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Some events on the Sun – notably active flares and coronal mass ejections (CMEs) – are extremely fast moving. If such features occur on the disc of the Sun, rather than on the limb, their high velocity towards the observer results in their wavelength being Doppler-shifted to a shorter wavelength than the 6562.8 Ångstrom H-alpha line to which the filter is tuned. If the velocity of the event is high enough, its Doppler-shifted wavelength could be outside the normal passband of the filter and the feature would not be visible. The T-Max bandpass tuner allows the observer to de-tune the solar filter’s passband slightly towards the blue wing of the H-alpha line to make such Doppler-shifted phenomena more visible.

All optical interference filters shift their passband towards shorter wavelengths when tilted. The T-Max bandpass tuner fits between the SolarMax etalon and objective lens at the front of the scope. Turning a rotary wheel on the T-Max body mechanically tilts the SolarMax etalon filter element by up to 0.7°, shifting the passband toward the shorter-wavelength blue wing of the H-alpha line. Any more of a tilt will shift the filter completely off the H-alpha line. As the filter tilts, Doppler-shifter filaments that were formerly invisible or barely visible will often start to appear or darken. Filaments that appear darker and prominences that appear brighter are moving towards your line of sight.

With a passive filter such as the SolarMax, its passband will broaden with increasing amounts of tilt, resulting in somewhat lower resolution and contrast on non-Doppler-shifted features when compared to their on-band images. There is also an increasing loss of transmission efficiency as the passband shifts away from its nominal center frequency on the H-alpha line. However, these minor shortcomings are more than offset by the increased number of features that become visible as you tilt the T-Max to scan back and forth across Doppler-shifted wavelengths.

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