Astronomy Specialist


$1,129.00 AUD
SKU: BA2961750

Baader Planetarium



The Baader Bessel filters are precision-grade photometric bandpass filters with transmissions according to the Bessel specification. The full set includes U, B, V, R & I filters and is available in 1.25", 2", 36mm, 50.4mm, 50 x 50mm, 65 x 65mm & 100 x 100mm sizes.



The UBVRI system was developed in the 1950s for the 0.9m McDonald Observatory telescope and initially consisted of a Johnson UBV filter set, later extended by Cousins to include the R and I filters for the red region of the spectrum. The filters of the extended UBVRI system according to Johnson and Cousins include:

  • U – Ultraviolet, with a transmission window between about 320 and 400 nm.
  • B – Blue, with a transmission window between about 400 and 500 nm.
  • V – Visible, with a transmission window between about 500 and 700 nm.
  • R – Red, with a transmission window between about 550 and 800 nm.
  • I – Infrared, with a transmission window between about 700 nm and 900 nm.

According to the state of the art at that time, the filters had different thicknesses and transmittance values as well as no sharply defined edges (if any). Their spectrum therefore corresponds to a curve instead of a plateau.

In 1990, M. S. Bessel looked into the subject in order to find a filter combination for the new CCD cameras that were becoming increasingly widespread at the time. These Bessel-UBVRI filters are still standard today and are also the most widely used in amateur circles such as the AAVSO. 

The new generation of Baader Planetarium UBVRI filters, introduced at the end of 2021, is a modern filter set that is both fully compatible with the characteristics of the original Bessel filters and at the same time meets modern demands for contemporary filters:

  • The photometric filters now have the same thicknesses as all other Baader filters.
  • They fit into any commercially available filter wheel, for which the original filters were often simply too thick and could not be fixed securely.
  • They are parfocal with all other Baader filters of the same size and now also available in sizes up to 100x100mm - large enough even for modern, professional sensors, or optimal for easy handling in the common filter mounts for amateur telescopes.
  • The coating of all photometric filters is manufactured in the same technology as the CMOS-optimized narrowband and LRGB filters.

Like all Baader CMOS-Optimized filters, it is designed to work very well with modern highly-sensitive astronomy cameras and telescopes with focal ratios between f/15 to f/1.8

Baader's CMOS-optimized filters have excellent halo and reflection control, increased contrast and even more narrow passbands than previous generations. These filters set the new standard for monochrome astrophotography with CMOS sensors.

Baader CMOS-optimized filters - advantages

  • Very high contrast due to steep edges of the coating
  • Reflex-Blocker coatings to minimize reflections caused by correctors, reducers and other optical elements
  • Standardized thickness for maximum possible parfocality - less need to refocus due to different filter thicknesses
  • Blackened edges with side markings in the form of a black outer rim on the telescope side
  • Each filter is individually fine-polished and coated with sealed coating edge, preventing moisture ingress
  • Life-Coat for hardness and age-resistance, making the filters able to withstand tougher conditions for longer


For more information on Baader's new generation of CMOS-optimized Bessel filters, check out this blog post


Transmission range
Transmission according to Bessel specification
Suitable for f/ratio f/15 to f/1.8
Reflex-Blocker hard coated, planeoptically polished
Filter thickness 1.25", 2", 31 mm, 36 mm 2 mm (without cell), 6 mm with cell
Filter thickness 50.4 mm, 50 x 50 mm, 65 x 65 mm, 100 x 100 mm 3 mm
Filter size 1.25", 31 mm, 36 mm, 2", 50.4 mm round, 50 x 50 mm square, 65 x 65 mm square, 100 x 100 mm square

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  • 1.25"
  • 2"
  • 31mm
  • 36mm
  • 50.4mm
  • 50x50mm
  • 65x65mm
  • 100x100mm

Optical design

Image circle

Focal ratio


Focal length

Optical thickness



Colour or Mono

Pixel size

Sensor dimensions


Sensor diagonal

Quantum efficiency

Read noise

Fullwell capacity

Bit depth


Camera backfocus

Filter size

Filter size

Bandpass (FWHM)

Bandpass (FWHM)

Glass thickness

Mount type

Payload capacity

Latitude range

Dovetail saddle

Slew speed


Periodic error correction

Counterweight shaft

Azimuth adjustment range

Internal diameter

Illuminated diameter

Compatible telescope external diameter





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