Preparations; Choosing A Dielectric Energy Rejection Filter - Baader Planetarium SUNDANCER II Manual

Choosing a Dielectric Energy Rejection Filter

A Dielectric Energy Re ection Filter
(D-ERF) is used to protect the actual
H-alpha lter from the intense solar
energy. In principle, it is a plano-op-
tically polished lter that only allows
red light in the vicinity of the H-alpha
line to pass and thus ensures that
the actual H-alpha lter does not
become too hot. It does not have to
be as narrow-banded as the actual
H-alpha lter; however, the same
demands are made on its surface
accuracy as on the telescope
objective. A white-light solar lter
(e.g. made of AstroSolar lm) cannot be used for this purpose because it also darkens
the H-alpha line, which is weaker than the rest of the solar radiation.
On all telescopes with more than 80 mm aperture as well as on telescopes with
secondary mirrors (e.g. Newtons or Schmidt-Cassegrains), a D-ERF protective
filter (not included in the scope of delivery) is absolutely necessary for long-term
observations (longer than a maximum of five minutes). A selection of suitable lters
can be found at baader-planetarium.com/derf; a suitable 3D-printed lens mount can be
created by yourself or by various service providers.
These D-ERF lters also serve as UV and IR blocking lters and re ect the incident
sunlight instead of absorbing it. If you want to use a smaller D-ERF lter diameter on
an obstructed telescope to simultaneously stop it down to a more favourable focal
ratio, choose the diameter of the D-ERF in such a way that it sits off-centre next to the
secondary mirror in the optical path. Otherwise you will lose aperture if the secondary
mirror sits behind the D-ERF. Do not try to put several smaller D-ERFs next to each
other in order to use more aperture despite having a secondary mirror – this also does
not give satisfactory results. Please note that a D-ERF must be tilted by approximately
3° when it is mounted off-axis.
A suitable mount speci cally for your telescope can be made by us only if you
purchase a complete system consisting of D-ERF and H-alpha lter. However, this is
relatively expensive due to the design and production time. For a quotation we need
the diameter of the mount with an accuracy of 1/10 mm. Instructions on how to const-
ruct your own mount can be found at baader-planetarium.com/derf.
The lter should be mounted as close as possible to the front of the lens so that no
warm layers of air can accumulate between the lens and the lter and impair the image
quality. Always make sure that the lter is rmly seated to avoid it dropping from the
telescope!
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Manual: SunDancer II

Preparations

Energy protection filters up to 180 mm aperture can
individually made for your telescope, if necessary also with adjus-
table iris diaphragm or fixed aperture diaphragm.