Are Astrophotography Colors Real?

Many of us have admired the beautiful images of nebulas, galaxies and other wonders. But do these objects really look like this?  The answer is…yes and no.

Three Ways to Get There

There are 3 main ways to do astrophotography. First you have basic black and white images like your typical Moon photo, shown below. The Moon, the Sun, and distant galaxies are mostly monochromatic (with some exceptions) so black and white is fine.

As anyone who has done a star party and shown people colorful double stars can attest, colors are subjective.  Some people will see a hot star as blue, others purple, and some don’t see color at all.  Younger eyes that dilate further may see the green glow from the Orion nebula while to an adult it looks grey.

Color Imaging

The most common color images you see are RGB (red/green/blue) images created either with DSLRs, one-shot color (OSC) cameras, or with filters.  A simple example of that would be a photo of planet like Jupiter, a broadband emission source like the Andromeda galaxy.

Objects emit light in one or more wavelengths.  Typical broadband emission sources are stars, planets (reflected starlight), galaxies and globular clusters.  With these types of objects a standard color camera works well.  

You sometimes see very dramatic colors in such objects, and this could be due to the photographer over-saturating colors for personal preference.  But in most cases, the colors of photos of planets, stars, and galaxies are similar to what the eye would see.

When you get to nebulas, it’s another story entirely. It gets tricky because there are different nebula types.  For example, some of them are reflection nebula, and these are emitting reflected starlight off dust.  In this case it concentrates much of light at the blue end of the spectrum, but it remains a broadband emitter.  

Examples of reflection nebula include the Pleiades and the Witches Head nebula.

Narrowband Imaging and the Hubble Palette

Other objects emit most of their light in one or more very specific narrow wavelengths, like emission nebula for example.  

A popular way to do this is with the so-called “Hubble Palette”.  In this case 3 narrowband filters are used consecutively on the same object.   Hydrogen Alpha (656nm), Oxygen III (501nm), and Sulphur II (672nm).  You end up with 3 black and white images. Then in software you assign a color to each wavelength and combine them into a full color image.  

The Hubble Palette assigns Red to S2, Green to Ha, and Blue to O3.  So by looking at the image done this way you can now get a sense of which wavelengths are most strongly emitted by the object. You sometimes see this designated as “SHO” for Sulphur2/Hydrogen alpha/Oxygen3.

One advantage of narrowband imaging is it allows you to do astrophotography in light polluted places, because you notch out all the undesired wavelengths.

SHO images are ‘false-color’ but beautiful. Consider the Rosette nebula. Below are two photos. One in broadband RGB, approximating its true appearance, and the other in SHO.

Which is a nicer photo?  The first one is truer to natural color, but the second one is beautiful and provide information (lots of blue means the object is a strong emitter at Hydrogen Alpha).  

As another example, consider the Tarantula nebula in the Large Magellenic Cloud. Here is a true color image from Wikipedia:

And here is one the author requested via Telescope.live and processed with the Hubble Palette:

So what have we learned?  RGB photos are closest to reality, but even then the red you typically see on a monitor is 630nm (deep red) vs the true red of the HA nebula at 656nm.  Narrowband images can be strikingly beautiful and convey information you simply don’t get in regular color images.