What about NII and H-a?
This is a bit complicated. It is not well known that most H-a filters pass both H-a and NII. H-a emits at 656.3 nm and NII emits most strongly at 658.4 nm (and weakly at 653.8 nm). These are very close together spectrally. Thus, most H-a filters are wide enough (e.g. 4.5 nm bandwidth and wider) to pass both emission lines as shown for the older Astrodon 6 nm filter above. Our 3 nm H-a begins to separate both emission lines and reduces the NII contribution significantly. In this example the 3nm filter only transmits 15% at the NII 658.4 nm wavelength, whereas the H-a remains unchanged. As mentioned earlier, some objects are enriched in NII, such as planetary nebula and Wolf-Rayet bubbles. The Dumbbell Nebula, M27, is a good example. The wispy clouds in the core of M27 are dominantly NII. A tricolor narrowband image is also shown below, mapping OIII to blue, H-a to green and NII to red to produce a beautiful color image. This information provides you with a choice based upon your light pollution, desire for more detail, or simply wanting all the photons you can get out
of your H-a filter.
Our NII filters have a center bandwidth of 658.4 nm.Bring Out The Faint Structures In Nebulae
Astrodon Narrowband filters set a new bar of performance and durability for imaging and research. We offer 5 nm and ultra-narrow 3 nm bandwidths.Astrodon Narrowband filters set a new bar of performance and durability for imaging and research. The narrow 5 and 3 nm bandwidths enhance contrast of emission targets by lowering your background signal. The guaranteed >90% transmittance at the emission wavelength provides you with the highest signal available. This guaranty is expensive to manufacture for such spectrally narrow filters, but it assures you in writing on each filter box that you will get what you paid for. These two factors combine to provide you with the highest contrast available. Our latest narrowband filters are typically achieving >95-98% transmittance. Astrodon Narrowband filters are renowned for minimizing halos around bright stars, even for long exposures of 30-45 minutes typical of narrowband imaging in astrophotography. Lastly, Astrodon Narrowband filters are coated to the edge of the part and are edge blackened. This is critical to minimize stray light for a filter that blocks most light except for the narrow bandpass.
Astrodon Narrowband filters for imaging are all about contrast. What do we mean by this? Contrast brings out faint features by reducing the background – the narrower the filter, the better. The problem is keeping the signal (%T at the emission line) constant as the filter becomes spectrally narrower. This is why our >90%T guarantee is so important, even though it becomes very costly to manufacture. But, this assures you that you can take advantage of the improved contrast with our narrower filters compared to the much less expensive 7- 8.5 nm filters on the market. You can see the increase in contrast in the sequence of equal-exposure images of the Crescent Nebula (NGC 6888) taken on the same system and on the same night. As the bandwidth becomes narrower, the nebula “pops” out of the background, as does the faint surrounding nebulosity. Actually, the >90%T is a legacy specification, since new production technologies employed over the past 3-4 years routinely produces 97-98%T at the emission wavelengths.
Random tests of a competitor’s low-cost 7 nm H-? filter showed %T values ranging from 86% down to 71%. Random selection of 5 nm Astrodon filters measured %T values at the H-? emission wavelength of 93, 98, 97 and 98%. You will never know that you are using their 71% filter compared to a 97% Astrodon filter because they do not guarantee this important performance parameter. They recently came out with a 3.5 nm H-? narrowband filter and those results are even worse, ranging from 3-70% at the emission wavelength. Similarly, analysis of different 2016 lots of Astrodon 3 nm H-? filters measured 97, 97, 97 and 96%T. Yes, our filters are more expensive, but you always know what you are getting with Astrodon Narrowband filters – the best performance in the market – guaranteed. Our recent blueshift data show that Astrodon 3 and 5 nm filters can be used with nearly all systems as fast as f/3.5 with virtually no loss in emission signal, so you do not need to buy custom “upshifted” filters if you have both fast and slow systems.
Step up to the quality and performance of Astrodon narrowband filters.
Astrodon Narrowband filters currently are provided off-the-shelf for:
Sulphur SII 671.6 nm 5 and 3 nm
Hydrogen-alpha H-? 656.3 nm 5 and 3 nm
Nitrogen NII 658.4 nm 3 nm only
Red Continuum RedCon 645 nm 5 nm only
Oxygen OIII 500.7 nm 5 and 3 nm
Other emission lines, bandwidths and sizes upon request
When we refer to bandwidth, we are referring to the spectral width of the transmission region at 50% of the peak transmittance (FWHM – full width at half maximum).
1.25? mounted, mounted in 31 mm rings (for QSI), 36 mm unmounted, 49.7 mm diameter and square unmounted (36 mm filters will not exceed 36.1 mm dia.)
3 +/- 0.025 mm thick (5 nm); 3+/-0.05 mm thick (3 nm) substrate prior to coating
Parfocal with all other Astrodon filters (depending upon telescope optics)
Peak transmittance guaranteed >90% at the emission line (97-98%T typical) f/10 to f/3.5 for both 3 nm and 5 nm
>90%T at the emission wavelengths for 5 nm OIII and H-a at f/3. >80%T for SII at f/3
>90%T for 3 nm OIII at f/3
Steep spectral profile minimizes halos around bright stars
>4 OD (out of band blocking) 300 – 1150 nm
Single, striae-free 1/4 wave fused silica substrate
30 arcsecond parallelism
Durable hard-oxide sputtered coatings with a 5-year limited warranty against delamination
All filters are coated to the edge and edge-blackened to minimize stray light contamination that can come though the uncoated “rim” in other brands
Please read our detailed Astrodon Narrowband Filters_FAQ (Frequently Asked Questions) by clicking on the link. We cover such issues as what is a narrowband filter, filter selection guidelines, contrast, bandwidth, out-of-band blocking, fast optics, effect of moonlight, separation of NII and H-a, etc.
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