Antlia CaK Filter, 3 nm to 393.3 nm, 1.25″ mounted, filter only

The CaK filter alone

The Antlia CaK 3 nm filter at 393.3 nm is offered here as a standalone item, in a 1.25″ mounted format. It is designed for solar imaging in the calcium K spectral region. This product listing does not correspond to the complete Antlia Herschel prism or a Solar Discover kit. It refers to the filter component that selects the CaK band in a suitable solar configuration.

A section dedicated to solar details

The multilayer coating produces a 3-nm bandwidth centered at 393.3 nm. Antlia intends this filter for imaging sunspots, surface granulation, and other structures visible in this region of the spectrum. The goal is to increase the contrast of these details in images captured by a camera. The manufacturer describes the filter used in its CaK system as having a heat-resistant substrate that is polished on both sides.

Documented spectral blocking

Antlia specifies an OD5 block, or 0.001 %, between 200 and 1200 nm, as well as an OD3 block, or 0.1 %, between 1200 and 1500 nm. These values describe the filter’s spectral selection. They should not be interpreted as authorization to point an instrument at the Sun using this component alone. The manufacturer’s website explicitly warns that a dangerous amount of ultraviolet radiation can reach the eye even when the observer perceives almost no light.

A photographic use, not a visual one

The human eye is not very sensitive to the CaK region and has difficulty distinguishing details there. Antlia therefore reserves its CaK system for imaging with cameras whose spectral response extends beyond human vision, and advises against visual observation. An image that appears very dark to the naked eye is not an indication of safety. Never look at the Sun through this filter alone, nor should its narrowband filter be considered a substitute for managing solar energy with a complete, appropriate system.

Check which items you already own

Antlia notes that this CaK filter is already screwed into its Herschel CaK prism. Therefore, there is no need to purchase it separately when using this set with its original filter. The offer presented here clearly identifies the filter alone, without specifying that a prism, Barlow lens, or Solar Discover assembly is included. Before any installation, refer to the instructions for the complete solar system being used: the specifications of this component alone are not sufficient to ensure the safety of the entire setup.

Technical specifications