Baader D-ERF Energy Rejection Filter

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

• D-ERF (Dielectric Energy Reflection Filter) in different sizes
• Pre-filter to reduce heat during H-alpha observation with SolarSpectrum Filters
• Plane parrallel 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 spun aluminum customized for your telescope can be ordered in addition (not available as standalone product)

Baader planetarium has introduced a new evolution in their premium energy rejection filters, from 70mm up to 180mm! These filters incorporate a new all-dielectric IR rejection coating that eliminates the heating from IR energy to produce a truly cold focus (hence, "Cool ERF"). As a result, the telescope and etalon remain cool and safe from excessive heating. The result is longer filter life, and freedom from tube currents caused by heating. Users with compound telescopes (SCTs, Cassegrains, Maks) will really appreciate the D-ERF, due to its reduced heat load on the mirrors and the reduction in tube currents caused by internal heating.

Unlike every other ERF produced up until now, the Baader D-ERF are produced from a high quality optical clear glass, ensuring freedom from the internal impurities that caused so much headache in producing the older red-glass ERFs. The red glass used in producing the older generation of ERFs is of very poor and variable internal quality. In order to produce ERFs from red glass, Baader had to complete and polish each ERF, then interferometrically inspect every one for internal defects, resulting in having to dispose of far more ERFs than they were able to keep. Its no wonder why other red glass ERFs have such a poor reputation, particularly in sizes over 3" in aperture.

D-ERF filters use a high grade BK7 optical glass which is free of any internal defects. The thick substrates are polished plane-parallel, smooth, and flat to 1/10 wave p-v over the entire surface. Only by using thick substrates can a high surface accuracy be achieved and held in use (for example, the 90mm ERF is 5.5mm thick. Thinner substrates are not capable of holding such high surface accuracy).

The energy rejection is now accomplished using ultra-hard dielectric coatings on both sides of the glass substrate, and also serve to protect the glass, making it highly resistant to scratches. The 43-layer dielectric DWDM-coating delivers a COOL-beam of pure red light, with a HBW of 80 nm! This reflects all the unwanted spectrum from 280 nm up to 1500 nm, while leaving open just an 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. Baader's 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 autocollimation on our Carl Zeiss optical bench.

D-ERF filters are sold un-mounted and are available in the following diameters: 70mm, 90mm, 110mm, 135mm, 160mm, 180mm. Please contact us for further specifics and availability.

Custom spun-Aluminum filter cells are also available to fit any telescope, on special order. Please contact us for pricing and availability.

IMPORTANT NOTE:
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
 

In the Box