* Astronomy

Backyard Calcium K filters range from a diagonal inserted into a telescope, rear mounted filter or a complete telescope system. All three designs do the same thing, block unwanted light and pass the Calcium K emission line. 
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Two emission lines are produced by Calcium just at the edge of the visible spectrum. The emission lines highlight Super Granulation Cells that are brightest and strongest in areas of high magnetic fields such as in sunspots and active regions. Calcium-K filters isolate the ultraviolet (~393.3 nm) spectral line to look deeper into the Sun's Chromosphere and reveal the intense magnetic fields.

Visually the Calcium K-line in the visual spectrum of the Sun is a pleasing blue/purple colour.

Baader Calcium K-Line Filter (395nm) 31.7mm 

£229.00
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Baader U2 allows UV from ~320nm to 390nm 

The BPU2 filter is a ultraviolet bandpass filter made so that only light in the 320 to 390 nm portions of the spectrum are transmitted; it will effectively block all else spanning 200 nm to 1120 nm. This U-Filter has improved transmission peaking with 85% transmission at 350nm with a 70nm bandwidth, and with a 5 stop reduction in the IR portion of the spectrum. This replaces the original U-Filter (BPU125) that was discontinued in 2007. Note however that it is only made available in 2 inch diameter.
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It was, and possibly still is, a common practice to engage in reflected ultraviolet photography by the simple expedient of placing the Wratten 18A over the camera lens and then "firing" away. In the days of film cameras this was an appropriate and most satisfactory approach but in the age of electronic sensors this procedure may yield unexpected and misleading results.
Part of the problem lies in the fact that CCD and CMOS sensors, unlike film, have a relatively high sensitivity to infrared forcing manufactures to include infrared absorbing filters ahead of the camera's sensor. It is stated, however, that these sensors do not have a very high sensitivity or response to ultraviolet. Trying to record an image by the residual response of the infrared filtered CCD or CMOS sensor by either infrared or ultraviolet is possible but generally calls for extreme exposure adjustment measures.
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In reflected UV photography the subject is illuminated directly by UV emitting lamps (radiation sources). A UV transmitting, visible light blocking filter is placed on the lens, that allows ultraviolet to pass and absorbs all visible light. Examples of these filters are Kodak Wratten 18A, B+W 403, Hoya U-340, Baader U-Filter and Kenko U-360. These filters are made from special coloured glass. Most types of glass will allow long wave UV to pass, but absorb all the other UV wavelengths, usually from about 350 nm and below. Only special developed lenses made of quartz (fused silica) or quartz and fluorite can be used. Using these lenses allow the camera to reach the range 180..200 nm. Lenses based purely on quartz show a distinct focus shift between visible and UV light, whereas the later developed fluorite/quartz lenses are fully colour corrected without focus shift. Examples of the latter type are the Nikon UV Nikkor 105 mm, the Hasselblad (Zeiss) UV Sonnar 105 mm and the Asahi Pentax Ultra Achromatic Takumar 85 mm.
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Calcium II H line and K-line Filters
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UVR Optical has developed a new ultraviolet bandpass filter - the 370BP60, also known as the Andrea 'U'. One advantage of the Andrea 'U' over other UV bandpass filters is the excellent transmission of the 370nm-400nm band - wavelengths which are not passed by most UV bandpass filters. 
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 The Wratten #18A allows UV from ~290nm to 400nm