Observing Project 6D The Invisible

In visible light, the Sun shows us dramatic examples of its activity level. Sunspots show signs of powerful action in the solar subsurface but beyond the photosphere there is little to see in visible light. Massive solar flares and prominences generally hide from view when they are on the solar limb and cannot be seen unless a total eclipse is in progress. At least that is if you are observing in visible light. At this point you're probably asking, "What else would I be looking at?" Is there something more perhaps to see?

Actually to see more of the Sun, you need to see less. The Sun produces energy across the entire spectrum and when we look at it through a telescope, we are seeing the entire visible band from deepest red to deepest blue or what is called "white light." Red dominates the solar chromosphere because hydrogen emits energy principally at the red end of the spectrum. When we limit our view to a very narrow part of the spectrum and eliminate everything else, amazing detail jumps into view. What we are most interested in is the light of what is called hydrogen-alpha. Hydrogen is the simplest element in the universe. A hydrogen atom consists of one electron orbiting one proton. That electron can orbit in one of several different orbits, which are numbered outward from the nucleus of the atom. When an electron gains energy, it jumps to a higher orbit and when it does so, it creates what is called an absorption line in its spectrum. When it loses energy or when a proton gains energy the electron drops to a lower orbit and creates an emission line in its spectrum. An electron that drops from the fourth orbital level to the se cond emits a special kind of light called hydrogen-beta. This kind of light is emitted by many deep space phenomena such as emission nebulae like the famous Orion nebula. Hydrogen-beta is a higher energy light so is visible more readily than is hydrogen-alpha. The Orion Nebula does emit hydrogen-alpha but is too faint to see in most cases. But this is not the case with the Sun. The Sun emits hydrogen-alpha that is very bright just as it does at every other wavelength. Hydrogen-alpha is produced when the electron drops from the third level orbit to the second. This causes an emission line to appear in the Sun's spectrum about 2 angstroms wide centered on 6,562.8 angstroms19. A hydrogen-alpha filter limits our view of the Sun to only that light emitted by hydrogen-alpha at 6,562.8 angstroms. These filters basically come in two types for your telescope, front mounted and rear mounted. The rear mounted filter is much more complex but offers much narrower bandpass, as little as 0.1 angstroms deviation from 6,562.8 angstroms. It will usually require an electrical power source to heat an internal oven. The front mounted filter is mechanically very simple requiring no electrical input, but is not as precise. The light passed through can be off by as much as 0.7 angstroms. This is important because if the bandpass gets any wider, then the detail of hydrogen-alpha will disappear and be overwhelmed by white light. What type you choose will be determined primarily by budget. Hydrogen-alpha filters start out at nearly $500.

All H-alpha filters systems consist of three main elements, an energy rejection filter (ERF), a telecentric lens system and the H-alpha filter itself. The energy rejection filter removes unwanted light from the ultraviolet and infrared bands. The tele-centric lens system straightens light prior to passing the filter. It typically consists of a 2x Barlow, an optically neutral spacer and convergent doublet. Light emerges from the telecentric system traveling straight and enlarged by a factor of three. In effect an f/10 telescope becomes an f/30. The final element is the actual H-alpha filter. When light strikes the filter, it first passes an antireflective element that enhances contrast and transmission. Light then passes through a narrow-band filter, which further restricts passage of light to very little beyond the H-alpha band. The heart of the filter is a Fabry-Perot etalon. The etalon consists of two panes of parallel glass or quartz coated with a reflecting material that passes only a very narrow band of

19 A nanometer is equal to exactly one-billionth of a meter. An angstrom is equal to one-tenth of a nanometer or about the size of an atom.

light. The two filter elements are separated by a spacer that maintains a very precise distance between the two elements. The spacing between the two etalon elements is critical because it very precisely determines what the final bandpass of the filter will be. The two elements are spaced by somewhere between 100 and 200 nanometers. Since such a simple etalon will pass multiple secondary wavelengths of light, a broadband filter backs up the etalon and eliminates the secondary wavelengths and a final antireflective glass element forms the end of the filter.

With the H-alpha filter installed, the Sun comes to life in dramatic new ways. Though sunspots are best viewed in ordinary white light, the active regions surrounding them become more apparent including surface features such as spicules that point radially away from sunspots, or fibrils and loops that occur when the magnetic field are stronger close to the surface. Solar flares are caused by extremely severe magnetic field stresses and are transient events that can last between a few minutes and several hours. Normally the matter blown out of the photosphere will settle back onto the surface in the form of a mist raining out of the corona. If sufficiently energetic, the rising solar material could be blown clear at escape velocity and leave the Sun in the form of a coronal mass ejection. This in turn can have serious impact on the magnetic and near-space environment here on Earth.

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