Products

Narrowband 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.

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.

 

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.

Typical 5nm Spectrum

Typical 3nm Spectrum

Typical H-a Blocking

Typical OIII Blocking

Red Continuum Spectrum

1.25" mounted H-a 5 nm 
(HA5_27R)

$275

1.25" mounted  OIII 5 nm (OIII5_27R) 

$275
1.25" mounted  SII 5 nm
(SII5_27R)

$275
1.25" mounted  Red Continuum 5 nm
(RedCon5_27R)

$275
1.25" mounted  H-a 3 nm
(HA3_27R)

$395
1.25" mounted  OIII 3 nm
(OIII3_27R)

$395
1.25" mounted  SII  3 nm
(SII3_27R)

$395

50mm dia. H-a  5 nm
(HA5_50R)  

$595

50mm dia. OIII  5 nm
(OIII5_50R)  

$595

50mm dia. SII  5 nm
(SII5_50R)  

$595

50mm dia. Red Continuum 5 nm
(RedCon5_50R)  

$595
50mm dia. H-a 3 nm
(HA3_50R) 

$895

50mm dia. OIII 3 nm
(OIII3_50R)  

$895

50mm dia. SII 3 nm
(SII3_50R)  

$895

50mm square H-a  5 nm
(HA5_50S) 

$695
50mm square OIII  5 nm
(OIII5_50S) 

$695
50mm square SII  5 nm
(SII5_50S) 

$695
50mm square Red Continuum  5 nm
(RecCon5_50S)

$695
50mm square H-a 3 nm
(HA3_50S)

$1055
50mm square OIII  3 nm
(OIII3_50S)

$1055
50mm square SII  3 nm
(SII3_50S)

$1055

1.25" mounted NII 3 nm 
(NII3_27R)

$395

50 mm dia. unmounted NII 3 nm 
(NII3_50R)

$895