Astrodon NII 3nm Filter - 31mm Round Unmounted
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.
- 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
|Accessory Type||Nitrogen II|
|Warranty||5 Year Warranty|