Any beam of light that will hit that uncoated surface under exactly 45° of incoming angle will undergo a "total reflection" (as soon as there would be a regular antireflection coating applied onto that reflective face, the performance would suffer greately). For ages Carl Zeiss had preferred the total reflection of prisms versus the reflective surfaces of mirrors, because until recently mirrors did tend to age (produce haze) very quickly.
For the BBHS T-2 prisms we needed perfect reflection also for very short focus telescopes and we wanted to produce a prism that could be used for a wide range of purposes - also for other reflective angles than exactly 45°. In this sense the BBHS-coating applied onto the T-2 prisms offers "another league" of functionality - and the hard silver coating will keep this optical surface to stay clean for ages, whereas the open glass surface of the 32 mm prism will be prone to collect some dust or even dew over time.
Still the 32mm prism is mounted within a precision metal housing, suitable to carry the weight of the Maxbright and heavier binocular viewers asf. It offers the very shortest light path for attaching - say - a Maxbright Bino onto any telescope due to its T-threads. In this way we regard it as a very good solution for any low-cost binocular viewer, since the optical polish of the prism faces is made very precise, with the extended optical distance in mind that adds itself into the light path when putting the added mechanical length of a binoviewer between the 90° prism and the eyepiece focal plane. Regular 90° prisms often feature a plastic housing and are only made to supply sufficient optical quality for an eyepiece to be mounted directly atop such a prism face.
Conclusion: each of these two prisms (32mm as well as BBHS T-2) are top performers within their respective price range. And if you plan to ever use a Mark V Binoviewer, the T-2 BBHS prism will be the first and most useful investment for an undeteriorated viewing experience.
Read also more on Baader BBHS Reflective Properties:
Manufacturer Baader Planetarium SKU (#) 2456005 Weight (kg) 0.16 Optical Design Star Diagonal Prism Inner Connection (lens sided) Thread, T-2 (M42 x 0,75) Inner Connection (eyepiece/-camera-sided) Thread, M34 Outer Connection (eyepiece/-camera-sided) Thread, T-2 (M42 x 0,75) Reflection surface Total reflection AR-Coating High transmission multi-coated (HT-MC) Image Orientation Erect image, Mirror inverted Optical tube length 35 Clamping System no clamp Deflection angle 90° Inner Diameter / Clear Aperture (mm) 32
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