The Altair 395nm NUV filter selectively passes light of the two main Calcium wavelengths emitted by the sun:
CA-K line at 393.37nm, and the CA-H line at 396.85nm.
A specially developed multilayer interference coating results in transmission of approximately 75% at 395nm NUV wavelength. Bandwidth is approximately 7nm (FHWM), resulting in extremely high contrast in the selected wavelength with efficient blocking of unwanted wavelengths.
Remaining UV and IR light between 400-1100nm is blocked at OD3 0.1%, and visible light between 400-700nm at OD4 0.01%.
Imaging the sun: The filter is to be placed in front of a high speed CMOS video camera for displaying detail in the upper solar atmosphere, at the boundary between the photosphere and the chromosphere. Structures which cannot normally be seen by the human eye or cameras are revealed, including fast-evolving Ellerman bombs (micro-flares) and Plages, (bright networks near sunspots). Solar granulation is also enhanced, as well as umbral detail around sunspots.
Imaging the planet Venus: The planet Venus also benefits greatly from higher contrast in the upper cloud layers, showing features normally invisible in white light. The high transmission allows fast frame rates.
IMPORTANT: The 395nm wavelength is harmful and almost invisible to the eye, therefore the NUV filter is intended for photographic use ONLY. A pre-filter of at least OD3.8 is required to reduce radiation damage to the filter. A Herschel Wedge, such as the Altair Imaging-Ready Solar Wedge can be used, or solar film, provided the manufacturer’s instructions are followed correctly. Refractors are usually employed for solar imaging, however reflectors can be used, provided they have a large blocking filter over the front aperture (usually solar film or glass filter because Hershel Wedges cannot be used with reflector telescopes). Imaging requires a monochrome CMOS camera. A typical example with false colour and selective contrast processing is shown opposite (HDR filter, Photoshop).
NUV transmission band, CWL 395nm FWHM 7nm
Filter substrate: 1.85mm thick BK7 optically polished glass with 30 arc sec parallelism
UVIR blocking: UV and IR light between 400-1100nm is blocked at OD3 (0.1%)
Visible light blocking: Visible light between 400-700nm at OD4 (0.01%)
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