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good test for diagonals?
I have acquired two 2" diagonals for use with my bare TMB: a cheap Synta
and a TeleVue Everbrite. So far, after about 8 hours of observation, I have been totally unable to tell them apart. What difference are you looking for? Optically, no real difference so far. Maximum magnitude is definitely the same. The light loss difference is not large enough for you to notice. The difference between 90% and 99% is not enough to see visually. That is not really the point of the Televue anyway. Andromeda and other DS objects look strikinly similar in my 30mm 2" Vixen EP. Bright stars seem to flare a bit less in the TV than in the Synta. Explain what you mean by "Flare"? If you mean coma, that is not the meaning of "flare". Astigmatism on the edge of the field is somewhat present in both cases albeit in a strangely different manner: the Synta shows astigmatism on the whole circumference while the TV only shows astigmatism on 1/3rd of the edge of the field at a time. Diagonals do not introduce astigmatism over only part of the field. Either all stars are astigmatic, in which case the diagonal is not truly flat, or all stars are round. Any astigmatism around the edge of the field is totally due to the eyepiece. Compare the view straight through with the same eyepiece and you will see the same astigmatism. basically my question is what should I look at to perceive the difference that must exist? The difference is that the coating on the Everbright is a dielectric which does not dim with age and is fully cleanable. This last feature is not a concern for casual users or for deep sky views, but becomes very critical when trying to eek out subtle detail on the planets. Diagonals get dirty in a hurry when used regularly, and this really kills planetary contrast. The dielectric coating can be cleaned repeatedly without introducing sleeks into the coating (which will happen with a normal aluminum diagonal). Sleeks, will of course permanently reduce the planetary contrast of your diagonal. Secundarily, is there any kind of objective test I can carry out? No. Roland Christen |
#2
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good test for diagonals?
In article ,
Pierre Vandevenne wrote: I am willing to admit that my eyes are not educated enough to see the difference therefore basically my question is what should I look at to perceive the difference that must exist? Secundarily, is there any kind of objective test I can carry out? You can test the collimation of the diagonal by putting in a recticle eyepiece, centering a star, turning the diagonal and seeing whether the star stays centered. You can do a star test with and without the diagonal in place. |
#3
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good test for diagonals?
Chris1011 wrote: .... The difference is that the coating on the Everbright is a dielectric which does not dim with age and is fully cleanable. .... The dielectric coating can be cleaned repeatedly without introducing sleeks into the coating (which will happen with a normal aluminum diagonal). Sleeks, will of course permanently reduce the planetary contrast of your diagonal. I have (reportedly 1/20th wave) Vernonscope enhanced silver diagonal which started to have tarnished spots on the perimeter. What can be done with it ? Who does enhanced silver coatings or can be the type of coating changed ? Thanks, David |
#4
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good test for diagonals?
Pierre,
Quoting from an earlier thread, TMBs are not designed for straight through viewing ;-) (not an attack against TMB, I am quite happy with its performance so far) I'm happy to hear that you are happy with your TMB telescope. But there is nothing that prevents any of our telescopes from being used for straight through viewing. All that is needed is to screw in another 100mm extension tube. They are available, along with 30mm and 60mm extension tubes, directly from TMB Optical. As for your diagonal problem, it sounds more like the eyepiece and or your eye's astigmatism at low power which is causing the problem. Try a higher power eyepiece, and see what happens. Thomas Back |
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