In many entries of
this blog I have discussed
astrophotography made using narrow band filters, i.e. those filters that
only let pass light of a very well determined color. The purity of the color is
given by how narrow is the slice of the light spectrum (the rainbow, so to say)
that the filters allows to pass. I use filters with 5 nanometers band width,
that is very narrow. The more "traditional" astrophotography with
monochrome CCD cameras is made using red, green and blue (RGB) filters, which
combined give a "natural" looking color image. When the
astrophotographer should use a set of narrow filters, or a set of broad band
filters, depends of many factors. In my case a critical factor is the light
pollution in my area, from where I image. This has led me to choose narrow band
filters in most of my images. This is because the filters, letting through only
a very specific color, have been designed by keep out most of the light emitted
by urban and industrial illumination.
However, not all
celestial bodies are equally brilliant at the wavelengths that the narrow band
filters let pass. The typical "emission nebulae", those red giant
clouds, are red because of the hydrogen emitting in the well known line of
635nm (hydrogen-alpha line), and they are very well captured by amateur
photographers. In general, emission nebulae do well with narrow band
astrophotography. But many other bodies have a continuous spectrum, that is,
like our Sun, they emit in all wavelengths, so yes they also emit at the
wavelengths that the NB filters let through, but proportionally a very small
fraction of their light. The bodies that
emit in a continuous spectrum are for example stars, therefore clusters and
galaxies (see my entry of the Andromeda Galaxy, M31, in narrow band filters),
and also reflection nebulae. It is impossible, or let's say, it does not make
sense, to try to capture a reflection nebula with narrow band filters.
In summer, shortly
after I started using my new Astro-Physics 130GT, I tried an experiment with
the Trifid nebula, or Messier 20. This nebula is well known for having an
emission portion and a reflection portion. It gives therefore a very nice red
and blue combination when imaged with RGB filters. With the hidrogen-alpha
filter, the emission portion show up in great detail, but the reflection part
is gone… So I thought, would it be possible to combine the H-alpha with a blue
(broad) filter? So tried, and took 28 frames of 10 minutes each with the
H-alpha filter, and 34 frames of 4 minutes each with the blue filter. Also,
this was the first image I took with the field flatenner from Hotech attached
to the GT. So I considered it only a small test. Of course, processing was teh
difficult part. In the blue channel I could not extract a lot of information,
having only about 2 hours and 16minutes.
I had to generate a synthetic green channel combining the H-alpha and B channels,
which then I assigned to red and blue, as expected. At the end, the result was
ok I think. The emission nebula tends to have an orange hue, but that can be
changed if desired to the more"classical" pinkish-red.
Then, in Novembre, I
came across anoter possible target which also has a combination of emission and
reflection. This object, or actually a collection of them, is far less known
and far weaker than M20. The field, however is dominated by the very brilliant
star Gamma-Cassiopeiae, a blue supergiant, which is obviously the bright thing
at the right of the image.
The two main nebulae
in the field have almost identical proximity to Gamma Cassiopeia, about 3
or 4 light years. They are Sharpless-185 (Sh2-185) or also called IC59 (left)
and IC 63 (below center). In pure RGB images they look a little different than
here. See for example :
IC59 seems to be
more dominated by reflection, and IC63 more by emission. In my
"mixed-band" image, you can also see more of the blue in IC59, but
they have a more similar appearance. In any case, both interstellar clouds are
being slowly eroded and dissipated by the UV radiation for the young and
energetic star nearby. Gamma Cassiopeae
is an erratic variable, about 40000 times more brilliant than teh Sun.
The star and teh nebulae are at about 600 light years from us.
For this photo I
have used the following equipment and conditions:
AP 130 Gran Turismo,
with Hotech field flattener (f/6.2, 800mm focal length)
Camera QSI 683ws
Mount ASA DDM60
Filters H-alpha 5nm
and B (Astrodon)
H-alpha: 11 hours
and 50 minutes, in subframes of 10 minutes
Blue: 8 hours and 40
minutes, in subframes of 4 minutes
Processed with
Pixinsight 1.8
In summary, I believe that in certain objects, a combination of broad and narrow band astrophotography can be a good choice. After all, many people are already doing LHaRGB or a similat combination of Halpha with RGB. In cases where reflection and emission nebulae are involved, HaB is an option.
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