Why does the Sun look so different in $\mathrm{H}\alpha$?

The origin of the line · 656.28 nm

Why does the Sun glow in $\mathrm{H}\alpha$?

It all starts with the simplest atom in the universe: hydrogen, which makes up almost all of the Sun.

The Sun is overwhelmingly made of hydrogen. And every chemical element has a kind of fingerprint: it only absorbs and emits light at very specific colors, not just any. Hydrogen has several of those characteristic colors, and one of them lies in the deep red, at a wavelength of 656.28 nanometers. We call that spectral line $\mathrm{H}\alpha$.

That color is produced by hydrogen when one of its electrons drops to a lower-energy orbital and releases exactly that amount of energy as red light. Because all hydrogen atoms are identical, they all emit and absorb at that very same wavelength: that is why the line is so sharp and so reliable as a reference.

Above the visible surface of the Sun there is a thin, faint layer called the chromosphere, made mostly of hydrogen. That layer glows precisely in $\mathrm{H}\alpha$. The trouble is that its light is completely drowned out by the dazzling white glare of the surface. A filter that lets through only the $\mathrm{H}\alpha$ color switches off that glare and finally reveals the glowing chromosphere: filaments, plages, and prominences.

The physics: the Bohr model and the Balmer series

In the Bohr model, the electron of hydrogen can only occupy orbitals with specific energies, labeled by an integer $n=1,2,3\dots$. The larger $n$ is, the higher and more loosely bound the electron. Energy is not continuous: it is quantized, which means the atom can only gain or lose energy in discrete jumps between those energy levels.

When an electron falls from a high level to a lower one, the atom emits a photon whose energy is exactly the difference between the two levels. The $\mathrm{H}\alpha$ line corresponds to the transition $n=3\to n=2$. Transitions ending at $n=2$ form the so-called Balmer series, which falls in the visible; $\mathrm{H}\alpha$ is the first and most intense of that series.

The energy of the jump sets the color of the light through the Planck-Einstein relation: the higher the energy, the shorter the wavelength. For the transition $n=3\to n=2$ that energy corresponds to $\lambda =656.28\mathrm{nm}$, the deep red of $\mathrm{H}\alpha$.

The same energy level explains two opposite things we see on the Sun. Over the disk, the cooler hydrogen of the chromosphere absorbs the white light coming from below and creates a dark line: it is one of the Fraunhofer lines, a black stripe across the continuous spectrum. At the limb and in prominences, by contrast, there is no bright background behind, and that same hydrogen emits its $\mathrm{H}\alpha$ light against the black sky: we see it in emission, bright.

$$E=\frac{hc}{\lambda }$$

The solar spectrum

The spectrum is not a smooth rainbow

If we carefully break up the Sun's light, the rainbow appears crossed by thin dark lines. $\mathrm{H}\alpha$ is one of them.

When we split sunlight into its colors we expect a continuous rainbow, but on closer inspection it is interrupted by hundreds of very narrow dark lines. These are the Fraunhofer lines, and each one marks a color that was absorbed by a specific element present in the Sun before its light reached us.

Each dark line is the signature of an atom: sodium, calcium, magnesium, iron, and, of course, hydrogen each have their own. The hydrogen lines in the visible range form the Balmer series: all are electron transitions ending at level $n=2$. $\mathrm{H}\alpha$ ($n=3\to 2$, $656.28\mathrm{nm}$) is the first and most intense; it is followed by $\mathrm{H}\beta$ ($n=4\to 2$, $486.1\mathrm{nm}$), $\mathrm{H}\gamma$ ($n=5\to 2$, $434.0\mathrm{nm}$), and $\mathrm{H}\delta$ ($n=6\to 2$, $410.2\mathrm{nm}$). Other dark lines in the same spectrum — such as sodium (Na D, $589\mathrm{nm}$) or magnesium (Mg b, $517\mathrm{nm}$) — belong to different elements and are not hydrogen transitions.

An $\mathrm{H}\alpha$ solar telescope is designed to do exactly the opposite of the rest of the spectrum: instead of discarding the $\mathrm{H}\alpha$ slit, it keeps only that one and rejects everything else.

Wavelength: $656\mathrm{nm}$

Selected: $656\mathrm{nm}$ — $\mathrm{H}\alpha$ — Hydrogen transition: $n=3\to n=2$ ($656.3\mathrm{nm}$)

Dark line on the disk, bright line at the limb

Over the solar disk, the bright white light of the photosphere travels through the chromosphere above it. The hydrogen in that layer selectively absorbs the $656.28\mathrm{nm}$ photons, subtracting them from the continuous background. That is why, in the disk spectrum, $\mathrm{H}\alpha$ appears as an absorption line: a dark notch on an intense background.

At the Sun's limb and in the prominences that rise above it there is no bright photosphere behind, only the dark sky. That same hydrogen, instead of absorbing, emits its own $\mathrm{H}\alpha$ photons. The result is a bright emission line. This is why prominences appear lit up at the edge while filaments (the same gas, but projected onto the disk) appear dark: they are two sides of the same phenomenon.

