Astrodon NII 3nm Filter
Introducing the 3 nm FWHM nitrogen [NII] filter for 658.4 nm emission line imaging, currently available in 1.25" mounted and 50 mm dia. unmounted sizes. Most imagers do not realize that their 4.5 - 10 nm FWHM H-a filter for 656.3 nm also includes NII emission at 658.4 nm. These emission lines are so close together that only a filter with 3 nm or less FWHM can readily separate their signals. Professional astronomy papers will state H-a + [NII], signifying that data from both emission lines are combined in the signal. The brackets refer to [NI]I as a forbidden transition, such as [OIII]. Many objects are enriched in nitrogen and have [NII] emission, such as planetary nebula, Wolf-Rayet bubbles and supernova remnants.
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. They can be used to image when the moon is up, thereby extending imaging time. They can be used in light-polluted locations.
The narrow range of wavelengths 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 NII 3nm 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: NII3-50R