Baader BBHS Prism Adjustable Star Diagonal - 2"

BBHS Silver Coating: 98-99% Reflection from 390nm to 2000nm

In contrast to simple prisms, Baader prisms carry an additional BBHS silver layer that is elaborately sealed against corrosion; Baader has long used sealed silver on its prism diagonals. BBHS stands for Broad Band Hard Silver: the silver layer is stabilised by a hard dielectric protective coating. Reflection is 98-99% from slightly above 390nm up to 2000nm, with no drop in reflectivity at different viewing angles and no stray light from the silver coating; the trained eye sees more colours, especially with short focal length APO refractors, and a conspicuous colour intensification on planets at the highest magnification.

Besides the sealed BBHS rear-surface coating, the prism has Baader's Phantom Coating Group anti-reflection treatment. Baader adds that the CNC-milled aluminium housing gives fast temperature adaptation and image stability.

2-inch ClickLock Clamp: A 20° Twist

Baader's 2" ClickLock clamp has no clamping screws: a 20° turn of the rotary lever with light pressure holds 2" accessories securely. Like a quick-action chuck, it holds the inserted item on three sides, and leverage multiplies the torque; a ratchet lets you hear and feel whether it is open or closed, and the massive tension ring holds accessories without scratches. Every eyepiece is aligned parallel to the axis automatically, even with diameter deviations.

47mm Clear Aperture and 100mm Optical Length

The clear aperture is 47mm and the glass path through the prism is 50mm.

The optical length is 100mm. The BBHS prisms can also be screwed directly onto a focuser or accessories for a tilt-proof connection with a short optical path.

Native S58, M55, and 2"/SCT Thread Connections

Unscrewing the 2" nosepiece gives access to a 2" SC thread for direct connection to a Schmidt-Cassegrain and an S58 ring dovetail. On a Schmidt-Cassegrain the ClickLock usually has to come off first (loosen its six small screws) so it does not hit the focusing knob; the separately available 2" Locking Ring (#2458270) can fix the diagonal in any rotational position. The S58 dovetail allows direct connection to Baader Diamond Steeltrack focusers in place of their 2" eyepiece clamp, which extends the available backfocus for long accessories such as binoviewers.

The telescope-side inner connections are M48, a 2"/SCT thread and M55, with a 2" bore and M55 on the eyepiece side.

M68 Conversion

Baader's M55a/M68i adapter (#2458234) can give the diagonal a female M68 thread on the eyepiece side, so the ClickLock can be replaced by an M68 quick changer, and the M55a/M68a adapter (#2956110) converts the 2" BBHS diagonals to the Zeiss system with an M68 male thread on the telescope side. Exposing the M55 thread requires the adjustable face spanner (#2450062).

Adjustable Prism Cradle

The BaK4 prism is mounted in a CNC-machined aluminium housing and rests stress-free on an adjustable cradle, so, like a refractor objective, it can be collimated. Baader sets the 3-point alignment at the factory so that the image shows no noticeable rotation around its centre when the diagonal is rotated by 180° in a correctly aligned eyepiece clamp, and you will not need to change it in normal use.

It is intended for fully adjustable systems such as binocular telescopes. Only perform this adjustment after all other potential sources of misalignment have been ruled out. Please note that user adjustment voids the warranty on the factory collimation, though all other warranty coverage remains unaffected.

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. As a general guideline, use prisms for telescopes of about f/6 to f/7 or slower, and BBHS mirrors for faster systems.