Astronomy Specialist


$249.00 AUD
SKU: BA2961700U

Baader Planetarium



The Baader SLOAN-u' filter is a precision-grade photometric bandpass filter with a central wavelength window around 355.1nm



In 2000, a new era in photometry began: with the Sloan Digital Sky Survey (SDSS), a 2.5m telescope at Apache Point Observatory (New Mexico, USA) surveyed one third of the sky in five wavelengths, with windows around 355.1, 468.6, 616.6, 748.0 and 893.2 nm.

This system was also used for the Pan-STARRS project; the broad data base of these two surveys makes them very well suited for calibrating images. Another advantage are the wider transmission windows, which allow fainter objects to be measured in a shorter time.

Even though the six Sloan filters have a different characteristic than the UBVRI filters, there are now the appropriate algorithms to compare the data with each other or to convert them into each other. Therefore, the Sloan photometry system is gaining more and more importance.

At present, UBVRI filters still dominate in amateur circles, but Baader not only offers unmounted Sloan filters for professional telescopes, but also in the sizes and mounts commonly used by amateurs. In the coming decades, it is expected that the Sloan system will also become more widespread in amateur circles, which is why Baader Planetarium's SLOAN filters are also available in the common filter sizes for amateur telescopes as well as in the large format 100x100 mm.

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 SLOAN filters, check out this blog post


Transmission range
Transmission according to SLOAN specification
Central wavelength
355.1 nm
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

Customer Reviews

Be the first to write a review


From $249.00 AUD


  • 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





View product