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Resolution, magnification and seeing.

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#26 Jon Isaacs

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Posted 02 February 2013 - 03:35 PM

3) I have eyepieces at 59X and 87X. I find the 4 stars easily split at 87X, which goes along with the naked eye test. I find the stars also split at 59X, which means that, theoretically, I can resolve objects at a naked eye separation of 2.26'.

I normally can convince myself that I see the split at 60x, there are nights when it has seemed possible at even lower magnifications. But it is definitely a clear split with space between the pairs at 90x and wide open at say 120x.

But this using a scope that itself is not challenged by the split, the pairs are widely separated at the focal plane and in the exit pupil. When one is working at the Dawes limit, double stars are very different objects..

By the way, the Dawes limit for a 5mm aperture is about 23 arc-seconds... According to Skytools 3, the separation of the pairs is 3.5 arc-minutes. This means that for a 5mm aperture optic, i.e. the eye, the epsilon 1 and epsilon 2 are approximately 10 times the Dawes limit, an easy split.

With "easy" splits like the double-double in a 3 or 4 inch scope, relatively low magnifications can be used. But the 2.3 arc-second separation of the pairs is the Dawes limit for a 50mmm scope, to resolve it and see that thin dark line, it takes considerably more than 100x. This is because a Dawes limit split is not well resolved at the focal plane or in the exit pupil.


#27 azure1961p


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Posted 02 February 2013 - 06:49 PM

Given say 120x in two scopes of apertures you mentioned, the larger telescope will provide more detail so long as it isn't overly bright. Irradiation caused by the greater aperture can have those lower power views seem less detailed if only because so much light is crammed into a small area. A neutral density filter does wonders here for low power bright objects through large aperture. The full thrust if the benefits of large aperture higher resolution are typically appreciated at 30x or more.

The low power full resolution magnification needs some clarifiers...

13x per inch is said to have all that can be seen in terms of resolution and its wonderful theory but higher magnification makes it so much better. Doublestars are a great example. Sure there are double stars I can barely detect at 120x as double but enough to just make the call but detecting is one thing, fruitfully observing is another. You don't want to JUST make out a detail ( tho as u kno at times its all we can do) when you can see the SAME detail PLAINER.

There's a whole lot to be said for the comfort and enjoyment of the plainer image had at higher magnifications. A trend among observers often is to see how little magnification is needed to make out a detail. I rarely bother with that. Instead I prefer to find what which makes it most apparent.

Like music -you don't want to just hear it - you want to listen.


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