The top image was taken with the Lunt 50 etalon, a 2x cemax barlow and the ZWO 290mm camera. The bottom image was takem with a Lunt50 etalon double stacked with a DAystar Quark and using the FLIR IMX174 camera. The difference in contrast on the filament is really quite obvious. I do use single stack some times, but the vast majotity of the time it is double stack all the way for me!