The Astronomik OIII visual filter is tailored for observation of OIII regions, particularly effective in both light-polluted or dark sky areas. It significantly enhances the contrast between objects emitting light at the OIII emission lines and the sky background.Specifically designed for gaseous and planetary nebulae, the Astronomik OIII visual filter features a narrow pass band for both OIII lines, ensuring substantial contrast gains even under optimal observation conditions. This attribute makes a critical difference in detecting faint supernova remnants and planetary nebulae, often revealing details that might otherwise be imperceptible.The filter's application spans aperture ratios from 1:3.5 to 1:15, most suitable for telescopes starting at 6" (150mm) aperture.The filter blocks emission lines from both high- and low-pressure sodium (Na)- and mercury (Hg)-vapor lamps and natural airglow, while allowing the OIII lines to pass through with high transmission. This selective filtering enhances the contrast between the celestial object and the sky background, crucial for all visual observations.Overall, the Astronomik OIII visual filter excels in enhancing the visibility and detail of OIII emission regions. It is an indispensable tool for astronomers aiming to maximize the observational potential of gaseous and planetary nebulae.Suitability of the Astronomik OIII visual filtersVisual observation (dark skies): Very good, huge contrast enhancement for OIII-emission nebulaeVisual observation (urban skies): Very good, huge contrast enhancement for OIII-emission nebulaeTechnical dataTypical transmission of 95% at 496nm (OIII)Typical transmission of 95% at 501nm (OIII)Transmission range: 494nm to 506nmFull-Width-Half Maximum (FWHM): 12nm Glass thickness: 1mm (except Astronomik OWB Type 3 and XT filters)Finely polished optical glass, free of any striae, residual stresses, and bubblesExtremely scratch-resistant and durable coatingCompletely resistant against high humidity and aging effectsAstronomik MFR coating (Astronomik Deep-Sky RGB, UV-IR Block L-1, L-2, L-3, and Narrowband-Emissionline filters)Parfocal with other Astronomik filtersDiffraction-limitedOptimized spectral windowsDelivered in a high-quality, long-lasting filter boxDue to the high-precision machining of the carrier substrate, all Astronomik filters are parfocal with wedge errors far below all common tolerances. Astronomik filters are diffraction-limited, ensuring that the optical performance (resolution and contrast) of your telescope is not compromised. Issues concerning focal distances and the exact alignment of images from each color channel are therefore effectively eliminated. All Astronomik filters are ideal for modern cameras, including both CMOS- and CCD-based models, ensuring you capture the best possible images with your equipment. Sharpness, contrast, and the absence of any reflections and halos are guaranteed. Even with bright stars in the field of view, nebulae and galaxies will be captured with the finest structures. The Astronomik MFR coating is designed for optimal performance across a wide range of focal ratios of the optics used.Every Astronomik filter undergoes a rigorous quality control process before delivery. Additionally, each Astronomik Narrowband-Emissionline filter is measured using a high-end PerkinElmer Spectrophotometer to ensure it meets the specifications published on our website. On the back of each Narrowband-Emissionline filter box, you will find a label detailing the precise spectral data for that specific filter, including the maximum transmission and the recommended range of focal ratios. Customer opinionJohn C.: »I bought your 2'' visual OIII filter from OPT and found it superior to the others I have tried. No red dots. Excellent contrast in the veil nebula. Thank you for making such a fine filter!«