Baader D-ERF Energy Rejection Filter (75 - 180mm)

Baader D-ERF Energy Rejection Filter (75 - 180mm)

£271.00✓ 2 year warranty


Out of stock due 10-15 working days

About this product

Model:  baader_2459241d
Part Number:  2459241D

D-ERF heat prefilter for H-alpha observation with surface filtering in the Solar Spectrum and Prominances (with IR-block coating)

Baader's 43-layer dielectric DWDM-coating delivers a COOL-beam of pure red light, with a HBW of 80 nm! The glass substrate in this case is no longer any kind of soft and streaky colored absorption glass, but clear BK7 of the highest homogeneity, fine-optically polished to the quality grade of the objective-lens surfaces, and with all the reflection performance coatings on one side. This reflects all the unwanted spectrum from 280 nm up to 1500 nm, while leaving open just that 80nm-wide spectral window around H-alpha. It is hellishly difficult to design such a performance coating, as well as the counter side anti-reflection multicoating, in such a way that no coating-induced stress builds up inside the glass substrate. If these coatings systems on either side of the plate are not done exactly right, to cancel out the stresses induced by the coating layers, the plane-parallel surfaces may easily be distorted, meaning the 1/10 wave flat plate deforms into an irregular curved substrate that acts as a (bad) lens. Our D-ERF filter-layers are designed with the utmost care, applied with the world‘s most advanced optical coating machines, and the final filters are all inspected and passed in auto collimation on our Carl Zeiss optical bench.

This level of care retains the full-aperture resolving power of the telescope lens while dramatically reducing the thermal energy that goes through the telescope to the Solar Spectrum filter. This not only is the main reason for a dramatic reduction of heat stress within the telescope, but also greatly reduces the thermally-induced seeing effects as well.


Baader D-ERF Energy Rejection filters are intended for use only as a Pre-Filter for Solar Spectrum H-Alpha filters. The D-ERF filter must always be present and mounted ahead of the telescope objective. The filter should be oriented with the primary coated face towards the sun (an arrow mark on the rim of the filter points toward this side). The D-ERF filter must be used together with the Solar Spectrum H-Alpha filter. Irreparable eye damage will result if the D-ERF is used by itself for visual observation.


  • D-ERF (Dielectric Energy Reflection Filter) in different sizes
  • Pre-filter to reduce heat during H-alpha observation with SolarSpectrum Filters
  • Plane parallel plate (without filter cell) with IR-Cut dielectric coating
  • non-ageing sealed coating edges
  • Half-Band-Width (HBW): 80 mm – see also graph
  • Filter WITHOUT cell – 3D-printed filter cells - customised for your telescope, can be ordered in addition (not available as standalone product)

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