Baader Ultra-Narrowband 4.5nm S-II Filters are precision grade filters for f/8 to f/3.5 optical systems.

Designed for imaging S-II emissions at an extremely narrow 4.5nm bandwidth. The enforced contrast results in much fainter stars providing more signal in the fainter nebula details, with the central wavelength of the filters being 673nm.

The manufacture of such ultra-narrow filters requires extreme coating homogeneity and very high center-wavelength (CWL) precision. Each filter is planeoptically polished and hard coated.

Filter sizes available:

• 1.25"  (2mm glass thickness)
• 31mm unmounted (2mm glass thickness)
• 36mm unmounted (2mm glass thickness)
• 2" mounted (2mm glass thickness)
• 50.4mm round (3mm glass thickness)
• 50x50mm square (3mm glass thickness)
• 65x65mm square (3mm glass thickness)

Filter-Spectrum

Below is the real filter-spectrum, taken with an MPI-DADOS-Spectrograph. Comparision with the solar-spectrum (including solar absorption lines).