Daystar Filters' 150mm-Aperture Energy Rejection Filter is designed for on-axis use with Refractor and Maksutov-style OTAs to reduce the heat load on 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 refractor's objective aperture or Maksutov corrector plate. Select the correct filter cap size by measuring the outside diameter of the OTA's dew shield and choose the ERF with the smallest inside diameter that will fit over the dew shield. A set of screws is included to secure the ERF into place. This ERF's cap has an inside diameter of 20 cm (7.874").