Coronado Telescopes

Part one of the two-part H-Alpha filter is an energy rejection filter and Fabry-Pérot etalon that is mounted in front of the scope’s objective lens. The Fabry-Pérot etalon consists of a transparent plate with two dielectrically-coated reflecting surfaces. It uses the principle of interference between the multiple reflections of light between the two reflecting surfaces to convert the uniform light output of the Sun to a series of peaks and troughs looking somewhat like a sinusoidal “picket fence,” as shown in the blue curve in the illustration (illustration not to scale). Constructive interference occurs if the transmitted beams are in phase, and results in a high-transmission peak. If the transmitted beams are out-of-phase, destructive interference occurs and results in a transmission minimum.

The energy reflection portion of the ERF/etalon assembly blocks the passage of UV and IR radiation into the scope to limit heat buildup within the optical tube. It also allows only the crimson portion of the Sun’s spectrum into the telescope.

Part two of the filter is a blocking filter built into the star diagonal supplied with the scope. This blocks all of the “picket fence” peaks except the one that is centered on the 6562.8 Ångstrom H-Alpha line of the Balmer ionized hydrogen series in the crimson portion of the Sun’s spectrum. By blocking the flood of light at all other wavelengths, you can observe only those solar features emitting or absorbing light in the chromosphere at the H-Alpha wavelength. Blocking all but this wavelength reveals faint details that would otherwise be lost in the thousand-times brighter glare of the Sun’s photosphere.

The narrower the passband of the filter, the greater the contrast on disk details. Stacking two etalons (double-stacking) reduces the passband from the <0.7 Ångstrom width of a single etalon to <0.5 Ångstrom, as shown in the red curve in the illustration (illustration not to scale). A <0.7 Ångstrom filter gives you a good balance of prominence and disk detail. If disk detail is of more interest to you, however, a double-stacked <0.5 Ångstrom filter will provide maximum contrast and visibility of active regions on the solar disk, but with some loss of faint detail in the prominences.