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

All configurations of the Pellucid Spectra H-beta 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 Spectra line, which suits imaging with long focal lengths. Anti-reflection coatings on both faces reduce halos around bright stars in the field.

A plastic case is light and stacks with other filter cases in an accessory box. This is the 2024 edition, which revised the cell knurling and the case insert of the Spectra line. Check the size in the specifications against your eyepiece barrels, star diagonal, or filter wheel before you choose.

What's in the box

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