Wavefront error questionPosted By Alex McConahay |
No, you get no usable data from adding, subtracting, or conjuring on the wavefront errors of primary and secondary and eyepiece.
It is possible that one is out one way, and another is out another way, and they cancel out (That is why we were able to fix the Hubble--and why Meade and Celestron match corector plates to primaries when they assemble SCT's.). But it is most likely just a mess, with no matching. In which case, they probably multiply--not add. There is just no way to generalize.
Lord Raliegh says that most of the energy of a pinpoint source of light is within the first diffraction ring, and to oversimplify, the Raliegh criterion says that if an optic is within one eighth of a vavelength of correct, you cannot tell the difference between it and perfect because it has that much amoung of light--or nearly that--in the first diffraction ring. At the eyepeice tests generally prove this true, although an experienced observer can in fact sense some difference.
Light, as you know, does not travel in perfectly straight lines, but "diffracts" around edges and in other ways bends. In short--nothing is ever perfect since diffraction will limit you. WHen there is no difference between as good as it gets with diffraction, and what your optics systems do, then you have "diffraction limited" optics.
And usually, when somebody quotes the wave rating of an optic, they are quoting it for the primary optic, not at the eyepiece--although theoretically, there is nothing that says they can't quote it for the eyepiece. (I am not familiar with the test that could render this number for you, though.)
There is a lot more to say on this subject (Such as what type of "Wave rating" ? Peak to Valley, Deviation from mean? What?
Alex
It is possible that one is out one way, and another is out another way, and they cancel out (That is why we were able to fix the Hubble--and why Meade and Celestron match corector plates to primaries when they assemble SCT's.). But it is most likely just a mess, with no matching. In which case, they probably multiply--not add. There is just no way to generalize.
Lord Raliegh says that most of the energy of a pinpoint source of light is within the first diffraction ring, and to oversimplify, the Raliegh criterion says that if an optic is within one eighth of a vavelength of correct, you cannot tell the difference between it and perfect because it has that much amoung of light--or nearly that--in the first diffraction ring. At the eyepeice tests generally prove this true, although an experienced observer can in fact sense some difference.
Light, as you know, does not travel in perfectly straight lines, but "diffracts" around edges and in other ways bends. In short--nothing is ever perfect since diffraction will limit you. WHen there is no difference between as good as it gets with diffraction, and what your optics systems do, then you have "diffraction limited" optics.
And usually, when somebody quotes the wave rating of an optic, they are quoting it for the primary optic, not at the eyepiece--although theoretically, there is nothing that says they can't quote it for the eyepiece. (I am not familiar with the test that could render this number for you, though.)
There is a lot more to say on this subject (Such as what type of "Wave rating" ? Peak to Valley, Deviation from mean? What?
Alex
