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Stargazing June 15th ✦ 6pm-10pm
Stargazing June 15th ✦ 6pm-10pm

Antlia 2.5nm Narrowband Oxygen III (OIII) Ultra Imaging Filter - 2" Mounted

by Antlia
SKU OIII-2in-2.5nm-Ultra

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The Antlia OIII 2.5nm Ultra filter is made with quartz glass and undergoes precisely controlled coating processes to ensure optimal performance. The 2.5nm Ultra series is designed with a FWHM (full width half maximum) of 2.5nm +0.3/-0.1nm and a transmission efficiency >93-95% at the Central Wavelength.

The quartz glass possesses an ultra-low thermal expansion coefficient with outstanding thermal and mechanical properties. When combined with advanced coating processes, these filters exhibit remarkable durability for imaging and research in the most challenging environmental conditions.

Compared to the 3nm Pro series, the 2.5nm Ultra series coated on quartz glass exhibits higher transmittance with OIII≥93%. These filters are estimated to improve signal by 20-30% in your images. Additionally, the Ultra series boasts a narrower FWHM of 2.5nm+0.3/-0.1nm, resulting in an exceptionally high Signal to Noise Ratio for deep sky photography.


  • High transmittance (OIII≥93%) and the narrow FWHM 2.5nm +0.3/-0.1nm to maximize contrast;
  • Flat-top spectrum minimizes the loss in transmission due to the Center Wavelength (CWL)-shift;
  • Out of band blocking specification is designed to OD5 (0.001%) to block interference from other wavelengths and minimize halos around bright stars;
  • Made of quartz glass, excellent durability and thermal stability, better fracture resistance and scratch resistance, improved coating resilience to cleaning;
  • Filters are resistant to scratches;
  • Identical filter thickness to existing standards and double-sided fine polishing , with utmost care for parfocality;
  • The OIII 2.5nm Ultra filter is recommended for use in optics with a Focus Ratio of ≥ f/3.5 for best optical performance;
  • 100% of production are scanned individually, all transmission and blocking (OD) data in the box are actual, measured spectra and transmission;
  • All Antlia filters are coated to the edge of the substrate and unmounted narrowband filters are edge blackened to eliminate internal reflections from stray light;
  • Warranty: 3-years against delamination.

Technical Specifications:

Oxygen III

  • FWHM: 2.5nm +0.3/-0.1nm
  • CWL (Central Wavelength): 500.7nm
  • Peak Transmission: 93%
  • Blocking: ≥OD5
  • Filter Thickness: 2mm+/-0.05mm

Filter Ring:

  • 2" Mounted (M48x0.75 thread)
  • Ultra-thin filter cell to minimize vignetting by maximize possible clear aperture
  • Black Anodized Finish
  • Laser Engraving No Fading

Not Recommended:

  • Solar imaging
  • Night visual observation

DO NOT LOOK AT THE SUN WITH ANTLIA FILTERS. Serious eye damage will result if you fail to observe the warning.