and the full-frame ASI6200MM-Pro gives a large enough Field of View to capture the whole nebula in one frame.

Technical Details

Telescope
Mount
Camera
Filters
Accessoires
Software

Takahashi FSQ-106, Sesto Senso
10Micron GM1000HPS, Berlebach Planet
ZWO ASI6200MM Pro, cooled to -15 ºC
Chroma 2” H-alpha (3nm), OIII (3nm) and RGB, all unmounted, ZWO EFW 7-position
Fitlet2 (Linux Mint 20.04), Pegasus Ultimate Powerbox v2, Flip Flat
KStars/Ekos 3.4.3, INDI Library 1.8.8, Mountwizzard4 1.04, SkySafari 6.7.2

Frames

The object is a typical object that needs to be photographed in both the H-alpha (flying bat) and OIII (flying squid) wavelengths. The signal, especially from OIII is very low, so a long exposure time was required. Exposures were 5 minutes for H-alpha and 10 minutes for OIII. And even with these long exposure times, total exposure of OIII needed to be much longer than usual to pick up some signal. To get some more enhanced star colours, a short series of RGB frames were shot at exposures of 2 minutes each.

Below is a list of frames taken in each session that made it to the final image.

Image

Even with the 10 minute exposures of OIII, the signal was still very weak. In total almost 20h of OIII data was collected to get some reasonable signal. The bortle 5 location was not perfect. This object is best captured from a real dark location. The 8h of H-alpha data seems short in comparison, but is also a pretty long overall exposure. With almost 30h of total exposure, this was by far the object with longest exposure time that was ever shot at the observatory.

After a little crop to straighten the edges, the final image has a resolution of 7000 x 5250 pixels, or 36.8 Megapixels. It covers a field of view of 2.84 degrees horizontally.

Processing

All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and stacked using the WeighedBatchPreprocessing script.

The H-alpha image came out quite well after stacking 96 frames, with a total exposure of 8h. A regular background extraction and MMT-based noise reduction gave a decent image. Much harder was it to obtain a proper OIII image. On an individual frame practically no image of the flying squad could be seen. It was only after all 118 images were stacked together, that the nebula stood out. But only barely against a fairly bright background. Because of the low signal to background ratio, a proper background extraction was hard to achieve and this appeared quite obviously when combining the channels. The OIII spectral line at 510nm is right in the middle between the Green and the Blue filter. The teal colour of this wavelength can therefore best be reproduced by assigning the OIII signal to both the Green and Blue channel. The H-alpha signal is assigned to the Red channel, creating a HOO image. In narrowband imaging often non-realistic colour palettes are used, but the HOO assignment is a fairly accurate colour representation.

Upon combining the two narrow-band images, the teal OIII signal was all over the place, causing a lot of noise, and hard to remove teal-coloured corners of the frame. Background neutralisation and Extraction helped a bit, but still not great. Another problem was camera angle rotation. Between the 4 observation sessions, it was intended to keep the camera angle the same, but apparently if was a bit off on one of the sessions. This caused some further artefacts in the corners. To help both issues, a pretty significant crop was made. This helped the edges of the frame, the overall noise-level and high background level still needed to be addressed.

After manually stretching the image using Histogram Transformation, luminance was extracted and separately processed. An extra round of MMT-based noise reduction was applied. Also the corners were darkened using Curves Transformation while the nebula was masked using the GAME script from Herbert Walter. Meanwhile the RGB image was convoluted to reduce the noise. The reds came out a bit too bright and artificial, which was corrected using Curves Transformation. The Luminance was brought back into the RGB-image using LRGB-combination and this revealed a much better overall image. To bring down the teal colours around the image a bit so the Flying Squid would pop out a bit more, an inverted squid mask was made using the GAME script. Using Curves Transformation and teal-targeted Saturation, the background teal colour was a bit reduced. This finished largely the image of the two nebulae.

The Red, Green and Blue data were only lightly processed, using Automatic Background Extraction and not further noise reduction. An RGB image was created using Channel Combination and Background Neutralisation was applied to balance the colour channels. The image was stretched and cropped to the HOO image. To ensure the luminance values of the RGB image was the same as the one from the HOO image, a Luminance was extracted from the HOO image and applied to the RGB image. With a star mask in place then, the stars from the HOO image were replace with stars from the RGB image using simple Pixelmath. It is always remarkable how subtle but nice effect this gives on the overall image. Since the image is about the nebulae, the stars were shrunk a bit using Morphological Transformation.

The final finishing touches were done in Photoshop and involved increasing a bit of the contrast, eliminating the last pieces of colour-vignetting in the corner and an overall small enhancement of brightness and colour.

This image has been published on Astrobin.