To reduce the half width at H-alpha solar telescopes and so clearly make out more detail on the solar surface visible, 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 to provide an existing H-alpha telescope with an additional H-Alpha Front filter to the lens. This process often leads not only to optical disadvantages, such as "ghosting", it is, above all, very expensive, since the front filter must have a large diameter.
The DSII system from Lunt takes a different approach: here a second etalon is built directly into the telescope. Since this second etalon is not placed in front of the lens, but far back in the beam path, it can substantially smaller and thus be cheaper. In the DSII modules a complete collimating lens system is integrated, the optical design ensures a perfect image and eliminates from the outset ghosting.
Since the original H-alpha telescope is already equipped with all the necessary power protection filters and DSII module is integrated in the telescope, can be dispensed with additional energy filter. In this way, the image of the sun remains brighter than it is typically with other double-stack systems the case. Also prominences, which are otherwise heavily attenuated when double stacking, remain so much clearer and detailed visible.
Finally, DSII modules are also equipped with the innovative "Pressure Tuner" system, whereby a high-precision tuning is enabled.