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  1. #1
    Ashley Culver's Avatar
    Ashley Culver Guest

    Default Plossl or Super Plossl??



    Dear Group,

    I am about to purchase my first extra lens for my Orion Optics Europa 150mm
    F5 scope. I have currently got the two lenses that came with it - both
    Plossls, a 10mm and a 25mm.

    I was looking at Saturn last night and decided to show my partner who
    pointed out it was very small (!) I am therefore thinking of getting a
    higher magnification eyepeice, and looked at Orion Optics website. They have
    both Plossls and Super Plossls, the SP's being only slightly more expensive.

    I wondered if anyone knew what the difference might be and whether it's
    worth the extra expenditure?

    Many thanks,

    Ashley
    Cambrisge



  2. #2
    Stephen Tonkin's Avatar
    Stephen Tonkin Guest

    Default Plossl or Super Plossl??

    Ashley Culver <zen@nowhere.com> wrote:

    Suggestions: Don't get another eyepiece, get a good (i.e. apochromatic,
    such as the Celestron Ultima or the TeleVue) x2 Barlow . This will give
    you the equivalent of a 12.5mm and a 5mm eyepiece. You will also find
    that eyepieces like Plossls will work better at f/10 (which is what you
    will have with the Barlow) than at f/5. (See Barlow tutorial on my web
    site.)

    If you are determined to get another eyepiece, and if the main purpose
    of your high-power eyepiece will be planetary observation, or other
    things where you are examining small objects, then you don't need a wide
    FoV. IME decent Orthoscopics tend to give better images than Plossls.



    Best,
    Stephen

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  3. #3
    Ashley Culver's Avatar
    Ashley Culver Guest

    Default Plossl or Super Plossl??

    Hi Stephen,

    Thanks for the advice. I had assumed that a Barlow would be quite expensive
    but looking at the Celestron Ultima it seems to be a comparable price to the
    Orion Plossls. I didn't realise that the eye relief of the Plossl would be
    affected by the F Number of the telescope either (I read your tutorial!) -
    my partner wears glasses so a longer eye relief provided by the Barlow would
    help her.

    Thanks again, I shall buy a Barlow I think :O)

    Cheers

    Ashley

    "Stephen Tonkin" <news05footfrommouth@astunit.com> wrote in message
    news:TRFy+3DNv2OAFwJ8@astunit.com...



  4. #4
    Stephen Tonkin's Avatar
    Stephen Tonkin Guest

    Default Plossl or Super Plossl??

    Ashley Culver <zen@nowhere.com> wrote:

    I think you may have misread it. What I state is that the eye relief of
    a Plossl is related to its (i.e. the Plossl's) focal length:

    "Many eyepieces have an eye relief (distance of exit pupil from eye
    lens) that is directly related to its focal length. For example, the eye
    relief of a Plössl is 0.73 × its focal length. Thus, with these
    eyepieces, for a given magnification there will be greater eye relief
    with a barlow than without."

    If you can see how I can re-word it to prevent it being similarly
    misread in future (or if you can just point out to me what it was that
    caused you to misunderstand it), I will attempt to improve the wording
    in order to reduce the possibility of similar future misunderstandings.
    Thanks!


    Best,
    Stephen

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  5. #5
    Alan's Avatar
    Alan Guest

    Default Plossl or Super Plossl??

    How about the following (I'm assuming that there are no eyepiece types for
    which eye relief is a nonlinear function of focal length - correct me if I'm
    wrong). Sorry there are more words - hack and chop or take ideas (or ignore
    them :-) - it's a free suggestion).

    Eye relief (distance of exit pupil from eye lens) is proportional to the
    focal length of the eyepiece. The constant of proportionality varies between
    different eyepiece types, but for a given type a longer focal length means a
    longer eye relief.
    For example, the eye relief of a Plössl is 0.73 × its focal length.

    Increasing magnification by reducing the focal length of the eyepice
    therefore also reduces the eye relief (this may be overcome to some extent
    if a different type of eyepiece can be found with a larger constant of
    proportionality).

    Using a Barlow lens to increase magnification works in a different way; by
    increasing the focal length of the telescope itself. In this case, the focal
    length of the eyepiece is unchanged, and the original eye relief is
    preserved.

    Alan

    PS - something troubles me about my last paragraph. I'm sure that the Barlow
    could equally be thought of as reducing the focal length of the eyepice
    while preserving the eye relief, because if you put the Barlow and eyepiece
    together in a sealed black box (hole either end obviously), you wouldn't be
    able to tell it apart from a different eyepiece design that had the same eye
    relief as, but greater magnification than, your orignial ep. So an
    alternative to the last paragraph would state that the Barlow converts the
    eyepiece to a different design of eyepiece with the same eye relief but
    shorter focal length. There must be a downside to that conversion?



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