ERF-prefilter E-120N80 for H-alpha solar monitoring | DayStar

An H-alpha setup does not start at the filter at the back of the telescope, but at the front of the telescope. An energy rejection filter like the DayStar ERF E-120N80 sits in front of the opening and blocks ultraviolet and infrared light and a large portion of visible light, while the red light around the H-alpha wavelength passes through.

This keeps the heat in your telescope and on the H-alpha filter limited. The pre-filter works together with a narrowband H-alpha filter at the rear, for example a DayStar Quantum PE.

Choose the ERF based on your telescope: the mount must fit snugly and securely on your tube or dovetail ring, without gaps where unfiltered sunlight can pass through. Therefore, first measure the outer diameter. Never look directly at the sun through an ERF alone and use it exclusively in conjunction with an appropriate H-alpha filter, as intended. Background can be found in view our guide on the sun.

Other sizes are also available, such as the ERF E-120N100.