Thousand Oaks LP-4 H-Beta Line Band Filter for 2" Eyepieces

Thousand Oaks LP-4 H-Beta Line Filter – 2" Eyepiece Format

Reveal the Hidden Detail of the Night Sky’s Most Elusive Emission Nebulae

The Thousand Oaks LP-4 H-Beta filter in 2" eyepiece format is a specialized tool for serious deep-sky observers—engineered to enhance the contrast of rare, hydrogen-beta–emitting nebulae that are otherwise nearly invisible to the unaided eye.

With an ultra-narrow 12 Ångstrom (1.2nm) passband, this filter isolates the hydrogen-beta emission line at 4861Å, allowing only the faint light from objects like the Horsehead, California, and Cocoon Nebulae to pass through. Everything else—light pollution, moonlight, airglow, and unwanted portions of the visual spectrum—is sharply cut off, thanks to the LP-4's very steep transmission curves on either side of the bandpass.

This isn't a general-use nebula filter. It's a precision instrument for hunting the sky’s most challenging “trophy” targets—objects that often require large aperture (10–12" or more), steady, transparent skies, and a long focal length 2" eyepiece to achieve a near-ideal 7mm exit pupil.

Key Features:

• Narrow 12Å Bandpass at 4861Å: Isolates the H-Beta emission line with extreme precision

• Very Steep Cut-Off Slopes: Blocks the entire remaining visual spectrum for maximum contrast

• Designed for Visual Use: Not intended for imaging or reflection nebulae

• Optimized for 2" Eyepieces: Compatible with long focal length eyepieces for best exit pupil performance

• Ideal for Difficult Targets: California, Horsehead, and Cocoon Nebulae stand out in dramatic relief

Recommended Use:
For best performance, use under dark, moonless skies with stable transparency and a 10" or larger telescope. A long focal length 2" eyepiece that produces a 6–7mm exit pupil will allow the faint H-Beta signal to be most effectively detected by the eye.

Not for Astrophotography:
This filter is optimized for visual observing only. It is not suitable for imaging or use with reflection nebulae, which do not emit significantly at H-Beta wavelengths.

How to Choose the Right Eyepiece for Your Filter

Not sure which eyepiece will give you the best view with this filter? Here's a quick tip:

The brightness of a nebula (and how well a filter performs) depends on something called the exit pupil—the size of the light beam exiting your eyepiece and entering your eye. For most filters, certain exit pupil sizes give you better contrast and visibility depending on your sky conditions.

To find the right eyepiece (your target eyepiece), just enter your telescope's focal ratio (like f/5, f/10, etc.) in the calculator box below. It will show you which eyepiece focal lengths work best under different skies:

• Dark Sky, Large Nebulae → Use a long focal length eyepiece for a large exit pupil (6–7mm)

• City or Suburban Skies → Use a medium eyepiece for a moderate exit pupil (3–5mm)

• Smaller Nebulae or High Contrast Needs → A shorter focal length eyepiece for a smaller exit pupil (2–3mm) may help

H-Beta Tip: This filter performs best with a long focal length eyepiece and a 6–7mm exit pupil in a 10–12" or larger scope. Dark skies are essential for success—this filter will reveal what no others can when the conditions are just right.

Tech Details: