Brightwell Dawnline OIII filter 36 mm, observatory grade

Specifications

Description

The Dawnline OIII filter is an oxygen-III nebula filter supplied as an unmounted 36 mm glass element, which fits a drop-in holder or a filter slot. Its passband is about 8 nm wide and sits around the doubly ionized oxygen lines at 496 nm and 501 nm, with a peak transmission of about 85 percent. The filter passes very little of the rest of the visible spectrum, so light from the sky and from artificial sources is largely excluded.

Emission nebulae that glow strongly in oxygen-III, such as the Veil Nebula, the Crab Nebula, and the Dumbbell and Ring planetary nebulae, gain the most with this filter. The sky background emits across the spectrum, so removing it while passing the nebula's oxygen light raises the contrast between the two. The nebula does not grow brighter, but the surrounding sky darkens, which can reveal faint outer structure. The effect is most noticeable under moderately dark skies with a telescope of 150 mm aperture or more. Because the filter is unmounted, you must fit it into a holder that matches the 36 mm clear aperture.

Galaxies, star clusters, and reflection nebulae shine across a broad range of wavelengths, so this filter mainly dims them. Hydrogen-alpha emission nebulae, such as the Rosette Nebula, emit little at these wavelengths and will appear faint. Stars are reduced in brightness and take on a slight blue-green tint, since the passband lies in that part of the spectrum. Handle the glass by its edges and keep the coated faces free of fingerprints, and use a clean holder or drop-in cell sized to 36 mm. Store the filter in its case when not in use.

About this configuration

This listing covers one configuration of the Brightwell Dawnline OIII filter 36 mm, defined by its grade, coating, case, and edition year. The observatory grade comes with a measured transmission curve for the individual filter in the box. Observatory grade glass has the tightest flatness tolerance in the Dawnline line, which suits imaging with long focal lengths. Anti-reflection coatings on both faces reduce halos around bright stars in the field.

The padded pouch protects the filter from knocks and fits in a jacket pocket at the telescope. This is the 2023 edition of the Dawnline line, the earliest version that is still listed in the catalog. A filter does not make objects brighter; it darkens the sky background more than the target, which raises contrast.

What's in the box

• OIII filter, 36 mm unmounted
• Plastic storage case
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Requires a 36 mm filter holder or drop-in slot, since the glass is unmounted and has no threads.
• Most effective in telescopes of 150 mm aperture and larger under moderately dark skies.