Baader Star Diagonal Prism w/ Zeiss Spec Prism & BBHS Coating - T-2

Who It Is For

This diagonal is especially suited to binoviewers with large prisms, and is the better choice over the 32 mm T-2 prisms for larger apertures such as the Mark V giant binoviewer. Compact T-2 prism diagonals also suit binoviewing in general, because their much shorter optical length often still allows focus when other mirror diagonals fail.

36 mm Zeiss-Norm Prism with Sealed BBHS Coating

The 36 mm BaK4 prism is made in accordance with Zeiss norms and has 1/10 lambda surface precision. Beyond the sturdier body and larger prism, its faces have about five times better planarity and angular accuracy than standard prisms in plastic housings.

In contrast to simple prisms, Baader's BBHS prisms carry an additional silver layer that is elaborately sealed against corrosion; this T-2 prism diagonal has always had such a sealed silver coating on its hypotenuse, and the result was noticed as better brilliance and richer colours. BBHS gives 98-99% reflection from slightly above 390nm to 2000nm and suits visual and photographic use, also in the infrared.

34 mm Clear Aperture and 38.5 mm Optical Length

Clear aperture is 34 mm, glass path 36 mm and optical length 38.5 mm. A further advantage of prisms is their much shorter optical length, which is why compact T-2 prism diagonals are recommended for binoviewers.

T-2 (M42 x 0.75) Threads on Both Sides

Instead of relying on 1.25" or 2" nosepieces, this diagonal features female T-2 (M42 x 0.75) threads on the telescope side and male T-2 threads on the eyepiece side. The eyepiece side also has an M34 inner connection.

Baader's T-2 star diagonals can be combined with the parts of the T-2 system in many ways. The most important adapters on the telescope side are the 1.25" to T-2 nosepiece (#2458105), which fits all 1.25" eyepiece clamps and has a 1.25" (M28.5) filter holder; the 2" to T-2 nosepiece (#2408150), which fits all 2" eyepiece clamps and has a 2" (M48) filter thread; and the 2"i/T-2a reducing ring (#2958244), which changes a 2" SC male thread into a T-2 male thread with just 1.5mm optical length.

On the eyepiece side, options include the ultrashort 1.25"/T-2 clamp (#2458121, 20mm optical length), the 1.25"/T-2 focusing eyepiece holder with 6.5mm focus travel (#2458125), the ClickLock 1.25"/T-2 eyepiece clamp (#2458100), the 2"a/T-2i expansion ring (#2958242), which changes a male T-2 thread to a male 2" SC thread, and the 2" ClickLock T-2 eyepiece clamp (#2956242). On both sides the Standard T-2 Changer System (#2456321) and the TQC/TCR Heavy Duty T-2 Quick Changing System (#2456322) allow compact connection and rotation of heavy accessories such as binoviewers.

• Telescope Side: Use a Baader T-2 to 1.25" (#2458105) or 2" (#2408150) nosepiece for standard focusers, or the 2"i/T-2a reducing ring (#2958244) on a 2" SC male thread.
• Eyepiece Side: Attach a T-2 eyepiece clamp like the Baader ClickLock 1.25" (#2458100) or the focusing eyepiece holder (#2458125).

Use with Binoviewers

This 36 mm prism diagonal is especially suited for binoviewers with large prisms, such as the Mark V giant binoviewer, while the 32 mm T-2 prisms suit smaller binoviewers up to the MaxBright II. The Standard T-2 Changer System (#2456321) and the TQC/TCR Heavy Duty T-2 Quick Changing System (#2456322) allow compact connection and rotation of heavy accessories such as binoviewers.

Housing and Weight

It has a CNC-milled aluminium housing and a net weight of 0.18 kg. The image is erect and mirror-inverted.

Prism or Mirror?

There is no general recommendation, but light passing through a prism is refracted, which behind a fast telescope can lead to colour aberrations; for fast telescopes designed for use without a prism a mirror is the better choice, with the limit somewhere around f/6 to f/7. Refraction in a prism also produces less stray light than reflection on a mirror, which is why many planetary observers prefer a prism, and a prism must adapt to the ambient temperature like the rest of the optics.