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OK no gimmick the y do make a difference, you think that if all the surfaces within your scope were optically that efficient that you would gain nearly 10-20% more light at the eyepiece depending on conditions. I used the original Meade 2" diagonal and a WO 99% one and showed 3 people at duckadang last year with the same object (a galaxy - Spanish dancing one (not Tuesday nights on Prime)) and they picked the Wo one as the brightest, I then told them. the other diagonal was the 97% reflectivity one.
JM2CW
oh forgot to say that at each optical surface there is light loss due to interference and loss within the medium, and dispersion, but some smart cookie will have the actual facts somewhere
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