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  1. #1
    wallcreeper's Avatar
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    Default resolving power and light gathering power questions



    Hi,
    I have some more newbie questions.
    1) I was comparing 6 and 8 inch reflecting telescopes and noticed that their listed resolving power was below 1 arcsecond for both. I also read that due to our atmosphere it is virtually impossible to get anything more than 1 arcsecond resolution no matter what your scope is. So, does it not make a difference whether you get a 6 or 8 inch telescope, if you are looking at something sufficiently bright like the moon or a planet?
    2) I understand that greater aperture means more light gathering ability. I also saw a table in 'The backyard astronomer's guide' that says that a 6 inch telescope can see down to 13.6 stellar magnitude and a 8 inch telescope can see down to 14.2 stellar magnitude. Is that a noticeable difference?
    Thanks!
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  2. #2
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    Default

    Most specifications written by the manufacture for telescopes were created under "laboratory" conditions. They are basically a "relative spec". As you stated greater aperture means more light gathering ability which will allow you to observe more detail in fainter objects such as nebula or galaxies. When observing a object like the moon there would not be any significant difference in what you can see. With further planets such Neptune there would still not be any significant difference between a 6 or 8 inch aperture.

    The biggest factor in the ability to "see" faint objects is your "seeing conditions" such as light pollution, atmospheric conditions. So basically to answer your questions. Yes the larger aperture is better, but would you notice a difference? It depends on what your looking at and what the conditions are on a given night.
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  3. #3
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    Default

    Hi there wallcreeper....

    On the point of comaparative light gathering, and magnitudes.....
    When comparing scope sizes, square the aperture, to give a direct comparison of light-gathering ability.
    So - 6x6=36, and 8x8=64 - which means that a 8" scope will gather 64/36 times as much light as a 6" - or about 1.8 times as much light.

    An increase of ONE magnitude of star brightness, equates to about 2½ times as bright.
    The difference between mag 13.6, and 14.2 as almost 2/3 of that figure of 2½
    Which pretty much agrees with the 1.8x difference between a 6" and 8"

    In practical terms - almost double the light-gathering can mean the difference between seeing no more than a fuzzy blob, and being able to see a bit of spiral structure, in galaxies such as m51, m81, etc...

    I can also attest that the difference in resolving power means that you can see the 'speckled' structure of globular clusters such as M13, with an 8", whereas a 6" will only show a few of the more prominent stars, on a backgtround 'haze'
    It's true that atmospheric conditions mean it's rare to get beyond a 1 arcsec resolution, but almost always the 'seeing' conditions swim in and out, giving glimpses of much better resolution - if you have the optics to take advantage.

    With an 8" you'll see better surface detail on planets, 'deeper' deep sky views, and will be able to split tighter double-stars.
    The jump from 6" to 8" is definitely worth it.
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  4. #4
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    Default

    Thanks for the feedback! This is very helpful.
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    I jumped from a 3" reflector to a nexstar 5se and the difference was amazing, and recently went up to a 10" lightbridge, now that was an eye opener (no pun intended) even from my light polluted area the difference was even more amazing.
    Just had to learn to let it cool down and collimate it as at first Mizar was bearly split and i was gatting frustrated not beiny able to split Epsilon Lyra as a quad. Once i realised it had cooled i collimated it and the views were much better. Stll haven't split Epsilon Lyra as a quad though, i'm sure i split it with my nexstar and i've read articles where they've split it with an 80mm and 90mm refractor. Not sure whats going on there.


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    Default

    Great answers. I'll just add that a part of the resolving part has to do with the figure of the optics, and contrast is affected to some degree by the size of the central obstruction. Resolution is mainly all in the apeture. Agree that these 'test bench' numbers are to get market share and not necessarily good overall quality numbers for a scope... what is really importnt with these is the quality of the mirror figure.

    After that, the focal ratio (f3.x... f8 or whatever) of the scope detemines what size of space the scope can take in... higher fl narrower, shorter fl larger. Generally longer fl better on planets in my experience. So not all apetures are created equal.

    The longer the focal length the higher power or magnification you will get for a given ep size, and narrower field of view, and with the reflectors I'd owned in the past(10+ years ago), the longer the focal length - the flatter the field.

    Think of it all as working hand in glove. From what I have seen there is a tendancy towards big mirrors with fast focal ratios these days to make larger apeture portable and take in a wider piece of the sky - but the 'faster' the scope (large apeture with short focal length comparatively) the more inherent curvature there is in the field of view and if going for that wider field with a wide field eyepiece the curvature (and associated problems) of the optics mean spending some $$ on ep's that don't show the problems or getting a Paracorr (see Mark M review here) or MPCC or something to try and clean it up at the eyepiece.

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