When it comes to Narrowband filters, no one knows more or does it better than Don Goldman of Astrodon Filters. 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.
Astrodon filters can be used to image when the moon is up, thereby extending imaging time and can be used in light-polluted locations.
The narrow range of wavelengths of Astrondon filters 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 has evolved its narrowband product line from 6 nm to 5 nm FWHM, significantly lowered the prices on all filters, and added the ultra-narrow 3 nm FWHM filters for H-a, OIII and SII. We added a Red Continuum filter (645 nm, 5 nm FWHM) that produces a star map without the H-a or SII emission in order to subtract stars from emission images. Astrodon NB filters are renowned for not producing halos around stars and not leaking NIR light.
The 3 nm FWHM filters are stocked and are not custom ordered. The 3 nm OIII filter provides the best protection from the effects of moonlight, producing half the background signal of a 90% T, 6 nm FWHM filter, resulting in an increase in S/N of ~21%. This will even be larger in comparison to wide 7 or 8 nm FWHM filters.
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. The Dumbbell Nebula, M27, is a classic example.
It is often asked whether the benefits of the narrower 3 nm filters are worth the extra cost over wider filters. Besides minimizing the effect of moonlight and light pollution, a primary goal for selecting the narrower filter is to increase the contrast in the shortest possible imaging time. The following comparison shows this improvement for M76, The Little Dumbbell Nebula, taken by Ken Crawford between an older Astrodon 6 nm OIII filter and the 3 nm OIII filter. They are each 3 x 30 min combined exposures taken with an Apogee U9000 CCD camera on a RCOS 20" RC telescope. The improvement in detail is dramatic with the 3 nm filter, especially in bringing out the faint nebulosity and fine structure.
Astrodon Narrowband 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