Pellucid Spectra H-beta filter 1.25 in, premium 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 premium grade uses polished glass with tighter flatness tolerances, which helps at high magnification. Premium grade filters are checked for the position of the passband before they leave the factory. The standard coating is applied to the filter side of the glass, with a single anti-reflection layer on the back.

The padded pouch protects the filter from knocks and fits in a jacket pocket at the telescope. 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
• Padded pouch

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.