The front of the converted Evostar ED150 f8 is equipped with a special energie rejection filter from Baader, the D-ERF.

Its main purpose is to reject the infrared rays of the sun, which normally heat the optical system and to pass only the red wavelenghts around the important H-alpha.

The UV light which may destroy the Solar Spectrum Filter SO-1 (19mm) 0.2A is also blocked.

The tube seeing is reduced and the contrast of the H-alpha picture is increased. The filter is combined with two apertures 150mm or 120mm, in order to adapt the ED optic to the various telezentric systems (TZ-3 or TZ-4) and the required optimal focal lenghts.

No Fabry-Perot-Solar-Filter must be subjected to “freeze all though”. The warranty may be void if damages are found to be a result of freezing.

Such an elaborate filter is calibrated to be used at a certain working temperature. The immersion oil which enables the large stack of Polarizers and optical elements to perform as one solid block of glass, free of internal reflections, may become “greasy”. As a result, the crystallizing liquid increases it's volume and forces optical surfaces within the optical stack to become apart, so that air can creep between the optical elements. The coatings applied to the 1/200 lambda plane etalen as well as the polarizers are subjected to enormous stress which can lead to forced aging.

At all times when any H-alpha Filter is not in use, it should be stored in a dry environment within a temperature range between +4°C to 36°C.
A dry box with including a generous pack of silica gel or zeolite is recommended.

H-alpha Solar

(ED150 telescope)