Baader D-ERF 160 mm Energy Rejction Filter for Solar Observation with H-alpha Filters

Baader´s
"GREAT" D-ERF Energy Rejection Filters are unique in the world - made
in Germany! Their 43-layer dielectric DWDM-coating delivers a COOL-beam
of pure red light, with a FWHM 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 counterside
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 a 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.

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 result if the D-ERF is used by itself for visual
observation.



This pre-filter is used for heat reduction for H-alpha observation with H-alpha filters in the tube and in prominence attachments.