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/
We highly advise against visual calcium observations of the sun - the 397nm of Calcium are already ultraviolet radiation, and UV-radiation can cause phototoxic reactions which can harm your eyes. In addition, even young people can't see these wavelengths very good, and sensitivity decreases with the age. Because of this, the eye doesn't feel if the intensity is much too high. If you now try to increase the amount of radiation (e.g. with another mirror) to achieve a brighter image, you will increase the risk for eye damage. According to the newest norms for eye safety and to ophtalmologist conclusions, the sun must not be observed visualy at 397nm.
We highly advise to only observe the sun in calcium photographically, then you also do not need a star diagonal. We can't understand why some manufacturers promote these calcium-filters for visual observations.
We also advise against placing a filter so close to the focus - any energy rejection filter belongs in front of the front lens, even if this sets high demands to the quality of such a large filter. Placing the filter close to the focus will save money, because you can use a much smaller filter, be we absolutely can't recommend it - because of our own conscience as well as because of EU-regulations. The filter would receive all the energy collected by the front lens. We can only recommend putting the energy rejection in front of the front lens, which is the ethically and technically correct method.
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.
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|Optical Design||Mirror Diagonal|
|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|
|Image Orientation||Erect image, Mirror inverted|
|Optical length (mm)||112|
|Inner Diameter / Clear Aperture (mm)||47.5|