Astronomik Filters HSO 6nm 31mm (85955)

H-alpha Filter: This filter is perfect for photographing hydrogen nebulae, whether you are in light-polluted areas or under dark skies. It greatly enhances the contrast between objects that emit H-alpha light and the background sky.

SII Filter: The SII filter is designed for imaging sulfur emission regions, both from light-polluted locations and dark-sky sites. It dramatically increases the contrast between objects glowing in the sulfur line at 672 nm and the sky background.

By combining a narrow 6 nm half-width with almost 100% transmission in the SII line, the filter provides maximum signal at the target wavelength and blocks unwanted light, ensuring an extremely dark background.

OIII Filter: The 6 nm OIII filter boosts the contrast between galactic nebulae and the sky background to an extreme degree. Its narrow half-width and high transmission in the OIII line allow only the desired signal to pass, creating a very dark background for observing faint nebulae.

The 6 nm half-width is specially optimized for CCD and CMOS sensors with very low dark current. These filters are the best choice for locations with strong light pollution or whenever faint objects need to be captured in star-rich regions of the sky.

Choosing the Right Half-Width:
When photographing under dark skies, images from DSLRs or many cooled CCD cameras are limited by the camera’s dark current, not by sky brightness. In such cases, making the filter narrower does not reveal more object details. The 12 nm filters generally make it easier to find tracking stars, especially for cameras with built-in tracking sensors.

If you have a camera with extremely low dark current and good cooling, the 6 nm filters provide maximum benefits: stronger suppression of sky brightness allows for longer exposures and deeper images. With such a narrow bandpass, stars appear smaller and faint stars may disappear, but even faint nebulae are displayed with high contrast—especially useful in star-dense regions like the Milky Way.

Summary:

• 12 nm filters are recommended for DSLRs and cameras with higher dark current.

• 6 nm filters are ideal for heavily light-polluted locations, cameras with very low dark current, or for imaging faint objects that require maximum contrast.

Specifications

• Half-value width: 6 nm

• Filter size: 31 mm

• Frame: Ring

• Mount material: Aluminium

• Optical coating: MRF

• Number of filters: 3

• Type: Line filter

• Series: HSO