DayStar Energy Rejection Filters reduce the heat load on your filter assembly by absorbing or reflecting UV and/or I light and transmitting light in the desired visual spectrum. These filters are sometimes Red or Yellow glass, or dielectric IR and UV, but pass light in the desired visual spectrum. The ERF can also serve as an aperture mask to accomplish a straight angle of incidence for light entering the filter.

DayStar filter assemblies operate best at f/30 and slower. Creating f/30 configuration can be done easily with the proper ERF and/or barlow. Visit our full list of reflectors or refractors or contact us for your exact model. Higher resolution, larger aperture configurations use a barlow.

In the order notes, please specify the telescope this ERF is to be used with, and measure (with a ruler) the outside (largest) diameter of your telescope's dew shield at the very front of the telescope. ERFs are custom machined on a one-off basis and so the accuracy of this measurement is very important. Failing to provide this measurement will cause your order to be put on hold until it is provided.