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Thread: Highest useful magnification

  1. #41
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    Default Re: Highest useful magnification



    Quote Originally Posted by Galileo3269 View Post
    Hello Skaven, Once again you amaze me at your vast wisdom. If i was half as smart as you i could have made something out of myself instead of a hat rack. So, if i get an ep that the scope calculator says is the best for my particular scope and if i,m still not happy with wat i,m seeing, what should i do then? Buy a bigger scope? The best detailed clearest views i,ve had so far has been with the "Explorer 2" 25mm ep. But, it,s the size of a pen head!!! Is it even possible to get a descent sized (detailed) view of Saturn with a 5in scope?? Thanks for your replies.

    Steve
    One thing that takes some time to get used to is the paradoxical "shrinking" effect that looking through a telescope eyepiece has. Consider what magnification is doing -- it's making the apparent size of an object larger, at the scale of the magnification factor. So, for example, if you see a boulder on the side of a mountain that appears, with the unaided eye, to be about the size of your thumbnail at arm's length, then looking through a scope providing 2x magnification would make that object appear to be twice that large.

    So let's take the full moon as a useful target for observability. Clearly you're able to see lots of interesting detail in the moon with the naked eye. The full moon is 30 arcminutes (or 1800 arcsec) in diameter. The disk of Saturn (sans rings) is about 18 arcsec. So in order to get the disc of Saturn to appear the same size as the full moon, you need to magnify it by 1800/18 = 100x. With your 5" f/7 scope (900mm focal length), you could use a 900/100 = 9mm eyepiece to achieve this level of magnification.

    HOWEVER -- when you do this, you'll look in the eypiece and think, "geez--- it's still so tiny! Just a pinhead!" But...it's not! You just have to learn to see...which takes patience and practice.

    Here's a fun experiment that demonstrates how misleading apparent sizes can be.

    Take a regular pencil (one with a pink eraser at the top) and hold it out at arm's length. Look at the size of the eraser. Hold it up next to other objects around you (particularly objects in the distance) and get a feel for how "big" that eraser is, when held at arm's length. Pretty small, right? Now, with the Moon out, do the same thing, and compare the size of that eraser head to the full moon. Whoah! They're pretty much the same size! In fact, the moon is slightly *smaller* than the size of that eraser.

    THAT was when it clicked for me just how much your brain tricks you into thinking something is "big" or "small", and there's no frame of reference around it to give your brain some clues as to how big it is.

    That's exactly what is happening when you observe something small through a telescope -- your brain, with nothing but the field stop to judge the size of what you're seeing, makes an assumption that what you're seeing is really really small. But it isn't! Practice ignoring the field stop, and concentrate on focusing intently on the planet's disc. Try very hard to ignore all the area around the planet, and after a while you'll find that you start seeing detail and texture you were missing before.

    That said, obviously piling on more magnification (within the limits of the scope) helps quite a bit! With your 5" scope, going to its day-to-day practical limit of 1mm exit pupil (7mm EP) gives you 128x magnification, which makes the disc of Saturn appear 18*128=~2300 arcsec wide, or 27% larger than the full moon. Add in the rings (which make Saturn's full diameter around 40 arcsec) and at 128x power you've got 40*128=5120 arcsec, or 2.8x larger than the full moon!

    You'll be very pleased with the views you get from Saturn with that 5" scope -- just give yourself some time to practice. Stable air and tight collimation help immensely as well. Remember that ultimately, the object you're observing is a tiny point of light in the sky. So any atmospheric turbulence has a much more pronounced effect on its image in the eyepiece than it does to larger objects like the Moon.
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  3. #42
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    Default Re: Highest useful magnification

    Hello Skaven,

    thanks for the comparison of the sizes, it is very helpful for the understanding,

    JG
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  5. #43
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    Default Re: Highest useful magnification

    Yeah that info is awesome, I have also found a really neat trick you can try so to how large objects in the scope actually are. This works better if you have a wider field ep but its not too hard with a plossl.

    Depending on the scope you are using. You can keep your other eye open which gives you one eye of scope view and the other of the outside. Sometimes you can extend your arm out and hold up a finger tip and have that image cross over into your scope view with your other eye...so you can sort of compare your finger tip to magnified objects in the scope and that really makes you realize the size of the objects viewed at different mags. I hope that made sense...
    Brett

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  7. #44
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    Default Re: Highest useful magnification

    Quote Originally Posted by brettecantwell View Post
    Yeah that info is awesome, I have also found a really neat trick you can try so to how large objects in the scope actually are. This works better if you have a wider field ep but its not too hard with a plossl.

    Depending on the scope you are using. You can keep your other eye open which gives you one eye of scope view and the other of the outside. Sometimes you can extend your arm out and hold up a finger tip and have that image cross over into your scope view with your other eye...so you can sort of compare your finger tip to magnified objects in the scope and that really makes you realize the size of the objects viewed at different mags. I hope that made sense...
    Great idea! I'm going to try that the next time I've got the scope out.

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    Default Re: Highest useful magnification

    Welcome to the forum.
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    Default Re: Highest useful magnification

    My scope is an f7.8 scope and I use a 5.1mm regularly and it works well. I used it tonight in fact, to look at Saturn (again).

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  11. #47
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    Default Re: Highest useful magnification

    Quote Originally Posted by skaven View Post
    Great idea! I'm going to try that the next time I've got the scope out.
    Yeah let me know how that works for you, it works for me real good. I was doing the math over and over and the objects still looked too small like you were saying earlier until i discovered that and then i was like "oh i guess the math was right" lol
    Brett

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