Appendix A

The Coronado Solar Telescope

Coronado solar telescopes are equipped with Hydrogen-Alpha solar filters to show you flares, prominences, spicules, filaments, plages, coronal mass ejections, and more – not just the sunspots you see through a conventional white light solar filter. With a Coronado solar telescope, you see the dynamic, living face of the Sun changing as you watch.

Part one of the two-part H-Alpha filter used on a Coronado solar telescope is an energy rejection filter and Fabry-Pérot etalon that is typically mounted in front of the scope’s objective lens. The Fabry-Pérot etalon consists of two carefully-polished thin glass plates with two dielectrically-coated partially reflective 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 that are tuned to slightly different frequencies that partially overlap (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 above (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 image brightness and faint detail in the prominences. Most observers willingly trade some image brightness for increased contrast.

The refractive index of air changes with altitude and to a lesser and more gradual extent as the temperature changes. This can affect where the filter’s passband falls in relation to the H-Alpha emission line. Accordingly, the filters include a mechanical tuning mechanism that lets you precisely center the filter on the H-Alpha line whether you are observing at sea level or in the Rockies, during the dog days of summer or in the middle of winter.

Optical interference filters shift their passband towards shorter wavelengths when they are tilted in relation to the light path. The filters are shipped with their passband falling slightly on the longer-wavelength red wing of the H-Alpha line. The tuning mechanism tilts the etalon, shifting the passband from the red wing toward the shorter-wavelength blue wing of the H-alpha line. This sweeps the filter’s passband across the H-Alpha line, no matter what the altitude or temperature, letting you tune the filter precisely on-band for the highest contrast and detail.