Energy Rejection Filters serve two purposes.  They reduce the heat load on your filter assembly by absorbing or reflecting UV and/or IR light and transmitting light in the desired visual spectrum. These filters are sometimes fabricated from Red or Yellow glass, or by using a precision coating to reflect light in IR and UV, but pass light in the desired visual spectrum.   DayStar filters can operate safely for short periods of time without an ERF, but the excessive heat and UV exposure will cause premature failure to your filter assembly.  The ERF also serves as an aperture mask to accomplish a strait angle of incident for light entering the filter.

DayStar filter assemblies operate efficiently with f/30 and slower optical systems. Creating a nominal f/30 optical system can be easily achieved by selecting the proper ERF listed below for aperture reduction. If you own a telescope that is not listed, please contact us for a quote.

Remember: to compute the proper aperture reduction mask, divide your telescope EFL or Focal Length by 30. For higher resolution, larger aperture configurations, consider using a barlow.  To compute the proper aperture using a barlow, use the adjusted EFL which includes multiplication by your barlow.  See formula below for details.

DayStar Filters LLC^™ 149 Northwest OO Highway   Warrensburg, MO 64093 - USA
Telephone: 660-747-2100 • jen@DayStarFilters.com • FAX: 1-866-791-0448