In order to reduce the half-width at H-alpha solar telescopes and so clearly make more details visible on the solar surface, a second etalon (H-alpha filter) must be used. This combination of two H-alpha filter is called "double stacking".
Previously it was necessary for this purpose to provide an existing H-alpha telescope with an additional H-alpha filter in front of the front lens. This process often leads not only to visual disadvantages, such as "ghosting", it is especially very expensive as the front filter must have a large diameter.
The DSII system of Lunt takes a different approach: here a second etalon is installed directly into the telescope. Since this second etalon is not placed in front of the lens, but far behind in the beam path, it can significantly smaller and thus also be cheaper. In the DSII modules a complete collimation system is integrated, the optical design ensures a perfect image and avoids from the outset ghosting.
Since the original H-alpha telescope is already provided with all the necessary power protection filters and the DSII module is integrated in the telescope, can be dispensed with additional energy filter. Thereby, the image of the sun is brighter than it is typically with the other systems of the double-stack case. Also prominences, which are otherwise very strongly attenuated when double stacking, remain so much clearer and detailed visible.
And finally, the DSII modules are also equipped with the innovative "Pressure Tuner" system, whereby a high-precision tuning is possible.