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  1. #1
    Original Mike's Avatar
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    Default How big do the planets look?



    I did this calculation and I want to run it by people who, you know, actually have a telescope () as a sanity check.

    The RASC handbook says Saturn right now has an angular size of 17 arc-seconds. If I view Saturn at 250x magnification, then the image of Saturn is 17" x 250 = 4250" = 1.2-degrees. If my eyepiece has an apparent field-of-view of 60-degrees, then Saturn fills only 2% (1.2/60) of the field-of-view (by diameter). Is this in the ball park of what you see when observing the planets?
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  2. #2
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    Yes.

    Yes, it is. In my ETX125, my 12mm astrometric eyepiece gives me a magnification of 158 diameters, has an AFOV of 40 degrees, and a TFOV of about 0.25 degrees, or 15 arc-minutes. The planet appears about the diameter of one of the graticle divisions in this eyepiece, and there are 50 divisions on the scale and some space at either end... call it 60 divisions total across...so a 17 arc-second diameter would be a bit over one division.

    This picture shows the reticle, but the star is just a star. Sorry...it's too late for Saturn, but I'll repeat the exercise for you and get an image of Jupiter under the same magnification and we can check your math again. But I think you did it right, as it accords with my observations.


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    ETX125AT guiding WO Megrez 90 Apo/CGEM mount w/ polar scope
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  4. #3
    Original Mike's Avatar
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    MUCH appreciated, alsetalokin. Jupiter right now has an angular size of 44 minutes. At 158x magnification this gives an image of 1.93-degrees. With an APOV of 40-degrees, this fills 1.93/40 = 4.8% of the FOV. If the 40-degrees is spread over 60 divisions, this should yield a size of almost 3 divisions (2.9). Jupiter is almost 3 times bigger than Saturn (2.6 actually) right now.

    The upshot of this is that for planetary viewing, it seems advantageous to have a smaller FOV eyepiece. I know it doesn't change the size of the image of the planet, but it might improve the WOW factor if you're filling more of the FOV. And in any case, there's no reason to pay for a wide FOV eyepiece.

    Is there any situation where you want a combination of high mag and wide AFOV?
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    alsetalokin's Avatar
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    Yes, absolutely. You want as wide a range and as many combinations of magnification and AFOV as you can afford. The objects you are looking at range from tiny dim things like the Ring Nebula in Lyra (1 arc minute diameter) to the Andromeda galaxy, which is fully three degrees or more in angular extent. And you are going to be looking at dim objects, bright objects, objects with very small angular separation but larger features as well....Epsilon Lyrae 1 and 2 for example...at Luna...maybe even at old Sol when he's feeling ornery. You need a wide AFOV and low mag for the Pleiades and Andromeda, a wide AFOV and high mag for ELyrae, a narrow FOV and high mag for Jupiter, a narrow FOV and low mag for detail in some nebulae....
    And so on.

    (In practice this often equates to a couple of telescopes, rather than a bag full of eyepieces and Barlows. Between the Megrez 90 and the ETX125, with my small assortment of EPs -- 5 and a Barlow --, I cover a pretty wide range in the FOV/Mag space.)
    Last edited by alsetalokin; 07-25-2010 at 08:42 PM.

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    Default

    It's always nice when the math works out.


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  8. #6
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    Quote Originally Posted by alsetalokin View Post
    It's always nice when the math works out.
    If it doesn't work out, you keep banging on it until it does.

    When I first did this little calculation, the answer seemed small compared to this demonstration. It appears, however nice the demo is at showing relative differences, the absolute size of the object in the eyepiece is exaggerated.

    Thanks much for this, alsetalokin. Appreciate it.
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    Default

    Here is a non scientific opinion: Most of the planets are not very interesting in a typical amateur telescope. Saturn and Jupiter are the best to look at. most of the others will just be a disc of colored light. Sometimes Mars shows the polar ice caps.
    There are limits to the magnification you can use. The size and quality of the telescope is one but freqently the viewing conditions (light polution, atmospheric disturbance) are the limit. In my area (Ohio) 150 to 200 is typical, Sometimes 250x. Don T.

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    You can use Stellarium to estmate the size too...
    Enjoy the Black cause they can't take it back............
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    In a *good* amateur telescope however, Saturn / Jupiter can be very interesting.

    As stated above, the image comes pre-blurred by the atmosphere. What this means is that a great telescope will manage about 220x in average conditions, a lesser telescope more like 160x

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    You will NOT see those BIG PICTURES of planets that you see in magazines. Those are either taken by an amateur using a camera and stacking hundreds of pictures to build up the image--- or by professional astronomers using LARGE professional telescopes.

    However in a good scope with good optics the planets will be visible and some surface details can be seen. Atmospheric turbulence plays a big part in properly "seeing" the planets. The best two are Jupiter and Saturn, Mars is usually very very tiny and TOUGH to see detail and Venus is just a white disk having different phases like the moon.

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