Any reflecting telescope with classic coatings can attain a lifetime of 20 or 30 years - correct handling provided. The same accounts for our protected silver-coatings, whereas the position of a stardiagonal within the optical train does provide much better protection against environmental influences, compared to the exposed surface of a primary telescope mirror.
We do regard the benefit of the BBHS hardsilver coating to be quite noticeable, compared to the reduced spectral range offered by a dielectric coating. Please check in the Internet for statements that would verify our position. Or look for the very extensive tests on star diagonals, performed by William Paolini (see tab "Downloads".
Please read more on the properties of our star diagonals and our design philosophy here: http://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/
2. Baader # 2456130 - T-2 / 90° Baader Roof-Prism with BBHS coating (https://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-t-2--90%C2%B0-baader-roof-prism-with-bbhs-r-coating-(t-2-part-02).html), or
3. Baader # 2456095 - T-2 Stardiagonal (Zeiss) Prism with BBHS coating (https://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-t-2-stardiagonal-(zeiss)-prism-with-bbhs-r-coating-(t-2-part-01b).html)
2) The roof prism will give you an erect, upright image suitable for terrestrial observations. It is one of the very few of its kind which also work at high magnifications. But there is alwaysa slight chance for reflections at the "edge" of the roof because of the optical design of these prisms.
3) The prism is similar in quality to the mirror, but there may be some colour aberrations in fast telescopes because the light passes through the prism. On the other hand, some lens telescopes were especially designed to work with prisms and delivered best results with prisms. Another benefit of prisms is that they don't require as much back focus as a mirror, that's why we recommend them e.g. for binoviewers.
For a comparison between mirror and prism, you may also be interested in the comparison at https://www.baader-planetarium.com/de/downloads/dl/file/id/160/product/1244/testreview_mirror_vs_dielectric_vs_prism_diagonal_comparison_w_paolini.pdf
Or, can you please describe to me where the optical path begins and ends on the diagonal body?
I need this information so I can accurately determine and measure backfocus distance.
The optical length is not always identical to the mechanical length, because the lenses in some parts change the focus point. It gives always the movement of the focus point when the part is used as intended.
as well as our post on BBHS reflective properties:
Are the threads (standard) for say a 2inch telescope eyepiece that could directly attach to this unit from the bottom barrel of it? Yes or no? .if not what is the diameter/threading of a T-2 fitting?
- Related Articles
|Optical Design||Mirror Diagonal|
|Outer Connection (lens sided)||Thread, T-2 (M42 x 0,75)|
|Inner Connection (eyepiece/-camera-sided)||Thread, T-2 (M42 x 0,75)|
|Reflection surface||BBHS coated, Dielectrically protected|
|Image Orientation||Erect image, Mirror inverted|
|Optical length (mm)||43|
|Clamping System||no clamp|
|Inner Diameter / Clear Aperture (mm)||33|