Pellucid Halflight OIII nebula filter 2 in, observatory grade

Specifications

Description

The Halflight OIII is a narrowband nebula filter in a 2 in threaded cell. It passes a band about 7 nm wide centered near 498 nm, which covers the doubly ionized oxygen lines at 495.9 and 500.7 nm, and it blocks most of the spectrum outside that band. The hydrogen-beta line at 486.1 nm and the hydrogen-alpha line at 656.3 nm fall outside the passband, so the filter darkens them substantially. Peak transmission is about 92 percent at the center of the band.

Emission nebulae shine strongly at the oxygen lines, while the sky background glows across the spectrum. The filter passes the nebula's oxygen light and blocks much of the background, so the sky around the nebula appears darker and faint emission structure stands out more clearly. The nebula itself does not gain brightness, and the effect depends on the nebula's surface brightness and on the sky conditions. Good targets include the Veil Nebula, the Cygnus Loop, the Dumbbell Nebula, and the Crab Nebula, where outer filaments and faint arcs show most clearly. Under moderately dark skies, the filter is most useful at low magnification with a wide exit pupil.

Galaxies, star clusters, and reflection nebulae emit most of their light outside the passband, so on those objects the filter mainly dims the view. Stars appear greenish-blue through it, since the passband sits in the blue-green part of the spectrum. Because the passband is narrow, the filter reduces the total light reaching the eye, so the view is dimmer than without it. Allow your eyes fifteen to twenty minutes of dark adaptation before observing. The filter threads into the barrel of any 2 in eyepiece or the nosepiece of a 2 in diagonal, and its glass carries a multi-layer interference stack on one side and an anti-reflection layer on the other.

About this configuration

All configurations of the Pellucid Halflight OIII nebula filter 2 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 Halflight line, which suits imaging with long focal lengths. Hard-sputtered layers keep their passband stable over time, which matters if you compare images taken years apart.

The filter ships in a clear plastic case that keeps dust off the glass between sessions. This is the 2024 edition, which revised the cell knurling and the case insert of the Halflight line. 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 in 2 in threaded cell
• Padded storage case
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Threads into 2 in eyepieces, diagonals, and filter wheels (M48 x 0.75).
• Most effective in telescopes of 100 mm aperture and larger under dark skies.