The full Balmer series covers all transitions to $n=2$ in hydrogen. Only $\mathrm{H}\alpha$, $\mathrm{H}\beta$, $\mathrm{H}\gamma$, and $\mathrm{H}\delta$ fall in the visible range; those for higher $n$ fall in the ultraviolet. The other dark lines in the solar spectrum (sodium, magnesium, calcium, iron…) correspond to electron transitions in those other elements and have no connection to the Balmer series.

The heart of the instrument

The etalon: how a single color is isolated

Keeping only a $0.5\text{A˚}$ slit in the deep red is an extreme optical challenge. The solution is the interference of light.

The key component of an $\mathrm{H}\alpha$ telescope is the etalon, also called a Fabry-Pérot interferometer. In essence it is two ultra-flat glass surfaces, almost like mirrors, very parallel to each other and separated by a tiny, exquisitely controlled distance. Each surface reflects part of the light and lets another part through.

When light enters that gap, it bounces back and forth many times before leaving. On each pass, the waves coming out overlap with those that left earlier. For most colors those waves end up out of phase and cancel each other (destructive interference). But for a few very specific colors, the waves come out perfectly in step and reinforce one another (constructive interference).

The result is that the etalon lets through only an extremely narrow slit of color. By adjusting the spacing between the plates, that slit is placed right on top of $\mathrm{H}\alpha$. It is like a choir of thousands of reflections that only sing in unison on a single note: the red at $656.28\mathrm{nm}$.

Fabry-Pérot: Airy peaks, FSR, finesse, and FWHM

The condition for constructive interference is $2nd\cos \theta =m\lambda$, where $d$ is the spacing between plates, $n$ the index of the medium between them, $\theta$ the ray angle relative to the normal, and $m$ an integer (the order). For each order $m$ there is a wavelength that satisfies the condition and emerges reinforced.

That is why the etalon's transmission curve is not a single clean box, but a series of very thin, regular peaks: the Airy peaks. The spectral distance between two consecutive peaks is called the free spectral range ($\mathrm{FSR}$). Because there are many peaks, the etalon alone is not enough: a prefilter is needed (the ERF and sometimes a broader-band filter) to pass the region around $\mathrm{H}\alpha$ and remove the other orders.

The width of each peak is measured by its $\mathrm{FWHM}$ (full width at half maximum). The finesse of the etalon is the ratio of $\mathrm{FSR}$ to $\mathrm{FWHM}$, and it depends above all on the reflectivity of the surfaces: the higher the reflectivity, the more effective reflections and the narrower the peaks.

For the chromosphere to stand out over the disk, the bandpass (the $\mathrm{FWHM}$) must be very narrow, typically below $0.5\text{A˚}$. If the window is wider, it lets in too much light from the photospheric continuum and the contrast of the filaments is lost: the disk looks smooth again.

$$2nd\cos \theta =m\lambda$$

Fine-tuning the window

Tuning: why what you see changes

The etalon's slit of color can be shifted a little. That small adjustment decides whether you see the disk or the moving material better.

The exact center of the color slit that the etalon lets through is not completely fixed: it can be fine-tuned slightly. We call that tuning. Shifting the window by only a few hundredths of an ångström surprisingly changes what appears in the image.

With the window centered right on $\mathrm{H}\alpha$ you see the disk better: the dark filaments, the bright plages, and the fine texture of the chromosphere. If instead we shift the window toward a wing of the line —toward the blue or the red— it enhances material moving at high speed, such as prominences and flares, because their light appears color-shifted by the Doppler effect.

That is why the same telescope can show two different Suns depending on how it is tuned: one centered on the still detail of the disk and another shifted to catch the jets and arcs of moving gas. Tuning is, in the end, choosing which story of the Sun you want to tell at that moment.

Pressure tuning, tilt tuning, and the Doppler effect

There are two common ways to tune an etalon. Pressure tuning changes the pressure —and therefore the refractive index $n$— of the gas sealed between the plates, which shifts the condition $2nd\cos \theta =m\lambda$ and moves the transmission peak. Tilt tuning tips the etalon by a small angle $\theta$: since $\cos \theta$ appears in the equation, tilting it shifts the peak toward the blue.

Moving material comes into play through the Doppler effect: gas approaching us emits $\mathrm{H}\alpha$ shifted toward the blue, and gas moving away, toward the red. By shifting the etalon's window toward a wing, we tune it to that Doppler-shifted gas and make it visible, while gas at rest is dimmed.

All of this implies a constant trade-off between contrast and light. A narrower, well-centered window gives more chromospheric contrast, but lets through less light and requires longer exposures or a darker image. Tuning well means finding the balance for the specific detail you want to observe.

The full chain

The optical system, piece by piece

The etalon does not work alone. It is part of a chain in which every link is essential, and some of them are essential for safety.

