Baader Filters D-ERF energy protection filter 290 mm (87870)

Plano-parallel round optical discs with both surfaces precision-polished to lambda/10 accuracy ensure excellent optical quality. An advanced infrared anti-reflection coating, based on a multi-layer dielectric interference system, prevents excessive heat from entering the telescope’s optical tube assembly. The plano-parallel D-ERF (Energy Rejection Filter) effectively blocks infrared radiation, even during extended observing sessions.

By rejecting heat before it enters the optical system, thermally induced air currents inside the telescope are minimized, preventing image degradation. Most importantly, the D-ERF protects the H-alpha filter located near the focal plane from premature aging caused by thermal stress. It also stabilizes the selected wavelength, which would otherwise shift due to temperature changes.

The D-ERF filters feature multi-layer dielectric coatings using one of the most advanced coating systems available, achieving a residual reflection of only 0.2%. This minimizes internal reflections that could otherwise bounce between surfaces, reducing contrast in solar surface details and prominences.

Producing a filter with a perfectly flat surface while maintaining that flatness after multiple coating layers requires exceptional manufacturing expertise. Only carefully selected, stress-free tempered glass blanks can be used. Inadequate production methods or outdated coating equipment can easily deform the glass, rendering the final product nearly unusable. Every step of the manufacturing process must therefore be executed with extreme precision.

These filters are manufactured using modern ion-beam coating technology, which minimizes thermal stress during coating and results in an extremely hard, scratch-resistant surface.

Important safety notice
D-ERF energy rejection filters are pre-filters and are only suitable for use with SolarSpectrum H-alpha surface filters and prominence systems. The filter must always be mounted at the front of the telescope, where light first enters. The correct orientation toward the Sun, as marked on the filter edge, must be strictly observed.
Observing the Sun in white light without an additional light attenuation filter will cause irreversible eye damage.

Please note that these filters are supplied unmounted.

Specifications

• Optical coating: Multi-layer dielectric

• Mounting: Unmounted

• Filter thickness: 20 mm

• Glass material: BK7

• Passband: 610–690 nm

• Half-value bandwidth: 80 nm

General information

• Weight: 3430 g

• Diameter: 289.9 mm ± 0.05 mm