Pellucid Spectra H-beta filter 1.25 in, observatory grade

Specifications

Description

The Spectra H-beta is a line filter in a 1.25 in threaded cell that passes a 9 nm band centered on the hydrogen-beta line at 486 nm. It is a specialized filter, used for a small number of nebulae that shine mainly in hydrogen-beta light and respond poorly to UHC and OIII filters.

The best-known target is the Horsehead Nebula in Orion, a dark cloud seen against the faint glow of IC 434. The H-beta filter darkens the sky enough to show the glow behind the Horsehead in telescopes of 250 mm aperture and larger under dark skies. Other targets include the California Nebula, IC 5146 (the Cocoon Nebula), and parts of the Orion Nebula.

Because so few objects benefit, many observers buy an H-beta filter only after they own a UHC or OIII filter. For the targets where it works, though, the difference can be large: a nebula that is invisible without the filter can become visible with it.

The narrow passband dims stars strongly, so finding the target before you add the filter helps. Large exit pupils work best: use a low-power eyepiece that gives an exit pupil of 5 to 7 mm.

The filter is made on polished optical glass with hard-sputtered coatings on one side and an anti-reflection layer on the other. The 1.25 in cell threads into eyepieces and diagonals.

In small telescopes, the dim view limits the filter's use. In a wide-field refractor with a low-power eyepiece, the California Nebula is a reachable target under very dark skies.

About this configuration

The Pellucid Spectra H-beta filter 1.25 in is offered in several configurations, and the notes below describe what this particular one includes. 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 Spectra line, which suits imaging with long focal lengths. The hard-sputtered coating resists scratches and humidity, so the filter tolerates cleaning better over the years.

An aluminum case with a foam insert keeps the filter safe during travel to a dark site. The passband in the 2024 edition matches the other years; the changes concern the housing only. A filter does not make objects brighter; it darkens the sky background more than the target, which raises contrast.

What's in the box

• H-beta filter in 1.25 in threaded cell
• Padded storage pouch
• Test report with measured transmission curve
• Aluminum storage case

Compatibility notes

• Threads into 1.25 in eyepieces and diagonals (M28.5 x 0.6).
• Works best with low-power eyepieces that give an exit pupil of 5 to 7 mm.