An $\mathrm{H}\alpha$ solar telescope is an ordered chain of components, and light passes through them in this order: objective → energy rejection filter (ERF) → etalon → blocking filter → eye or camera. If any of them is missing, the system stops working or stops being safe.

The objective gathers the Sun's light. Next, the ERF (Energy Rejection Filter), placed at the front, rejects the vast majority of the energy: nearly all the infrared, the ultraviolet, and much of the heat, letting through mainly the red region around $\mathrm{H}\alpha$. Without it, that concentrated heat would destroy the etalon and be extremely dangerous.

The etalon then acts, selecting the $\mathrm{H}\alpha$ slit of color. But at its output there is still residual out-of-band light and the etalon's other orders. The blocking filter takes care of that, right before the eye or camera: it blocks the leftover light, leaves a clean $\mathrm{H}\alpha$ image, and protects the observer. Only then does the light reach the eye or sensor safely.

What happens if the ERF or the blocking filter is missing

Without the ERF, all the solar energy gathered by the objective —including the invisible infrared and heat— would reach the etalon concentrated. The etalon, a delicate and very thin optical part, would heat up, warp, and be ruined; and the energy built up along the path is a serious risk of burns and fire. The ERF is the first line of defense that discards the bulk of the energy before it enters the instrument.

Without the blocking filter, even though the etalon has selected $\mathrm{H}\alpha$, the etalon's other orders and potentially harmful out-of-band light would still pass through, along with a residual glow that would ruin the contrast. For the eye that is dangerous, because some of that light is intense and invisible. That is why an $\mathrm{H}\alpha$ telescope is never used without its corresponding blocking filter or diagonal: it is not an optional accessory, it is part of the safety system.

Three methods, three problems

Comparing solar filters

Not all solar filters do the same thing. Each method solves a different problem and goes in a different place.

There are three main ways to look at the Sun safely, and it is important not to confuse them because they work in very different ways. Certified eclipse glasses (ISO 12312-2) are placed in front of the eyes and are only for naked-eye viewing, without any instrument that concentrates the light.

The white-light solar filter is placed in front of the aperture of the telescope or binoculars, before the light enters the optics. It attenuates the whole spectrum evenly down to a safe level and shows the photosphere: the disk, sunspots, and granulation. It is like wearing very dark sunglasses for the whole spectrum at once.

The $\mathrm{H}\alpha$ telescope is a complete dedicated system, not a mere filter: it combines energy rejection, the etalon, and the blocking filter to keep a single slit of color and reveal the chromosphere. The first attenuates everything equally; the second selects one color. They are tools for different purposes, not interchangeable.

Eclipse glassesWhite-light filterHα telescope

Filter position

In front of the eyes, without any optical instrument.

Spectrum it passes

The entire visible spectrum, heavily and neutrally attenuated (density filter).

Solar layer it reveals

For naked-eye use only. They reveal no solar layer in detail.

Neutral attenuation versus energy rejection plus spectral selection

A white-light filter acts, in essence, like a neutral density (ND) filter: it attenuates all visible wavelengths more or less equally down to a safe level, preserving the shape of the spectrum but greatly reducing its intensity. What reaches you is a heavily darkened version of the whole Sun, in which you see the photosphere.

An $\mathrm{H}\alpha$ system does something qualitatively different in two steps: first it rejects energy (the ERF discards the bulk of the flux, especially infrared and ultraviolet) and then it selects the spectrum (the etalon and blocking filter pass only the $\mathrm{H}\alpha$ slit). It is not a uniform attenuation: it is a surgical selection of one color, which is exactly what lets you see the chromosphere rather than the photosphere.

Solar safety

Rules that never change

• Eclipse glasses are never used with optics

  Certified eclipse glasses (ISO 12312-2) are only for naked-eye viewing of the Sun. They are never used with a telescope, binoculars, or camera: the optics concentrate the light and would burn through them instantly, with the risk of permanent blindness.

• Every optical instrument needs its own filter

  Any telescope, binoculars, or camera pointed at the Sun needs a proper front solar filter or must be part of a dedicated solar system. The light must be filtered before it enters the optics, not after.

• An $\mathrm{H}\alpha$ telescope is not used without its blocking filter

  The blocking filter (or blocking diagonal) is part of the safety system of an $\mathrm{H}\alpha$ telescope. It is never removed, and you never observe without it: it lets through residual light that is dangerous to the eye.

• Solar filters are never improvised

  X-ray film, CDs, smoked glass, sunglasses, or homemade filters do not protect against the Sun. Only solar filters manufactured and certified for that purpose may be used.

• Finders are used by projection

  Never look through a telescope's finder when pointing at the Sun. Locate the Sun by the tube's shadow or by projection, or use a dedicated solar finder.

• Inspect filters before every session

  If a filter is dented, scratched, loose, punctured, or does not stay firmly attached, it is not used. A solar filter is inspected before the instrument points at the Sun.

• Golden rule: when in doubt, cap the telescope

  Do not leave a solar instrument pointed at the Sun without supervision, especially in public sessions or around children. If any part is uncertain, cover the objective and stop observing.