Adding a Lunt Hydrogen-Alpha solar filter to your telescope will show you solar 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 Lunt H-Alpha solar filter, you see the dynamic, living face of the Sun changing as you watch. A complete Lunt H-Alpha solar filter consists of three parts.
Part one is an energy rejection filter and Fabry-Pérot etalon that mounts in front of your scope’s objective lens. These are available in several diameters to match your scope aperture and type and provide you with the desired resolution. 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. This results in a transmission minimum.
The red energy reflection portion of the ERF/etalon assembly blocks the passage of UV and IR radiation into your scope to limit heat buildup within the optical tube. It also allows only the crimson portion of the Sun’s spectrum into your telescope.
Part two is a blocking filter built into a star diagonal that must be used in place of your normal star diagonal. 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. Different diameter clear aperture blocking filters are available, depending on the focal length of the telescope you will be using, in order to provide an unvignetted solar disk image with your particular scope. In addition, with a Lunt solar filter, you get your choice of either a 1.25” or 2” star diagonal at the same price.
Part three of the filter is an optional adapter plate that is needed to safely mount the etalon on the front of your telescope. These adapters are custom-made to match your scope type, brand, and aperture and to hold the specific diameter etalon you have chosen.
The narrower the passband of the filter, the greater the contrast on disk details. Stacking two etalons (double-stacking) reduces the passband from the <0.75 Ångstrom width of a single etalon to <0.55 Ångstrom, as shown in the red curve in the illustration (illustration not to scale). A <0.75 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 <0.55 Ångstrom filter will provide maximum contrast and visibility of active regions on the solar disk, but with some loss of 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 Lunt solar 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.