Brightwell Dawnline OIII filter 1.25 in, observatory grade

Specifications

Description

The Dawnline OIII is a narrowband nebula filter in a 1.25 in threaded cell. It passes a band about 8 nm wide centered near 499 nm, which covers the doubly ionized oxygen emission lines at 496 and 501 nm, and it blocks most other visible wavelengths, including much of the glow from artificial lighting. Peak transmission at the center of the passband is about 88 percent.

Oxygen-III emission is strong in planetary nebulae, supernova remnants, and some high-excitation emission nebulae, while city lighting contains little energy at these wavelengths. By blocking the background and passing the oxygen light, the filter raises the contrast between the nebula and the sky. The nebula itself does not gain brightness, but faint outer regions and fine structure become easier to see against a darker background. On galaxies and star clusters, the filter simply dims the view, so it is most useful on targets that emit strongly in the oxygen lines.

Good targets include the Veil Nebula, the Dumbbell and Ring nebulae, the Crab Nebula, and the Eskimo Nebula. The Veil is a large, faint supernova remnant that benefits from a low-power eyepiece and a wide field. The effect is most noticeable in telescopes of 100 mm aperture or more under moderately dark skies. Through the filter, stars appear blue-green, because the passband lies in the blue-green part of the spectrum.

The filter screws into the barrel of any 1.25 in eyepiece or the nosepiece of a 1.25 in diagonal. The glass is coated on one side with a multi-layer interference stack and on the other with an anti-reflection layer. Because the filter is a thin interference stack, it should be kept away from moisture and abrasive cleaning, and it should be stored in its case when not in use.

About this configuration

All configurations of the Brightwell Dawnline OIII filter 1.25 in share the same filter glass design; the options change the grade, the coating, and the case. 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. The standard coating is applied to the filter side of the glass, with a single anti-reflection layer on the back.

A padded pouch is easy to open with cold fingers, which helps when you swap filters during a session. The passband in the 2024 edition matches the other years; the changes concern the housing only. The transmission figures in the specifications apply to every configuration of the Dawnline line.

What's in the box

• OIII filter in 1.25 in threaded cell
• Plastic storage case
• Test report with measured transmission curve
• Padded pouch

Compatibility notes

• Threads into 1.25 in eyepieces, diagonals, and filter wheels (M28.5 x 0.6).
• Most effective on planetary and emission nebulae under moderately dark skies.