ALTAIR F125NUV395, 7nm, 1.25" NUV filter - Calcium and Venus K-line.

The Altair 395nm NUV filter selectively passes light from the two main Calcium wavelengths emitted by the sun:

• The CA-K line at 393.37 nm
• The CA-H line at 396.85 nm.

A specially developed multilayer interference coating results in a transmission of approximately 75 % at a NUV wavelength of 395 nm. The bandwidth is approximately 7 nm (FHWM), resulting in extremely high contrast at the selected wavelength with efficient blocking of unwanted wavelengths.

The remaining UV and IR light between 400 and 1100 nm is blocked with an OD3 of 0.1 %, and visible light between 400 and 700 nm with an OD4 of 0.01 %.

Sun imaging:

The filter is placed in front of a high-speed CMOS video camera to image details of the upper solar atmosphere at the boundary between the photosphere and chromosphere. Structures that cannot normally be seen with the naked eye or cameras are revealed, including fast-evolving Ellerman bombs (micro-flares) and Plages (bright grids near sunspots). Solar granulation is also enhanced, as well as details of the umbral zone around sunspots.

Images of the planet Venus:

The planet Venus also benefits greatly from increased contrast in the upper cloud layers, which show features that are normally invisible in white light. The high transmission allows fast frame rates.

IMPORTANT: The 395 nm wavelength is harmful and nearly invisible to the eye, therefore, the NUV filter is designed 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, or a solar film may be used, provided the manufacturer's instructions are followed correctly. Refractors are generally used for solar imaging, however, reflectors can be used, provided they have a large blocking filter over the front aperture (usually a solar film or glass filter because Herschel wedges cannot be used with reflecting telescopes). Imaging requires a monochrome CMOS camera. A typical example with false color and selective contrast processing (HDR filter, Photoshop) is shown in the image on the right.

Technical specifications:

• NUV transmission band, CWL 395 nm FWHM 7 nm
• Filter substrate: 1.85 mm thick optically polished BK7 glass with 30 arc second parallelism
• UVIR blocking: UV and IR light between 400 and 1100 nm is blocked at OD3 (0.1 %)
• Visible light blocking: visible light between 400 and 700 nm at OD4 (0.01 %)