Narrow bandpass filter assemblies for solar telescopes

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#The various objects, features, and advantages of the present inventions may be better understood by examining the Detailed Description of the Preferred Embodiments found below, together with the appended figures, wherein:

#FIG. 1 shows an exemplary schematic of filter elements according to one preferred embodiment of the invention;

#FIG. 2 shows an exemplary schematic of filter elements employing an angled, reflective-type polarizer according to another preferred embodiment;

#The preferred embodiments will now be described with respect to the drawings. To facilitate the description, any numeral identifying an element in one figure will represent the same element when used in any other figure.

#For convenience, the invention will be described as used for Hα solar observation. It will be clear to those of ordinary skill in the art that similar elements and techniques of the invention would apply for other visible wavelength selections, such as Hβ, calcium K-line, or other atomic or molecular species of interest, with the straightforward adjustment of bandpass regions to accommodate viewing of these alternate lines.

#One aspect of the present invention is significant emphasis on stray light reduction. This is particularly important for solar viewing because of the extremely high background signal (full sunlight exposure) vs. the extremely narrow atomic emission band of interest. Recall that typical glass surfaces reflect about 4-6% of the incoming light. If this stray reflection is not contained or eliminated, it can easily propagate through the optical system via multiple bounces and reflections from the optics. This can easily blur the image from the true light path due to off-axis contributions of stronger stray light. Alternately, the precise signal from the emission band will be obscured if too much additional light processes through the optical system. While one extremely narrow bandpass filter is theoretically sufficient to select the emission band of interest, such filters are not perfect out-of-band blockers in reality. Therefore, a careful combination of additional filter elements is used to remove additional stray and excess light either sequentially or cooperatively.