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/
the diagonal onto the OTA: https://www.baader-planetarium.com/en/baader-2%22-nexstar-locking-ring..html
Baader 2" BBHS ® Mirror Diagonal with 2" ClickLock Clamp
Baader 2" BBHS ® Prism Star Diagonal Prism with 2" ClickLock Clamp
And yet, the mass of them is indicated the same, is there any error here? The prism should be heavier? Thank you for your responses!
Usually prism diagonals are better suited for long focal length refractors and mirror diagonals for short focal length mirror telescopes.
This is because the total reflection with prisms is suboptimal when using short focal lengths due to nonlinear light reduction with high angle side light rays and chromatic prism effects. Also there are shifts in backfocus and it can influence the correction of high quality optics due to the glasspath (the speed of light is lower inside a dense medium like glass in comparison to air).
On the other side mirrors don't have the same maximum reflection values like a prism's lossless physical total reflection. Also a mirror is much more sensitive to transmission reduction when exposed to the air (sensitive silver layer getting weak very soon).
But with the Baader BBHS self protecting silver reflection layer on both diagonals (mirror and prism) the backsides of both principles are well compensated, this means the mirror diagonal will not loose any light due to aging anymore and the general light angle sensitivity of the prism diagonal due to reliance on total reflection alone is compensated by the additional BBHS layer below the total reflection hypotenuse area.
So the decision between diagonal mirror or prism is not as critical as it was until now.
But still a short focal length instrument favours a mirror diagonal.
You could have the eyepiece sleeve plus mounted filter reach up to 5 mm into the housing without touching the mirror. We did allow this to happen in order to provide maximum depth for eyepieces.
Or better: a Self Centering Adapter version ClickLock-QuickChanger. 2" and T2.
You might order the T-2 Astro-System adapter #27 (2" to T-2): http://www.baader-planetarium.com/en/baader-adapter-2%22-(male)--t-2-(male).html
Remove the clicklock clamp by opening the 6 pc of headless set screws and thread the said adapter into the 2" thread inside the dovetail ring that did hold the clicklock in place. Then thread on the T-2 changer that came with the bino.
This will be the absolutely shortest solution to attach the Bino onto any 2" Baader-Diagonal.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glaspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error.
- Related Articles
|Optical Design||Star Diagonal Prism|
|Inner Connection (lens sided)||Thread, M48, M55, 2" (50,8mm)|
|Outer Connection (lens sided)||Barrel, Dovetail ring, S58, 2" (50,8mm)|
|Inner Connection (eyepiece/-camera-sided)||Thread, Clamp, M55, 2" (50,8mm)|
|Reflection surface||BBHS coated, Dielectrically protected|
|AR-Coating||Phantom Coating® Group|
|Image Orientation||Erect image, Mirror inverted|
|Optical length (mm)||100|
|Inner Diameter / Clear Aperture (mm)||47.5|