Is A True Color Image?
Image with intrinsic color relationships.
Same image roughly convolved with the
response of the human eye.
What is a true color astronomical image? Is it
what an astronaut would see if there was an eye
piece on, say, the Hubble Space Telescope? Or is
it one that captures the intrinsic colors emitted
by the stars, nebulae, and gas clouds in galaxies?
For the Hubble Heritage image of globular cluster,
M15, we constructed an image which attempted to
captured these intrinsic colors as closely as possible
given Hubble Space Telescope (HST) data. The detail
on the left above, highlighting Planetary Nebula
K648, is an example of this approach.
Our goal was to maintain the relationships between
the intrinsic energy output at a selected wavelengths.
HST Wide Field and Planetary Camera 2 data consists
of black and white exposures taken through filters.
(Please visit "Is
that what they really look like?" by Zolt
Levay, for an in-depth explanation of image
construction.) Each filter is centered on a certain
wavelength and selects a range of wavelengths in
spectrum. For M15 we used black and white exposures
taken through 5 different filters.
For all our images we assign a color to the image
in each filter and combine the color images. If
the color assignment increases with increasing central
wavelength, then we call this assignment
chromatic order. Usually we don't worry if the
assigned color exactly matches the filter's central
wavelength nor if the difference in brightnesses
between filters is maintained. But in the case of
M15 we did worry about these things.
First we calibrated the black and white data so
that we knew how much energy, on average, came through
each filter and was detected at each pixel. (For
those who are curious, the units were erg/sec/cm*cm/angstrom.)
Then we made sure that when we mathematically manipulated
the brightness levels, to bring up the faint light,
that we applied the identical manipulation to each
filter. This way we could maintain the intrinsic
relationships between the different parts of the
Next we assigned a color to each filter's image.
Two filters capture the light from ionized gas;
ionized hydrogen appears magenta and ionized oxygen
is green to the human eye. Three of the filters
capture the light from the stars. One filter has
a central wavelength that is blue, another has a
central wavelength that is a green which is just
a tad yellow. And the last filter, ah well, please
forgive us... it has a central wavelength that is
in the infrared! That's from part of the spectrum
that the eye does not see. Since there were no data
through a filter which matched the red color seen
by the eye, this one would have to be our substitute.
Combining these color images results in the image
on the left above. (For the
Dying Star in Globular Cluster
M15 image we de-emphasized the oxygen filter
so that you could more easily find the pinkish planetary
nebula. Here you can see that the pink (hydrogen)
and green (oxygen) mix to form a yellowish color.)
Clearly some stars are more blue than others, indicating
that they have higher surface temperatures. But
a surprise was that the planetary nebula appeared
more pink than green. Most people looking through
a large telescope report that, in general, these
nebulae appear green. However independent measurements
of this specific nebula demonstrate that it does
emit more light due to ionized hydrogen (magenta)
than ionized oxygen (green)! (Adams
et al. 1984). Our image has captured the intrinsic
relationship between these 2 colors emitted by the
So what would be the "true" color, if instead that
was defined to be the color seen by the human eye?
Roughly, it could be the image seen on the right
above. At low light levels
the human eye is most sensitive to green light,
then blue, and lastly red. But this brightness relationship
is not in color! For example, small faint blue stars
simply appear white and brighter than similar red
stars. The color sensitive part of the eye responds
most to the color yellow and slightly more to red
than green. Mixing the brightness relationship with
this color response we would expect to see a ghostly
green nebula, as reported.
Thus trusting our eyes we would never know that
there was more emission from magenta, ionized hydrogen
than green, ionized oxygen. And we can barely see
a difference between hot (bluer) and red (cooler)
So consider the Hubble Heritage image
of M15, a representation of what you could see
if astronomical objects were brighter and if your
eyes were sensitive to a slightly larger range of
the electromagnetic spectrum. And besides maintaining
the intrinsic relationships between the colors emitted
by the stars and the nebula in M15, don't you think
this image has a striking glow?
English and Forrest
rendition of Planetary Nebula K648, see Howard
at Space Telescope.