How a Hydrogen alpha telescope works
   

The outer layer of the Sun is a wonderful thing.  Here magnetic fields burst out of the star’s surface and hurl glowing material thousands of miles into the void.   The tops of millions of convection fountains make the surface appear to boil and rivers of glowing plasma snake through the tenuous atmosphere before they plunge down into vast holes that are big enough to swallow our planet.

Yet all this activity is invisible because of the blinding light created by lower layers.   But there is a way of revealing the magnificence of the Sun.

In the chromosphere it is cool enough (6000oC) for atoms to form.   90% of the Sun is made of the simplest atom –Hydrogen.    When the hydrogen atoms’ electrons jump from their third possible orbit back to their second they each give out a photon of red light with a wavelength of 656.3 nanometres (billionths of a metre). This is hydrogen alpha (Ha) light and is the main wavelength of visible light given out by the outer layer of the Sun.

 

Normally the Ha light is impossible to see because it is swamped by the colossal output of visible light from the underlying layer – the photosphere.   

Ha filters are able to precisely cut out a section of the spectrum less than one ten billionth of a metre across with the Ha wavelength inside.   It is a feat like trying to separate a needle from a planet sized haystack! 

To do the job, the heart of the filter, the ‘etalon’, needs to be manufactured to tolerances four times finer than that required for the optics of the Hubble Space Telescope.

As light enters the Ha telescope it goes through an energy rejection filter (ERF).   This is designed to filter out Ultra Violet and Infra Red wavelengths.   Next comes the Fabry-Perot etalon.   This is the most precisely made part of the scope.  Two extremely flat pieces of glass with special coatings are placed at such a distance that Ha light constructively interferes with itself and passes through. Other wavelengths are reflected backwards and forwards between the surfaces destructively interfering and cancelling each other out.   The Our Star * telescope etalon can filter out a bandwidth of just 0.07nm .

The etalon does create harmonic wavelengths but these are easily filtered out by the blocking filter in the diagonal at the rear of the telescope.