Milky Way from Scutum to Ophiuchus

On this page a 50°×16° wide-field view of the Milky Way that covers large parts of the constellations Scutum, Serpens Cauda and Ophiuchus is presented in different color composites. That part of the sky is characterized by HII regions near the galactic plane, the large emission nebula SH2-27 (Zeta Ophiuchi Nebula), and dense star clouds, more or less hidden behind dark nebulae.

All pictures below are downscaled versions. Full resolution images with more than 100 megapixels can be loaded with a Javascript viewer by clicking on the images in the first section. Selected details are shown in the second section. The third section presents some discoveries. Image and instrument data can be found at the end of this page.

Full views

Click on the images to load a full resolution version with more than 100 megapixels using a JavaScript viewer.

Milky Way from Scutum to Ophiuchus in H-alpha, blue continuum and red continuum — This image is a false color composite in which H-alpha (including red continuum) is mapped to red, blue continuum (including [OIII] and H-beta emissions) is mapped to green, and red continuum (without H-alpha) is mapped to blue. Reflection nebulae appear green to blue, while HII regions are red. Stars in the continuum channels are partially subtracted to make the faint nebulae visible.

Milky Way from Scutum to Ophiuchus in H-alpha — This visualization is a pseudo-color image that only uses the H-alpha data (including some red continuum). It shows many more details of the emission nebulae than the image above.

**Color composition:** After partial star subtraction, the dynamic range was compressed using a non-linear high-pass filter. This results in a compression ratio r, which is used to calculate the color as depicted in the legend. (The legend shows the compression c:=1-r). Blue regions are compressed the least, while white regions are compressed the most. Luminance is determined by the tonal curve-corrected result of the dynamic range compression.

Legend for false color image of Milky Way from Scutum to Ophiuchus in H-alpha — This visualization is a pseudo-color image that only uses the H-alpha data (including some red continuum). It shows many more details of the emission nebulae than the image above.

**Color composition:** After partial star subtraction, the dynamic range was compressed using a non-linear high-pass filter. This results in a compression ratio r, which is used to calculate the color as depicted in the legend. (The legend shows the compression c:=1-r). Blue regions are compressed the least, while white regions are compressed the most. Luminance is determined by the tonal curve-corrected result of the dynamic range compression.

Milky Way from Scutum to Ophiuchus region in RGB — An almost-true color image. Unlike to the other images, the stars are not subtracted. This improves the visibility of dark nebulae that absorb the light from the stars behind.

Due to the limited resolution of continuum channels, the image is only presented at half resolution.

Selected details

Here are a few details that also can be seen using the JavaScript viewer.

Galactic plane near M16 in H-alpha, blue continuum and red continuum — The galactic plane near the M16 (Eagle Nebula), the bright orange nebula on the left side. The pinky regions are clouds of unresolved stars which are more or less obscured by dark nebulae.

Galactic plane near M16 in H-alpha — The region around M16 (Eagle Nebula) in H-alpha as pseudo-color visualization. M16 and the nebulae around may be a part of a larger complex which appears to be dissected by the dark nebulae that lie in front. These dark nebulae are also responsible for many details in the image above.

SH2-27 (Zeta Ophiuchi Nebula) in H-alpha, blue continuum and red continuum — SH2-27 (Zeta Ophiuchi Nebula) is a large (about 15°×10°) emission nebula which is ionized by Zeta Ophiuchi, the bright star in the center of the nebulae. Its distance to earth is about 370 light years. SH2-27 lies behind or within interstellar dust which partially absorbs the light of the ionized hydrogen. The dark nebulae obviously lies in front. The matter that causes the bluish haze (reddish in reality because red continuum is mapped to blue) is either thin or lies within the emission nebula. (It cannot lie behind because it partially absorbs the light from the emission nebulae.)

SH2-27 (Zeta Ophiuchi Nebula) in H-alpha — Same view of SH2-27 as above, but here as pseudo-color visualization calculated from H-alpha data. This image reveals much more structures (probably of the dark nebulae the lie in front or within SH2-27).

Bow-shock at Zeta Ophiuchi — Zeta Ophiuchi, the ionizing star of SH2-27, is a running star. It is assumed that this star was a component of a binary system and was ejected by the supernova explosion of its more massive companion. The high velocity relative to the interstellar medium creates a bow-shock (at the boundary where the speed of the stellar wind abruptly drops due to the interaction with the interstellar medium).
That bow-shock is visible in both variants shown here. (left side: H-alpha and continuum, right side: H-alpha pseudo-colors).

Discoveries

The views above show many HII regions that cannot be found in catalogs. (The JavaScript Viewer allows identifying objects using catalogs or SIMBAD and defining new objects.) Some (probably not all) of these unexplored nebulae have been collected in the list below. Click on the following links for a presentation.

Notes

In the JavaScript viewer, the object outlines can be toggled on and off by pressing the '2' key. This can be helpful to make certain structures (e.g. rings) visible.
False color images containing H-alpha and continuum:
- H-alpha is mapped to red, blue continuum is mapped to green, and red continuum (without H-alpha) is mapped to blue.
- Color of molecular clouds and reflection nebulae in the false color image is something between green (bluish in reality) and blue (reddish in reality).
- HII regions (ionized hydrogen) appear red to orange, depending on the amount of OIII (doubly ionized oxygen) detected by the blue continuum filter.
SIMBAD queries for certain object types can be made easily in the JavaScript Viewer by drawing a circle and pressing a shortcut key or via the menu
A repository with the discoveries can also be found on GitHub

Image data

Images where captured with a camera array which is described on the instruments page.

Image data are:

Projection type:

Stereographic

Center position:

RA: 17h48, DEC: -7°30'

Orientation:

Above:	North is right
JavaScript viewer:	North is up

Scale:

10 arcsec/pixel (in center at maximum resolution)

FOV:

50°×16° (RA×DEC, through center)

Exposure times:

Sum of exposure times of all frames used to calculate the image.

H-alpha:	8.3 d
Continuum channels:	6.1 d

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

Bias correction, dark current subtraction, flatfield correction
Alignment and brightness calibration using stars from reference image
Stacking with masking unlikely values and background correction
Star subtraction
Denoising and deconvolution both components (stars and residual)
RGB-composition (same factor for stars and residual for the true color composite)
Dynamic range compression using non-linear high-pass filter
Tonal curve correction

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