To prevent damage, we recommend using an Energy Rejection Filter on telescopes of 80mm aperture or more, or when tracking the Sun for long periods. This can be a UV/IR cut filter mounted before the diagonal, or a full aperture red or yellow glass ERF mounted in front of the telescope.
When to use a Front Mounted ERF
Telescopes upon which we do not recommend UV/IR cut filters:
- Some larger aperture (130mm and higher) and/or all-day tracking applications where the Quark / Quantum are susceptible to overheating.
- SCT telescopes
- Petzval refractors (a concentrated beam will fall upon unknown coatings)
- Certain older oil spaced triplets prior to adoption of UV resistant oils.
- Any instrument for which the owner is not comfortable with the UV/IR cut solution.
These users should consider front mount energy rejection. This is accomplished using a colored glass optical window before the objective or corrector plate.
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Colored glass ERF's must maintain an optical figure. This means they must be somewhat thick and must be polished to 1/4 wave accuracy. This factor drives the cost of front mount Energy Rejection Filters. Colored glass ERF's block all Ultraviolet light and all visible light below the 500nm range. Red glass blocks all visible light below 600nm (making it red). We choose Yellow 500nm cut point glass in the event that an owner might later acquire a Sodium or Helium D3 line filter. The same glass can be used for both types of filters, but Red glass can only be used for Hydrogen. Colored glass front mount ERF filters do not pass Calcium 393nm light. Calcium owners must use UV/IR cut filters only or no viable image will pass. All front mount Energy Rejection Filters must be fabricated to fit the front dew shield of the telescope. Due to the thousands of designs and modifications of telescopes over the decades, the owner must measure his/her own dew shield to determine the best inside diameter size offered which fits best over his exact instrument. IT MUST BE NOTED THAT A UV/IR CUT FILTER, NOR A FRONT MOUNT COLORED GLASS FILTER IS NOT SUFFICIENT FILTRATION FOR SOLAR VIEWING ALONE.
