The Optolong L Ultimate is an ultra high performance dual band filter built for astrophotographers who want exceptionally clean and high contrast nebula images with a colour camera. It uses an extremely narrow 3 nm bandpass in both Hydrogen Alpha and Oxygen III, which isolates the pure emission lines from nebulae while removing almost all unwanted sky glow. This produces deep colour, crisp detail and a very strong improvement in signal to noise ratio even under heavy city light pollution.
 
The L Ultimate is ideal for capturing complex emission regions where fine structure matters. It delivers smooth backgrounds, strong contrast and beautiful colour separation that is normally possible only with dedicated mono cameras.
 
Correct Optical Compatibility
Because the L Ultimate uses an ultra narrow 3 nm bandpass, it performs best on telescope systems of f4 and slower. Ultra fast setups can shift the wavelength enough to reduce transmission.
Best performance: f4 to f10
Acceptable: f3.5 to f4 (minor shift)
Not recommended: f2 to f3 systems such as RASA or HyperStar
This ensures that you set the right expectations and get the full performance the filter is designed for.
 
Technical Features
Ultra narrow 3 nm bandpass for both Ha and OIII
Ha centered at 656.3 nm and OIII centered at 500.7 nm
More than ninety percent peak transmission
More than ninety nine percent blocking from 300 to 1000 nm
Substrate made of B270 optical glass
Thickness 1.85 mm
Surface quality 60 to 40
Wavefront RMS lambda by 4
Parallelism 30 seconds
Multilayer coatings that minimise halos and maintain flat transmission curves
 
Performance and Use
The L Ultimate works with colour CMOS cameras, DSLRs and mirrorless sensors and is available in both 1.25 inch and 2 inch formats. It is highly effective on refractors, Newtonians, SCTs and astrographs with moderate focal ratios. It is especially useful in bright urban conditions where traditional dual band filters struggle to maintain contrast.
 
The filter produces smooth gradient free backgrounds and helps reveal faint filaments, shock waves and delicate structural features in nebulae that wider filters often miss.