Astrodon 5nm OIII Narrowband Filter - 36mm Round Unmounted

Astrodon 5nm OIII Narrowband Filter - 36mm Round Unmounted
Item # AN-OIII5-36R
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  • Parfocal to f/4 and parfocal with Astrodon LRGB.
  • The Astrodon 36mm Round OIII 5nm Narrowband Filter has a guaranteed >90% transmittance.
  • Crafted with 1/4 wave, striae-free fused silica substrates.
  • The Astrodon 36mm Round OIII 5nm Narrowband Filter feature hardest available sputtered coating for durability.
  • Designed to have lower blue spectral shift with faster optics.


Astrodon 36mm Round OIII 5nm Narrowband Filter

When it comes to Narrowband filters, no one knows more or does it better than Don Goldman of Astrodon Filters. Narrowband (NB) filters enhance contrast of emission objects by accepting only a narrow range of wavelengths around the emission lines of hydrogen (H-a, 656 nm), oxygen (OIII, 501 nm), sulfur (SII, 672nm) and others.

Astrodon filters can be used to image when the moon is up, thereby extending imaging time and can be used in light-polluted locations.

The narrow range of wavelengths of Astrondon filters is defined as the FWHM (full-width at half- maximum intensity). Narrower filters decrease the background noise. However, narrower filters are more difficult to manufacture consistently, and are thus more expensive. Furthermore, it is difficult to maintain high transmission through the bandpass of the filter as it becomes narrower. If the peak transmission decreases as the filter is made narrower, the emission signal decreases and the gain in S/N (signal-to-noise) is not realized. Astrodon has achieved this goal of high transmission for narrower filters.

Astrodon has evolved its narrowband product line from 6 nm to 5 nm FWHM, significantly lowered the prices on all filters, and added the ultra-narrow 3 nm FWHM filters for H-a, OIII and SII. We added a Red Continuum filter (645 nm, 5 nm FWHM) that produces a star map without the H-a or SII emission in order to subtract stars from emission images. Astrodon NB filters are renowned for not producing halos around stars and not leaking NIR light.

The 3 nm FWHM filters are stocked and are not custom ordered. The 3 nm OIII filter provides the best protection from the effects of moonlight, producing half the background signal of a 90% T, 6 nm FWHM filter, resulting in an increase in S/N of ~21%. This will even be larger in comparison to wide 7 or 8 nm FWHM filters.

Astrodon 36mm Round OIII 5nm Narrowband Filter Features.

  • Peak transmission guaranteed >90% assuring you of the highest performance.
  • Bandwidths of 5 nm for general use and 3 nm for highest contrast, moonlight rejection and light pollution rejection
  • > OD 4 (0.01%T) out-of-band blocking
  • Hard-oxide coatings on clear 1/4-wave substrates go to the edge, eliminating uncoated rims that leak light.
  • Sides are blackened to minimize reflections
  • 3 nm filters can be used with fast optical systems to f/3 with little signal loss
  • Parfocal with all Astrodon filters
Astrodon Product Number: OIII5-36R
Additional Information

Additional Information

Manufacturer Astrodon
Type of Filter Oxygen III
Filter Usage Imaging
Single or Set? Single Filter
Filter Size 36mm
Filter Shape Round
Filter Mounted? Unmounted
Warranty 5 Year Warranty
Included Items

Included Items

  • Astrodon 36mm Round 5nm OIII Filter
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