Baader Planetarium T2 star diagonal
Looking for a reliable and optically precise 90° diagonal for your telescope with a T2 interface? The Baader Planetarium T2 star diagonal is a solution that will be appreciated especially by more demanding observers - whether you focus on visual observation of planets, the Moon or double stars. The combination of outstanding optical quality, premium surface treatment and a compact T2 interface makes it one of the most sought-after diagonals in its class.
BBHS coating - up to 99% reflectivity
The prism features BBHS (Baader Broadband High-Speed) surface treatment, offering reflectivity of up to 99% - a value comparable to the best dielectric coatings on the market. Unlike standard semi-transparent or aluminium mirrors, the BBHS coating provides significantly higher contrast and more accurate colour rendition. This is particularly important when observing fine planetary details or in astrophotography where faithful image reproduction is critical. The optical precision of the prism reaches lambda/10 and higher according to Zeiss standards, ensuring minimal aberrations and a sharp image all the way to the edge of the field of view.
Clear aperture and optical path length
The clear aperture of 35 mm (with an effective light beam transfer diameter of 34 mm) ensures that the prism will not become a limiting element even in refractors or Cassegrains with a moderately large light cone output. The optical path length of 38.5 mm is compact enough not to interfere with focusing on most telescopes equipped with a T2 adapter.
T2 interface and compatibility
The prism is fitted with T2i (input, internal thread M42x0.75) and T2a (output, external thread M42x0.75) threads, allowing direct connection to telescopes and eyepiece rotators with the T2 system without the need for additional adapters. This standardised interface is the foundation of the Baader Planetarium ecosystem and guarantees seamless compatibility with a wide range of accessories - from eyepiece rotators and filter sliders to camera adapters. The prism body is manufactured in a black finish, resistant to reflections inside the optical path.
Image orientation
The prism corrects vertical image inversion (the image is erect), while horizontal mirroring remains present - standard behaviour for a diagonal prism of this type. This is fully satisfactory for astronomical visual observation, where image orientation plays no practical role.
Who is this diagonal ideal for?
The Baader Planetarium T2 star diagonal is an ideal choice for owners of refractors, apochromats or Cassegrains with a T2 output, who are looking for an optically uncompromising solution for visual observation or astrophotography. This premium 90° diagonal with BBHS coating will be especially appreciated by those who demand maximum reflectivity and precision without compromise - whether for planetary observation, double stars or demanding lunar photography. Its compact dimensions and standard T2 interface make it a versatile component for any Baader Planetarium setup.
Technical specifications
| Parameter |
Value |
| Diagonal type |
90° prism |
| Surface coating |
BBHS (Baader Broadband High-Speed) |
| Reflectivity |
up to 99% |
| Optical precision |
lambda/10 (Zeiss standard) |
| Clear aperture |
35 mm (light beam diameter 34 mm) |
| Optical path length |
38.5 mm |
| Input thread (T2i) |
M42x0.75 internal |
| Output thread (T2a) |
M42x0.75 external |
| Body finish |
black, anti-reflection |
| Image correction |
vertically erect, horizontally mirrored |
About Baader Planetarium
Baader Planetarium is a German manufacturer of astronomical optics and accessories with decades of experience in the field. The brand's products are designed with emphasis on optical precision, mechanical reliability and long service life - and are popular with both amateur astronomers and professional observatories around the world. The T2 system developed by Baader has become a de facto industry standard for modular telescope accessory setups, ensuring high compatibility and flexibility when building your own observation system. By choosing Baader Planetarium products, you are investing in quality that lasts and that will not let you down even on the most demanding nights under the starry sky.
