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Creating "Color" Images From Narrowband Data

Narrowband images are also called "false color".  Color is used in astronomy to convey important information on the composition of astronomical objects. We take advantage of the brain's ability to discern color differences to convey information on the relative distribution of the components of nebula and other narrowband targets.

Since my particular location is plagued by light pollution and frequent clouds, I prefer to take narrowband images. I can take narrowband images on any clear night as long as the moon is not right on top of the object.  They are not affected by light pollution. Unfortunately targets that consist mainly of stars (galaxies and star clusters) can not be photographed with narrowband filters. I use a different technique for them.

Creating a Narrowband Image

In a Narrowband image I collect separate data for each filter I am using as a series of exposures.  I then register and integrate these exposures together possibly combining them further using HDR if the object has many ranges of brightness (like M42). Each filter only allows light from a single energy state change in the atom.  Quantum mechanics requires each of these changes to be a a specific energy level which directly maps to wavelength.  Since all of the energy is concentrated in a single wavelength, a narrow filter will capture virtually all of the energy. Since light pollution and moonlight is spread over the entire spectrum (with the exception of LPS) most of the unwanted light is discarded.

I use 4 different narrowband filters. In this drawing Nitrogen emission overlaps with Hα

emission spectra


Constructing an Image

A false color image is made by taking 3 grayscale (black and white) images and assigning each to a color channel.  In this example I have used the CFHT or  HON palette
This assignment is referred to as a palette.  Much like an artist's palette it defines what colors are available. The goal is to produce an image that will easily show how the various elements are distributed.

As an example the following shows the 3 gray scale images that were combined to make the final RGB for the West Section of the Veil Nebula.  Note the differences in the nebula between the different elements.  This represents differences in the distribution of these elements in the blast debris.

Click any of the images below for a larger version.

Hydrogen = RED

Hydrogen assigned to Red
Oxygen=Green

Oxygen assigned to Green

Nitrogen= Blue

Nitrogen assigned to B

HON Palette Image

HON


Color Palettes

If you give up on the notion of trying to reproduce spectrally correct color and instead use color to convey information that gives you a number of possibilities.  With my equipment I have 4 narrowband filters that I can use to encode information 3 filters at a time.  Assigning the output of filters to a color is called a palette.  In the following I describe the palettes by how they map to R, G, and B.  Remember the ultimate goal is to produce the most interesting picture.  Thus unbound from any usage convention I will use the palette or palettes that best display my subject.

Standard Palettes

Two palettes are in common use in the professional community.

The Hubble Palette

Hubble
              Palette

NHO

Hubble Palette

NHO veil

One of the most famous palette in use in astronomy is the Hubble Palette.  These are the color assignments in use for narrowband images in the Hubble telescope.
  • Red = Sulfur
  • Green = H alpha
  • Blue = O III
The picture on the upper left is the most famous of all Hubble pictures.  It is a false color image of a portion of the Eagle Nebula.

The Ha and N emissions are very close and most narrowband filters combine them. My filters split Ha from NII.  Depending on the Object I may replace Sulfer with NII.  For example my image of the Veil (lower left). 


In objects strongly dominated by Ha this results in a sickly green image, but sometimes this is what is required to see the structure.

SH 274 also gives an example of using the Hubble Palette using Sulfur instead of Nitrogen.


CFHT Palette

HOS HON
CFHT
HON


NGC 6888
                in HON

NGC 281 HON
This is the palette used by the Canada  France, Hawaii Telescope
  • Red = H alpha
  • Green =  O III
  • Blue = Sulfur (or N)
For objects that contain a lot of hydrogen it results in a less fake looking image since the red hydrogen is still red.  Of course Sulfur and Nitrogen are also red so this is not real color.

The top photo is NGC 6888. The bottom is NGC 281. Both use Nitrogen.

In NGC 281 note how much more visible the O III enhanced center is when compared to the HNO image below.

SH274 was processed with the HOS palette. Constrast this image to the SHO image above.

Non Standard Palettes

On occasion neither of the two standard palette highlight the features of the object.  Note the difference in NGC 281 between the CFHT and HNO palettes.  The CFHT palette better shows the detail of the object. The HNO palette better shows the texture of the clouds in the vicinity of the nebula.

HNO HSO

HNO


6888 as
                HNO


NGC 281 HNO
An alternative palette to CFHT when the majority of the signal is Hydrogen and the Nitrogen/Sulfur signal is also strong.  Examples are NGC 281

  • Red = H alpha
  • Green = Nitrogen (or Sulfur)
  • Blue = O III
Again NGC 6888 is on top and NGC 281 is on the bottom.

While this particular palette does not do justice to NGC 6888 itself see the impact on the surrounding gas clouds here



Narrowband Filters

Hydrogen Alpha

When Hydrogen gas is excited it glows in a pinkish color.  The principle component of that emission are a series of lines discovered by Balmer.  The most intense of these is a line at  656.281 nm which is in the deep red.  Because of physics the wavelength of each line is very narrow (like a tuning fork). If your filter is just wide enough then it will capture all of the emission filtering everything else out. 

Hydrogen Spectra (from
      wikipedia)
wikipedia

Hydrogen is present in stars, but it is also one indication of star forming regions.
HA

Most Ha filters being used by amateurs also capture the nearby N II line.  By using the 3 nm filter I can isolate N II from Ha.

Nitrogen II

Nitrogen is produced by the CNO process in stars larger than our sun.  It is released into space by Planetary Nebula and some Wolf Rayet Stars.  Nitrogen has several emission lines.  My filter looks at the line at 658.4 nm   Due to its proximity to the Ha line most Ha filters include this line in their images.

N II filter

Oxygen III

Oxygen  is also produced by the CNO process in stars.  It is concentrated in areas where it is produced as nebula.  Oxygen III is an emission that can only occur in space since it requires the gas to be closer to a vacuum than is possible to produce on earth. This produces two aqua colored lines.  I am imaging the one at 500.7 nm.

O III

Sulfur

Sulfur II is the third color on the two most popular narrowband palettes.  Web sources differ on the exact emission lines.  Some say 671:6nm and 673.1nm. Others say 672.4nm. The center of Hubble's filter is 673.2nm but it is about 5 nm wide so it would have a transmission slightly broader than my Astrodon filter.

sulfur

Sulfur is formed when Oxygen is consumed by stellar processes. Thus it represents a later stage of stellar evolution than Oxygen does.