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