Daystar Filters' 80mm-Aperture Energy Rejection Filter is designed for on-axis use with larger Schmidt-Cassegrain style OTAs to reduce the heat load on the internal components within the optical path, as well as your filter assembly, by absorbing or reflecting UV and/or IR light while transmitting light in the desired spectrum. This ERF can be used with H-alpha, Sodium, and Helium D3 solar filters, but is incompatible with calcium wavelengths.
Choose the ERF aperture size to approximately match your OTAs front lens. Select the correct filter cap size by measuring the outside diameter of the OTA's front retaining ring, including fastener heads, and choose the ERF with the smallest inside diameter that will fit over the end. A set of screws is included to secure the ERF into place. This ERF's cap has an inside diameter of 31.1cm (12.244").