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DayStar 90mm Clear Aperture Energy Rejection Filter


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DayStar 90mm Clear Aperture Energy Rejection Filter

DayStar Filters - DayStar 90 mm Energy Rejection Filter (ERF)

A DayStar Clear Aperture ERF, or energy rejection filter, has its rule. First and foremost, the DayStar Energy Rejection Filter must cover the front objective of any telescope using rear-mounted DayStar T-Scanners or Quantum H-alpha filters. They are "telescope specific" designed"!

What makes a DayStar Energy Rejection Filter work? ERF's reduce the heat load on your filter assembly by absorbing or reflecting ultra-violet and/or infra-red light and transmitting light in the desired visual spectrum. These filters are sometimes red or yellow optical glass, or dielectric IR and UV, but pass the light through in the desired visual spectrum. The DayStar ERF can also serve as an aperture mask to accomplish a straight angle of incidence for light entering the filter.

Because they are a specialty item, DayStar Energy Rejection Filter assemblies operate best at focal ratio of f/30 and slower. Creating an f/30 configuration can be done easily with the proper DayStar ERF and/or a Barlow lens. Higher resolution, larger aperture configurations (such as full or half aperture) use a Barlow lens, which is sold separately. In order to compute the proper aperture reduction mask, divide your telescope EFL or Focal Length by 30. To compute the proper aperture using a Barlow, use the adjusted EFL which includes multiplication by your Barlow.

Choosing a DayStar Energy Rejection Filter

All DayStar Energy Rejection Filters are priced by clear aperture, and include the chosen aperture glass, a cell to fit the diameter of your telescope OTA, and a telescope end cap.

Here is a list of common telescopes that can use a 90 mm DayStar ERF in either full, half, or reduced configurations:
  • Meade ETX 90 - Full with 2X PowerMate
  • 3.5-inch Questar - Full with 2X PowerMate
  • Celestron 11-inch SCT - Reduced
  • Takahashi Mewlon 250 - Reduced
  • Takahashi Mewlon 300 - Reduced
  • Takahashi FRC 300 - Reduced
  • Konus 90 mm Mak - Full with 2X or 2.5X PowerMate
  • Bushnell 90 mm NorthStar - Full with 2X PowerMate
  • AP 92 mm f/7 Stowaway - Full with 4X PowerMate
  • AP 130 mm f/6 Starfire - Full with 4X PowerMate
  • Tele Vue NP 127 - Full with 4X PowerMate
  • Celestron 90 mm f/12.3 Astromaster - Full with 2.5X PowerMate
  • Borg 101 f/6.3 - Full with 4X PowerMate


Remember, you are not done ordering your ERF until you have specified the telescope you plan to use with your DayStar Quantum or T-Scanner (rear mounted) filter. If the telescope is listed above, the make and model (i.e. Meade 8-inch LX200-ACF) is sufficient. However, if you do not see your telescope listed here, please give the make and model as well as the front edge diameter of the OTA. Put this information in the "comments" section of your shopping cart when ordering. If we need more information, we will contact you.

OPT Product Number: DF-ERF-90




Accessory TypeFilters
Warranty10 Year Warranty