Optolong L-eXtreme F2 fast optics Filter 2"

The Optolong L Extreme f2 Filter is a high performance dual narrowband imaging filter engineered specifically for very fast telescopes and astrographs with focal ratios around f2 to f3. By isolating two powerful emission lines, Hydrogen Alpha at 656 nm and Oxygen III at 500.7 nm, and using an extra narrow 7 nm bandwidth, it delivers superior contrast and cleaner deep sky data even under strong light pollution.
 
This filter is ideal for imaging emission nebulae with fast optical systems. Fast telescopes naturally shift the angle of incoming light, which can reduce transmission in standard narrowband filters. The L Extreme f2 is optimized for this environment, maintaining tight bandpass performance while minimising star haloing and colour shifts. It is a favourite choice for modern astrographs, fast refractors, wide field rigs and systems that operate at low focal ratios.
 
The ultra narrow bandpass dramatically reduces unwanted urban glow, mercury and sodium contamination, giving you cleaner backgrounds, richer nebula detail and stronger signal to noise for shorter total exposure times. This makes the L Extreme f2 particularly effective for observers who want deep sky data quality even from bright or suburban skies.
 
Technical Features
Dual narrowband transmission: Ha at 656.3 nm and OIII at 500.7 nm
Bandwidth: 7 nm optimized for fast optical systems
High peak transmission: greater than ninety percent in pass bands
Blocking depth: greater than ninety nine percent for unwanted wavelengths from 300 to 1000 nm
Substrate: high quality B270 optical glass
Thickness: 1.85 mm
Surface quality: 60 to 40
Transmitted wavefront RMS: lambda by 4
Parallelism: 30 seconds
Precision coatings: multi layer anti reflection coatings for minimal scatter and halo control
 
Compatibility and Performance
The Optolong L Extreme f2 is specifically designed for:
Very fast telescopes from approximately f2 to f3.5
Fast astrographs, wide field refractors and specialized imaging rigs
Colour one shot colour cameras, cooled CMOS sensors, DSLR and mirrorless setups
Urban or light polluted skies where contrast and background control matter
 
This filter is especially beneficial for imaging emission nebulae where strong nebular signals are present but broadband filters struggle due to rapid sky glow and short exposure limits. It delivers excellent contrast, minimal star bloating and reliable colour separation in challenging imaging environments.