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How to Make a Hydrogen Alpha Filter

Fire is hypnotic. Sit any group of campers around a nighttime fire, and you̵7;ll notice their attention drawn by the constantly shifting interplay of form and color. Observing the sun offers that same fascination, enhanced by the knowledge that the flames of the sun support all life on Earth. The roiling, popping, swirling outer surface of the sun is best seen with one particular narrow part of the spectrum, the hydrogen alpha line, which is one particular wavelength of red light. The wavelength is 656.28 nanometers, or 656.28 billionths of a meter. The principles behind a filter that will isolate that wavelength are clear.

Things You'll Need

2 high-precision front surface mirrors
Precision adjustable mounting stages
Precision translation stage
Bandpass filter

Instructions

- 1
  Put the mirrors in the mounts and set up the mounts so the reflective sides of the mirrors face each other. This kind of cavity is called a Fabry-Perot etalon. One mount should be placed on the precision translation stage.
- 2
  Adjust the mirrors so they face each other perfectly. You can do this by shining a laser from the back side of one through the pair of mirrors and looking at the transmitted beam. Adjust the angle until any transmitted beams are right on top of each other.
- 3
  Move the precision stage to adjust the spacing for maximum transmission at 656.28 nanometers. The transmission for a Fabry-Perot peaks when the spacing is equal to an integral number of half-wavelengths. That is, the distance is equal to m x lambda/2, where lambda is the wavelength and m is a whole number. M will be, in general, a very large number. For example, to filter for hydrogen alpha light with a spacing around 25 mm, m would be 76,187.
- 4
  Put a blocking filter in front of your mirror assembly. The blocking filter is necessary because when your Fabry-Perot is perfectly set up to enhance the transmission of 656.28 nm hydrogen alpha light for m = 76,187, it will also enhance the transmission of 657.89 nm light with m = 76,000, for example. So you put a less precise blocking filter in front of your etalon to get rid of background wavelengths that would otherwise make it through.

Astronomy