Nebulae / NGC6888 - The Crescent Nebula
NGC6888 - The Crescent Nebula

Object Description (Thanks to the Chandra X-Ray Observatory for the description):
This is the Crescent Nebula, also known as the "Dividing Cell Nebula", or the "Van Gogh's Ear Nebula".  This image reveals dramatic details of the Crescent Nebula, a giant gaseous shell created by powerful winds blowing from the massive star HD 192163.

After only 4.5 million years (one-thousandth the age of the Sun), HD 192163 began its headlong rush toward a supernova catastrophe.  First it expanded enormously to become a red giant and ejected its outer layers at about 20,000 miles per hour. Two hundred thousand years later "a blink of the eye in the life of a normal star", the intense radiation from the exposed hot, inner layer of the star, began pushing gas away at speeds in excess of 3 million miles per hour!

When this high speed "stellar wind" rammed into the slower red giant wind, a dense shell was formed. The force of the collision created two shock waves: one that moved outward from the dense shell, and one that moved inward to produce a bubble of million degree Celsius X-ray emitting gas. The brightest X-ray emission is near the densest part of the compressed shell of gas, indicating that the hot gas is evaporating matter from the shell. The massive star HD 192183 that has produced the nebula appears as the bright dot at the center of the full-field image.

HD 192163 will likely explode as a supernova in about a hundred thousand years. This image enables astronomers to determine the mass, energy, and composition of the gaseous shell around this pre-supernova star. An understanding of such environments provides important data for interpreting observations of supernovas and their remnants.

Date Captured:
7/26/2006 through 8/16/2006

Equipment Used:
 - TAK TOA-150, with 67 Flattener
 - STL-6303, self-guided, 10-second guiding integrations
 - Astrodon narrowband filters
 - FLI-PDF
 - Paramount ME

Exposures (MaxIM for camera control and capture):
 - Ha, 5x30min (2.5h) + 9x15min (2.25h) = 4.75h
 - OIII, 17*30min = 8.5h
 - SII, 10*30m = 5h

Processing:
CCDStack:
 - calibration
 - bloom removal
 - channel sub registration
 - normalization and data rejection
 - mean combine
 - hot/cold pixel removal
 - gentle deconvolution, where appropriate
 - DDP, no sharpening
 - registration of final sub masters

Photoshop CS2:
 - color creation: full tri-color narrowband variations, Ha/sG/OIII bi-color versions as per descriptions
 - curves for adjusting color treatment to taste


NGC6888-Ha-OIII.Hb-OIII
This version uses a manual synthetic green (sG) approach.  This one mixes 20% Ha with the OIII data.  I put it in the green channel, and played with curves a bit.  Probably the most esthetically pleasing of the bunch.
NGC6888-sG Version

This sG version is done with Noel Carboni's PS actions.  This interpretation does:
- a mix of the OIII and Ha data, strongly weighed towards the OIII
- spreads out the high and low end of the cyans and blues to accentuate both, making one end more blue, the other more green.  Same thing with the oranges and reds.
- does a curves adjustment, followed by a user-controlled levels adjustment to allow for personal taste.

This strives for a synthetic green that allows for a more natural appearance than one trying to generate a synthetic SII.


Hubble Palette
A true narrowband tri-color, done with the "Hubble Palette".  That uses SII for red, Ha for green, and OIII for the blue channel assignments.
NGC6888-CFHT version
This narrowband tri-color interpretation uses the famous "Canada-France-Hawaii Telescope" (CFHT) palette.  It assigns Ha to the red channel, OIII to the green, and SII to the blue.
Narrowband Palettes
Here is a comparison of ALL of the various combinations of narrowband palette choices.
Narrowband Component Data
NGC6888-Ha

Ha channel component data:

 - 5x30min (2.5h) + 9x15min (2.25h) = 4.75h


NGC6888-OIII

OIII channel component data:

 - 17*30min = 8.5h


NGC6888-SII

SII channel component data:

 - 10*30m = 5h