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Thread: Need help with eye piece

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    Default Need help with eye piece



    I purchased an Orion XT8 as my first telescope recently because I had a big amazon gift card given to me through a workplace award. I’m obviously very new to telescopes. I’ve been enjoying it as far as looking at the moon goes and can see the rings around Saturn. But my question is, what eye piece or eye pieces should I get to really be able to see the planets better and with more detail than just the orange dot I see when looking at Mars, for example?
    I have more amazon money to use but just don’t know what I should get. So I did a google search for astronomy forums hoping I could get some input from experienced users.

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    Default Re: Need help with eye piece

    Hello and welcome to the forums
    This may help out with choosing different focal lengths: Article: What Astronomy Eyepieces should I get for my Telescope?
    Mars as well as most of the planets are not well placed for good observing at the moment.
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    Default Re: Need help with eye piece

    Welcome to the forum! What EPs you already have?

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    Default Re: Need help with eye piece

    Welcome to the forums.

    I prefer to think about eyepieces in terms of exit pupil rather than magnification. The exit pupil is the diameter of the parallel beam of light that exits the eyepiece and enters the entrance pupil of your eye. Your eye when dark adapted can't dilate more that about 7mm of you are young and healthy, perhaps 5-6mm in middle and old age. So my wide angle limit for any telescope is always a 7mm exit pupil.

    Your scope is an 8" f6 scope. 1200mm focal length.

    EXIT PUPIL = EYEPIECE FL ÷ F stop
    or rearranging -
    EYEPIECE FL = F stop x EXIT PUPIL
    so on an f6 scope, a 42mm eyepiece will yield a 7mm exit pupil

    Also
    MAGNIFICATION = TELESCOPE FOCAL LENGTH ÷ EYEPIECE FOCAL LENGTH


    Highest magnification is a vexed question. It depends on atmospheric stability (called seeing), quality of your optics, thermal equilibrium of the optics and tube, and your eyes. I have a 6" f7 reflector designed for visual observing with excellent optics and an 8" f9 Vixen cass which is set up for photography (big diagonal) and not as good for visual as the 6".

    My 6" optics are good enough that I have used it with a 2mm eyepiece (525x) with razor sharp images. But here's the kicker, I've only ever experienced atmospheric conditions stable enough to do this a few times in 40 years. Twice where conditions were absolutely stable, other occasions where the image will shimmer or boil then stop momentarily.

    You could consider say a 3mm eyepiece, yielding a 0.5mm exit pupil, 400x as long as you have the funds to hold this eyepiece and only use it on rare occasions when atmosphere is stable, mirror is cool etc

    The shortest eyepiece I now own is a 5mm Baader Hyperion. I also have 7,12,17,22,31mm Naglers and barlow.

    With my 6" f7(1050mm focal length) newt, or my 8"f9 cassegrain (1800mm focal length), I mostly find myself using eyepieces in the 7-17mm range observing Moon and planets and only rarely using very short focal lengths.

    On my 18" f5.6 (2500mm focal length) I rarely use the 7mm and mostly use 17mm and the 12mm on a few occasions.

    When conditions stabilize, it's nice to have a short FL eyepiece at hand.

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    Default Re: Need help with eye piece

    My most used eyepiece is an Explore Scientific 18mm 82 degree. Great all-around. Don't know how much you'd like to spend, however. Here's a link:

    https://www.explorescientific.com/co...oducts/82-18mm
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    Default Re: Need help with eye piece

    Mars is only about 8 arcseconds in diameter now. And still going further away, so it is becoming smaller. That's one of the reasons why it is becoming harder and harder to observe.

    If you want Mars to see as big as the full Moon ( 1800 arcseconds) with the naked eye ( measured at arm length: 0.5 cm), you will have to magnify it 225X. Don't expect to see as much details as on the full Moon with the naked eye, since details on the Moon have far more contrast than those on Mars. As a matter of fact even for experienced observers it will be difficult to see one or two somewhat darker areas, that's all.

    You will need a 5mm eyepiece or a barlowed 10mm eyepiece for that in your telescope. Your cooled down and well collimated telescope can handle that, but the question is if the atmosphere can. Most of the time not. The last kilometers of light-travel in our earthly atmosphere is severely spoiled by it.

    You'll be surprised how small Mars will be, even with 225X magnification. You'd better get used to it, for the coming years it won't become any better.

    Many experienced observers prefer an even bigger image of a planet. 3 or even 4 times as big as the full Moon with the naked eye. A small calculation gives 675X on Mars at this time. Your telescope will not handle that much, contrast in details is lost and all you will see is a blurred orange. A planet like Jupiter for instance, which measures approximately 45 arcseconds, will need 40 times magnification to see it as big as the naked eye full Moon, and 160X to see it 4 times bigger than the naked eye full Moon. Much better.

    Surprise yourself: measure the full moon at armlength with a ruler....to your surprise it won't be bigger than 5 mm...

    If you have got a barlow: use it together with your 10mm eyepiece on Mars. The dissapointing image you will see has nothing to do with your telescope however, and everyting with the small diameter of Mars at the moment and the turbulent atmosphere above you.

    Good luck and clear skies!
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    Default Re: Need help with eye piece

    The planets are on the other side of the earth in the winter time here in the states, not much viewing. I also have the xt8, great scope. You might want to try a 15mm or 10mm eyepiece. Making sure your collimation is right on will give you great views. I normally use 2" eyepieces on my xt8, I get great views with these and the xt8 handles the 2" very well.
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