In a number of posts I will be studying the composition of the Milky Way, with the goal of better understanding the sources of the light from objects in the galaxy observed here on the surface of this planet we call Earth. I will focus on radiation that I can capture in my telescope –the visible spectrum.
In parts 1 through 3 of this series we have learned that the Milky Way’s interstellar medium is approximately 15% of the total mass in the galaxy. It is richer in heavy elements than the galaxy in general, with small but important amount of Carbon, Nitrogen and Silicate dust. We explored the distribution of stars and nebulae in the sky as seen from our planet, Earth.
Figure 1: North America and Pelican Nebulae, image processed to remove all stars and emphasize the nebulosity. Image captured at Lac Teeples 2020-08-14. Telescope was the RASA-8.
While star light, reflection nebulae and dark nebulae emit over a wide range of wavelengths, emission from ionized gas near extremely powerful energy sources is at particular spectral lines or wavelengths. There are actually a vast number of spectral lines observed with professional equipment, but in practice most astrophotographers are limited to a small number of options.
Spectral Lines and Filters in Practice
Narrowband astrophotography involves using filters with very narrow passbands to look at specific emission lines from nebulae. Narrowband astronomy is dominated by three particular lines and filter combinations. The dominant spectral lines are Hydrogen Alpha (H-alpha), Oxygen III (O-III) and Sulphur II (S-II). These are shown in Table 1.
Table 1: Spectral Lines, wavelength and likely filter options
Spectral Line | Wavelength (nm) | Filter to match (example) & comments |
H-alpha | 656nm Red | Baader H-alpha Ultra-Narrowband-Filters CMOS-optimized, 3.5nm passband |
O-III
| 501 and 496 Blue-green (two nearby lines) | Baader O-III Ultra-Narrowband-Filters CMOS-optimized 4nm passband. 496 line is not in passband |
S-II | 672 and 673 (two nearby lines). Red. | Baader S-II Ultra-Narrowband-Filters CMOS-optimized 4nm. Both lines passed. |
Unusual below | | |
H-Beta
| 486 Blue | Easily found but uninformative. Used in visual, rarely in Astrophotography |
He-II | 468 Blue | Possibly Edmund Optics |
He-I | 587 Yellow | Possibly Edmund Optics |
O-I | 630 Red | Possibly Edmund Optics |
Ar-III | 713 Infrared | Possibly Edmund Optics |
Rare and Exotic below | | |
Ne-III | 370 Ultraviolet | ? Camera sensitivity at this wavelength an issue. |
Ar-III | 775 Infrared | ? Camera sensitivity at this wavelength an issue. |
S-III | 953 Infrared | ? Camera sensitivity at this wavelength an issue. |
N-II | 658 Red | Astrodon sells a filter for this, passband 3nm |
| | |
The ZWO ASI6200 MM camera has a quantum efficiency of approximately 70% at 400nm, but this drops quickly as wavelengths shorten in the UV. In the infrared it drops slowly, from 50% at 700nm, to 30% at 800mn and 15% at 900nm. The camera efficiency peaks between 450nm and 550nm at around 90% (blue light).
Given this, some filters would be useless if used with this (typical) CMOS camera. For example, a filter for S-III would be useless since the camera would be very insensitive to this wavelength of light.
Concluding Remarks
In this series "What Light Through Yonder Telescope Breaks?" I have researched the spatial distribution of stars and nebulae in the sky, the material within the Milky Way Galaxy and the types of nebulae that are observed. This discussion led to the detailed review of spectral lines observable with commercially-available filters.
Looking up at the night sky we see stars distributed evenly in nearly all directions because apparent brightness is dominated by proximity to us. If the thickness of the Milky Way were reduced such that it was only one star thick we'd see a line of stars in the sky. When we are in areas with little light pollution and can see dimmer stars, the pattern of the Milky Way disk emerges into view. Galaxies are distributed evenly over the sky (to a good approximation) but are only visible away from the disk of the Milky Way because the material in the Milky Way blocks the light from more distant objects. Dying stars and supernovae (observable to amateurs) are distributed similarly to the stellar neighborhood, with some greater density in the galaxy disk. Bright nebulae with star forming regions are quite tightly clustered along the disk of the Milky Way.
The galaxy is primarily composed of Hydrogen and Helium, with many other elements present. Most of the mass in our galaxy is in the form of stars. Most stars are smaller than our Sun, but there is a distribution of masses which leads to a distribution of brightness and surface temperatures. A very few are extremely large and bright, and where such an extremely intense source of radiation exists, gases nearby are ionized and particular emission lines can be observed. Otherwise, the gases are not observable unless they contain dust grains which may block light (dark nebulae) or reflect light (reflection nebulae).
Where we do see emission lines, whether in star formation regions, planetary nebulae or supernova remnants, there are three main spectral lines of interest. It's unclear if these spectral lines are significantly more important than others, but filters are available for them and they are of scientific interest because they reflect differing physical properties within the nebulae of interest. Astrophotographers will utilize Red, Green, and Blue filters for light from stars and reflection nebulae, supplemented by the three primary narrowband filters.
Figure 2: RGB filters from Astronomik used to capture star light and reflection nebulae.
Figure 3: Narrowband filter from Astronomik used to capture the Hydrogen-alpha emission line at 656nm (only). This is a relatively wide passband at 12nm.
What Light Through Yonder Telescope Breaks?
Comments