Used Baader S-II 2" f/2 Ultra-Highspeed CMOS Optimized 4nm Filter


  • Sale
  • $324.95
  • Regular price $499.00

Yes, our inventory is accurate!

Qualifies for Free Ground Economy Shipping (CONUS Only)

Not Returnable Please note that used items are not returnable.

This is a Baader S-II 2" filter in excellent condition.  The optics and body are clean.  It comes in it's original jewel case.  




Baader has designed this 2" SII ultra-narrowband filter to be precision-grade and provide astrophotographers an excellent complement to their imaging setup. With 4 nm FWHM, this ultra-highspeed filter from Baader is recommended for optical systems from f/3.5 to f/10. The filters are designed to allow for 1:1:1 exposures. This matches the typical CMOS quantum efficiency and s/n ratio. The filters are optimized for CMOS cameras, but they work great with CCD cameras as well.

SII filters are suitable for imaging cosmic bodies like nebulae that are along the singly ionized sulfur (SII) wavelengths. This narrowband filter works well both in conditions of light pollution or dark skies! SII filters block all wavelengths from UV to infrared, including the emission lines of sodium/Na and mercury/Hg artificial lighting, except for the SII singly ionized sulfur wavelengths which occur as a doublet at 671.7 nm and 673 nm.

This Baader Planetarium SII ultra-narrowband filter is CMOS optimized with Life-Coat. This coating is a hard coating that will not peel, flake, and will not physically degrade even in the most extreme weather conditions. This coating is also designed to withstand repeated cleaning with fine optical cleaning equipment. This coating is so durable that Baader is confident it will last the life of the filter.

Not only do the filters have sealed coating edges with the Life-Coat, but they are also hard coated with Reflex-Blocker and planoptically polished. All of the edges are blackened for increased contrast and eliminate any reflection that would come from light falling on the edge of the filter. The filters are also equipped with a filter-lead-side-indicator in the form of a frontside outer rim.

Baader Ultra-Narrowband 2" CMOS Optimized Filter Features

  • Transmission Ranges: SII
  • Filter Thickness (w/out glass) 2 mm
  • Half Bandwidth: 4 nm
  • Central Wavelenth: 671.7 nm
  • AR-Coating: Reflex Blocker hard coated and planoptically polished
  • Filter Size: 2"
  • Threaded: Yes
  • Usage: CMOS, CCD, SII
  • Mounted: Yes
  • Filter Type: Narrowband
  • Filter Shape: Round
  • Feature Highlight: CMOS-Optimized with Life-Coat
    Product specification
    SKU Number
    Model Number
    Single Filter, Set, or Accessory
    Single Filter
    Filter Type
    Filter Size
    50.8mm (2")
    Filter Shape
    Filter Insert Style
    Baader Model Number

    Filter Wavelength (Center or Range)
    671.7 nm
    Filter Bandwidth/Bandpass
    4 nm
    Filter Optical Thickness
    2 mm

    Baader - Limited Lifetime


  • Baader S-II f/2 Ultra-Highspeed Filter – Premium Grade
  • 4 nm FWHM, recommended for extremely fast optical systems from < f/2.3
  • Specially designed for the special filter requirements when used with extremely fast f/2 optics like the Celestron RASA astrographs
  • For highest contrast even at strongest light pollution, minimizes star size, emphasizes finest nebula detail – indispensable under Bortle 9/8/7/(6) skies
  • Reflex-Blocker™ hard coated and planeoptically polished – with sealed coating edges (Life-Coat™)
  • Blackened edges all around, with filter-lead-side-indicator in the form of a telescope-sided black outer rim
  • Optimized for modern CMOS cameras, but likewise excellent for CCD camera technologies

This new generation of Baader CMOS-filters features:

CMOS-optimized Baader Filters
  • Increased contrast
  • Ever more narrow passbands
  • Reflex-Blocker coatings, for largest ever freedom from halos, even under most adverse conditions concerning aux-optics
  • FWHM on each filter category carefully designed to allow for 1:1:1 exposures, matched for typical CMOS quantum efficiency and s/n ratio
  • Identical filter thickness to existing standards, with utmost care for parfocality
  • Blackened edges all around, with filter-lead-side-indicator in the form of a black frontside outer rim, to additionally eliminate any reflection due to light falling onto the edge of a filter
  • Each filter coated individually, with sealed coating edge (NOT cut out of a larger plate with coatings left exposed, read more)
  • Life-Coat™: evermore hard coatings to enable a non-aging coating for life – even in a most adverse environment