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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 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.
* 28 mm dia (1.25" mounted)., 50.4 mm dia, 49.7 x 49.7 mm square unmounted
* 3 mm (+/-0.025) thick
* Parfocal to f/4 and parfocal with Astrodon LRGB depending on telescope optics
* H-a, OIII, SII, NII, Red Continuum
* 5 nm FWHM filters guaranteed >90% transmittance (typically 94-97%)
* 3 nm FWHM filters typically 90-92% (scan included)
* >4 O.D. (out-of-band blocking) 300 - 1150 nm
* 1/4 wave, striae-free fused silica substrates
* 30 arcsecond parallelism
* sides are blackened
* hardest available sputtered coating for durability
* 3 nm filters can be used on systems to f/3
* designed to have lower blue spectral shift with faster (e.g. f/3) optics.
Astrodon's Narrowband Filter FAQThe H-a filter is unique in that most H-a filters (4.5, 5, 6 ,7, 8, 12 nm) pass BOTH H-a and NII (nitrogen). These emission
lines are very close together. The 3 nm filter eliminates about 75% of the NII that normally gets captured in the wider filters.
Going to a 3 nm Ha-filters:
PROS
1. Better contrast, i.e. detail of fine structure
2. Better for light polluted locations because narrower filters remove more light pollution and moonlight. This means the noise
(N) goes down with narrower filters.
3. We don't lose signal because the filter is narrower. We have developed unique processes where the throughput through the
filter is about the same between the 3 and 5 nm. Normall the narrower the filter the lower the throughput. This is our
advantage.
CONS
1. More expensive
2. By eliminating much of the NII, the signal (S) goes down in some objects like planetary nebula (e.g. Dumbbell).
3. Since the S goes down due to loss of NII but the N goes down due to lower background, the S/N does not improve with 3nm H-a
for these objects. Don't lose, but don't gain.
4. For normal emission objects like the Pelican or N. American, the S/N goes up since there is little NII there in the first
place.
5. Many pretty picture images don't care if NII is in their H-a. They want all the signal they can get and would be better off
with the 5 nm.
The important filter to get 3 nm is the OIII which is most prone to moonlight, where many images use these narrowbands. There
are no other emissions to lose with OIII or SII so narrower is always better.
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