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The Filter

The H-alpha filter itself makes use of the natural transition of neutral hydrogen from the n=3 state to the n=2 state. This was the first transition to be discovered (and observed by Balmer) and is the first transition of the Balmer series.

90-degree adapter

90-degree adapter

Due to the high temperatures in the solar atmosphere (of which this line originates) the hydrogen electrons there are raised to a higher level, 3 in this case, but due to the chromosphere being so tenuous they cannot sustain this level, or exceed it by absorbing more radiation, hence no absorption lines in a spectrum analysis. Instead there is an emission line because of the release of radiation by this transition. It occurs at one specific wavelength, 656.28nm, just to the red of visible. Unfortunately the photosphere tends to drown out this emission but the filter uses consecutive layered coatings to isolate this emission line and observe the results.

The Plossl 26mm lens

The Plossl 26mm lens.

The design of the coatings is based on the principles of the Fabry-Perot interferometer. They are highly reflective and non-absorbing so when incident light is upon the filter most is reflected back, some however, does goes through. The second coating is an even number of wavelengths separate from the first so as the transmitted light reaches this layer more light is reflected (and acts destructively with that, that comes in) leaving only a narrow waveband of light (656.28 +/- 0.07nm) to pass. This not only provides an excellent, unobstructed view of the solar surface but also makes the solar disk safe to observe, through a lens, with the naked eye. Now the lens cap on the back of the telescope can be removed, as the major hazard of blindness has been eliminated, and the solar disk focused on, through a 90-degree angle adapter and a Plossl, 26mm eye piece.