Medicals Telescopes Tools

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)., 49.7 mm dia, 49.7 x 49.7 mm square unmounted
• 65 mm square filters for SBIG STX  -   H-a 5 nm in stock!
  
• 36 mm dia. for SBIG ST-8300 - H-a, SII, OIII 5 nm  in stock!
  
• 28 mm dia. mounted in 31 mm inserts for QSI cameras in stock!
• 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 and 3 nm FWHM filters guaranteed >90% transmittance(typically 92-97%)
  • Measurement of three 7 nm H-a filters from a competitor showed 86%, 74% and 71% transmittance at the H-a emission wavelength compared to typical 95% transmittance for our 5 nm narrowband filter. Since our filters are also spectrally narrower, your background noise will be lower, thereby improving your overall signal-to-noise.  Our  guarantee means that you can be assured of the highest performance compared to hit-or-miss low cost filters. See ourNarrowband FAQ for more more details.
• >4 O.D. (out-of-band blocking) 300 - 1150 nm
• <1/4 wave, striae-free fused silica or other optical-quality glass substrates
  • Actual measurements on our substrates prior to coating show <0.05 transmitted wave front for ~5x better than 1/4 wave!
• Coatings go to the edge and the sides are blackened
  • Coring or cutting from larger high-quality substrates prevents light leakage through an uncoated "rim" on individually coated filters  (see Narrowband FAQ)
• 30 arcsecond parallelism
• hardest available sputtered coating for durability - edge sealing not needed
• 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
• Proudly made in the U.S.A.

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.  The Dumbbell Nebula, M27, is a classic example.  The emissions from 3nm Astrodon OIII, SII, H-a and NII shown below are significantly different.  Click here to see a full color image made from these filters.

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. 

You can see a Flash overlay of the 3 vs. 6 nm images here.