Lunt Solar Systems 18mm Calcium-K Solar Eyepiece Filter (18mm Blocking, 2" Straight)

Specifically engineered for the 393.4nm wavelength, Lunt Solar Systems' 18mm Calcium-K Solar Eyepiece Filter is primarily used to more clearly resolve sunspots and active areas of the Sun. This wavelength is slightly outside the visible spectrum on the ultraviolet side and highlights areas that are slightly lower and cooler than the layer viewed using hydrogen-alpha filters, so this Ca-K filter makes an ideal companion accessory to give observers a more diverse viewing experience. The straight tube viewing configuration fits into 2" focusers, and accepts any user-supplied 1.25" eyepieces and accessories while providing a blocking 18mm aperture. This model is recommended for use with optical tube assemblies (OTAs) with a focal length of up to 1800mm.