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  1. #1
    D. Buck's Avatar
    D. Buck Guest

    Default 25mm eye piece vs 10mm eyepiece??

    Could someone explain something to me, I'm pretty clueless when it
    comes to telescopes and eyepieces? I have a Celestron 114mm telescope
    with two eyepieces, a 25mm SMA 1 1/4" and a 10mm SMA 1 1/4". They are
    what came with it. Why is it, things are larger with the 10mm than
    with the 25mm?

    Also, I tried to look at Mars the other night and all I saw was an
    orange dot (Iowa, USA 11:30pm 8/23/03). What can I expect to be able
    to see with a 114mm telescope. Can I buy an eye piece that will allow
    me to see more detail, or is my telescope just too small?



  2. #2
    D. Buck's Avatar
    D. Buck Guest

    Default 25mm eye piece vs 10mm eyepiece??

    Thank you very much for the info.


  3. #3
    SPQR's Avatar
    SPQR Guest

    Default 25mm eye piece vs 10mm eyepiece?? (D. Buck) wrote in message news:< com>...

    The little orange dot is Mars. It looks bigger in the 10mm eyepiece
    than in the 25mm eyepiece because the 10mm eyepiece gives your more
    magnification. Shorter eyepiece = greater magnification.

    Do a bit of math.

    Your 114mm scope has an aperture of 114mm, or, approx 4.2 inches.
    Rule of thumb is that the maximum magnification you can expect from
    your scope ranges from 50-60 times the aperture. Thus, your maximum
    magnification will be on the order of 4.2 x 50 = 210X to 4.2 x 60 =

    Consult your owner's manual and find the "focal length" of you scope.
    I found the Celestron 114 on a web site and the focal length is listed
    as 900mm.

    Magnification equals scope focal length divided by eyepiece focal

    Thus, if your scope has a focal length of 900mm, and you use a 25mm
    eyepiece, your magnification is 900/25 = 36X. With the 10mm eyepiece,
    your magnification is 900/10 = 90X. Thus, Mars will be 2.5 times as
    big in your 10mm eyepiece as in your 25mm eyepiece.

    Assuming, then, that your maximum magnification is around 225X, then a
    4mm eyepiece will get you to your maximum magnification.

    However, it may not be the smartest thing to do to purchase a 4mm
    eyepiece because all it will do is give you your maximum magnification
    -- and when you encounter poor atmospheric conditions, you may not
    want max mag. Instead, get an 8mm eyepiece and a 2X Barlow. The
    Barlow multiplies the power of each eyepiece by a factor of 2. Thus,
    a 25mm eyepiece with 36X, when coupled to a Barlow acts like a 13mm
    eyepiece at 72X. An 8mm eyepiece will give you 900/8 = 112.5X; with a
    2X Barlow, the same eyepiece will yield 225X -- about the most you
    will get from your scope.

    The BArlow effectively doubles the number of eyepieces you have. Your
    eyepieces with a 2X Barlow will give:

    25mm = 36X; with Barlow = 72X
    10mm = 90X; with Barlow = 180X
    And if you add an 8mm, you will get:
    8mm = 112.5X; with Barlow = 225X

    So -- with 25, 10, and 8 eyepieces and a 2X Barlow you will have
    magnifications of 36X, 72X, 90X, 112.5X, 180X, and 225X.

    I use an 8-inch Dob with 1200mm focal length. My 7mm eyepiece yields
    171X, with a Barlow I get 340X from the same eyepiece. Under normal
    atmspheric conditions, 340X is pushing it for Mars where I live, which
    is a high-humidity area and if I try to view Mars at much above 250X,
    it starts to wash out in the atmospheric haze. Folks with bigger
    scopes and better atmosphere are seeing Mars at 500X and up.

    If you purchase an 8mm eyepiece and a 2X Barlow, or if you buy a 4mm
    eyepiece, add a set of planetary filters to your order. I have found
    Mars is improved by using an orange filter (#21), light red (#23), red
    (#25), and a Sirius Planetary Enhancement Filter (try HandsOn Optics
    -- I ordered one from them and received in in 4 days). Mars is very
    bright; the filter will knock back some of the brightness so the
    brightness does not wash out the details, and, the colors in the
    filter enhance or reduce certain colors so you can see surface

    Your 10mm eyepiece yields 90X, which is just not enough to see details
    on Mars -- you need to get up around 150X.

    Also, remember -- no matter how clearly your scope shows Mars, no
    matter which filters you use, you WILL NOT SEE MARS THE WAY THE PHOTOS
    SHOW IT. Those photos were made either from satellites that flew past
    Mars or from huge, earth-based telescopes shooting long (several
    hours) exposures, or from the Hubble.

    You will see a round disk with the polar cap and some dark, shadowy
    areas - and what you see depends on how high Mars is in the sky, the
    amount of crap in the atmosphere, and the amount of light pollution at
    your observing site.

    By the way, you are welcome to post an article on this newsgroup
    asking for advice about purchasing eyepieces. You will get LOTS of
    advice and recommendations, some of it contradictory, most of it

    You may want to consider a University Optics 7mm orthoscopic eyepiece
    (they don't make an 8mm). 7mm will give you 128X, with a Barlow =
    256X -- which is pushing your scope but should work. The orthoscopic
    eyepieces require you to get your eye right up against the eyepiece
    but I have UO 9mm, 5mm, and 4mm and have had excellent results with
    them on Mars. The price is reasonable.


  4. #4
    Jarle Aasland's Avatar
    Jarle Aasland Guest

    Default 25mm eye piece vs 10mm eyepiece??

    > Also, remember -- no matter how clearly your scope shows Mars, no

    Several hours exposure of Mars... That's a bit much, don't you think?




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