Deep in the South constellations Apus and Octans lies a spectacular field of nebulosity part of the Galactic Cirrus. Not far from the South Celestial Pole (at declination -80º), this nebulae are very rarely captured, especially in full colour.
The Galactic Cirri are veils that surround our galaxy – made of dust and gas in the interstellar space. It was first noticed on optical glass plates recorded at Palomar Observatory and subsequently cataloged by B. T. Lynds, in 1965. In the 2000s, Steve Mandel noticed faint cirrus in deep, wide field photographs near the North Celestial Pole, and labelled the nebulosity as the IFN, or the Integrated Flux Nebula. [1] It has incredibly low surface brightness, at ~22-28 mag/arcsec² (fainter than the darkest sky background on Earth), thus it is not easy to capture!
This image was captured on my dark site using really modest gear (full-frame DSLR + telephoto lens on a sky tracker, unguided), but it was not without its challenges. The clouds are extremely faint, and even at 22ºS latitude, it never rises above 30 degrees from my location. The low altitude is further complicated by nearby trees make this frame only 3-10º above my local horizon! There are only a handful of images that probe this region, especially in full colour*. I think this may be due to its immense challenge (requiring excellent sky conditions and careful calibration/post-processing) and deep southern latitude. It was a challenge to process – I tried my best, but extracting the sky gradient is very difficult (I still think the upper right corner might have been a bit over-subtracted). I have also imaged the Western part of this frame
before. After some tries, I decided to let this image go and post my effort here, hoping to revisit the region with more data in the future.
A feature I would like to point out is the different colours of the clouds, in different shades of brown-yellow to grayish, especially the faint cloud running vertically in the center. A useful strategy to reach for those faint clouds is to use a deep Luminance master to boost SNR in the clouds, at the expense of colour information. I could not find other sufficiently deep images to confirm if these color differences are real or an artifact of imperfect calibration/processing. In any case, to my eyes it almost looks like this grayish cloud is behind the denser brown clouds, and gives a sort of 3D look.
Finally, I recommend checking the starless version of the image (Rev. B). I am usually not a fan of starless images as final pictures (but they are extremely valuable during processing). In this case, though, I think it looks beautiful, almost surreal. What do you think?
Constructive criticism, comments and suggestions are more than welcome in the comments section!
Date: 17 June 2021
Location: My Observatory, MG, Brazil. Rural Skies (Bortle 3, calculated SQM ~21.4)
Camera: Canon EOS 6D (mod) at ISO 1600
Optics: Samyang 135mm f/2, operated at f/2.4
Mount: Sky-Watcher Star Adventurer, tracking, unguided
Exposure Detail: 180x60s. Total 180min = 3h.
[1] R. Jay. GaBany. Galactic Cirrus. At:
cosmotography.com/images/galactic_cirrus.html*Note: I recommend checking out the widefield images by Gerald Wechselberger and Hisayoshi Kato around this area.
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