Aurel Tallgrass H-beta nebula filter 2 in, observatory grade

Specifications

Description

The Tallgrass H-beta filter is a 2 in threaded filter with a passband about 8 nm wide, centered on the hydrogen-beta emission line at 486 nm. Its peak transmission is 85 percent. The aluminum cell has an M48 x 0.75 thread, so it screws into the filter threads of 2 in eyepieces, diagonals, and filter wheels. The filter blocks most other visible light, including much of the broadband glow from artificial lighting.

Emission nebulae radiate at specific wavelengths, while the sky background is spread across the spectrum. A narrow passband removes much of that background and keeps the nebula's emission at 486 nm, which raises contrast without making the nebula brighter. Hydrogen-beta is weaker than hydrogen-alpha and the oxygen-III lines in most nebulae, so the gain is usually modest. Under moderately dark skies, you may notice more structure in faint emission regions, particularly in telescopes of 150 mm aperture or more.

Stars and star clusters transmit little of their light through a 486 nm passband, so they appear dim and tinted blue-green. Galaxies and reflection nebulae shine in a broad range of colors and are dimmed by the filter rather than enhanced. Planetary nebulae are often better served by oxygen-III filters, since their brightest lines lie away from 486 nm. Keep the filter in its case when not in use, and clean the glass only with a blower or a soft brush.

The glass carries a multi-layer interference coating on one side and an anti-reflection layer on the other. At 2.0 mm thick, the glass is sized to sit flat in its cell. You can use the filter with 2 in eyepieces, or thread it into a 2 in filter wheel, depending on your optical train.

About this configuration

All configurations of the Aurel Tallgrass H-beta 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 Tallgrass 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 2023 edition uses the same coating design as later years, with small differences in the cell markings. The transmission figures in the specifications apply to every configuration of the Tallgrass line.

What's in the box

• H-beta filter in 2 in threaded aluminum cell
• Plastic storage case
• Test report with measured transmission curve
• Aluminum storage case

Compatibility notes

• Threads into 2 in eyepieces, diagonals, and filter wheels (M48 x 0.75).
• Most effective in telescopes of 150 mm aperture and larger.