Orion region

On this page a 24°×34° wide-field view of the constellation Orion is presented in different color composites. This region is full of emission nebulae and Molecular clouds.

The 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.

Orion region 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.

Orion region 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 orion region 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.

Orion 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.

Orion region: M42, Running Man Nebula, Horsehead Nebula, Flame Nebula — A region full of many kinds of nebulae. Most famous ones are (from top left to lower middle): NGC 2071 (cyan), Flame Nebula (pinkish, almost white), Horsehead Nebula (pink), Running Man Nebula (mostly cyan) and M42 (yellowish).

Orion region: Barnards loop and Witch Head Nebula — A wider view which also shows the Which Head Nebula (bottom right) Barnards Loop (the red arc across the whoole image).

Orion region: SH2 264 — Orions head, SH2 264 in H-alpha.

Orion region: Detail of Barnards loop — A detail of Barnards Loop in H-alpha. Flame Nebula and Horsehead Nebula can be seen too.

Orion region: M42 region — The M42 region and a part of Barnards Loop in H-alpha.

Observations and discoveries

Click on the following links for presentations of interesting observations using the JavaScript Viewer.

SH2 264 and Lambda Orionis Ring with distance estimations using Gaia EDR3 data.
Molecular clouds and HII regions in the southern part of Orion (containing Barnard's Loop, the Orion Nebula and the region behind the Horsehead Nebula). These HII regions are ionized by several sources which makes it difficult to estimate their distance. By considering interactions between molecular clouds and emission nebulae and by searching objects with known distance that are related to the molecular clouds (using SIMBAD), it is possible to find distance constraints or even estimates for HII regions.

The views above show many nebulae 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.

Objects that are not cataloged by now and objects the are related to them:
- A1-A8 in H-alpha and Continuum
- A1-A6, A8 in H-alpha
Uncataloged HII rings and arcs. Some of these structures may be random, some may be projections of spherical shells (bubbles). Such structures are always circular in the image because stereographic projection was used. The outlines of these structures (all but AA4) are completed to circles. It may be helpful to toggle these plots on and off by pressing the '2' key in order to improve the visibility.
- AA1-AA4 in H-alpha
- AA1-AA3 in H-alpha and Continuum

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:
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.
- HII regions can be obscured (i.e. their radiation is scattered and/or absorbed) by molecular clouds that lie in front of them. However, emission nebulae do not obscure molecular clouds. This means that a HII region lies farther away if it appears to be hidden by a visible molecular cloud or a dark nebula. If molecular clouds and HII regions look somehow mixed (pinky red), the HII region is either nearer, or the molecular cloud is very thin. The latter aspect is the reason why it is difficult to determine whether an unobscured HII region lies in front of the molecular cloud.
  An improved visualization of the correlation between HII regions and molecular clouds is planned as soon as better data are available, namely NIR data, which better indicate regions that are optically thick for H-alpha emission.
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: 5h28, DEC: 1°
Orientation:	North is up (exactly)
Scale:	10 arcsec/pixel (in center at full resolution)
FOV:	25°×34° (through center)
Exposure times:	H-alpha: 7.7d, continuum channels: 4.3d (sum of exposure times of all frames used to calculate the image)

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:

H-alpha only: bias correction, photon counting
Dark current subtraction, flatfield correction, noise estimation
Alignment and brightness calibration using stars from PPMXL catalog
Stacking with masking unlikely values and background correction
Extracting stars
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 (except of the true color composite)
Tonal curve correction