Choosing An Energy Rejection Filter - Baader Planetarium SolarSpectrum RG18 Manual

H-alpha-filter

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Table Of Contents

Table of Contents

A Dielectric Energy Reflection Filter (D-

ERF) protects the H-alpha-filter from the

intense heat of the sun. In principle, it is

a plano-optically polished filter that only

allows red light close to the H-alpha line

to pass and thus ensures that the actual

H-alpha filter does not become too hot. It

does not have to be as narrow-banded as

the actual H-alpha filter; however, the same

demands are made on its surface accuracy

as on the objective. A white-light solar filter

(e.g. made of AstroSolar film) cannot be

used for this purpose, as it also darkens the

H-alpha line, which is already weaker than

the rest of the solar light.

You can find a selection of suitable filters on baader-planetarium .com/derf; a mat-

ching filter cell can e.g. be 3D-printed by yourself or several commercial 3D-printers.

These D-ERF filters also serve as IR blocking filters and reflect the incoming sun-

light instead of absorbing it. If you want to use a smaller D-ERF filter diameter on an

obstructed telescope to simultaneously stop it down to a more favourable focal ratio,

choose the diameter of the D-ERF so 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. Placing several D-ERFs next to each other in order to use more aperture

despite having a secondary mirror also does not give satisfactory results in practice.

Please note that the H-alpha filter must be mounted tilted by about 3° when placed

off-centre.

A suitable filter cell specifically for your telescope can only be made by us if you

purchase a complete system consisting of D-ERF and H-alpha filter. 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. You can find instructions on

how to construct your own mount at baader-planetarium .com/derf.

The filter should be placed as close as possible in front of the lens so that no warm

layers of air can accumulate between the lens and the filter and impair the image quali-

ty. Always make sure that the filter is firmly seated!

Tip: Our TriBand telescopes are based on Schmidt-Cassegrain telescopes with aper-

tures of 8", 9¼" and 11". These telescopes do not need an additional energy protec-

tion filter, it is already integrated in the coating of the Schmidt plate.

Manual: SolarSpectrum H-alpha-Filter and Baader Planetarium Telecentric Systems

Preparations