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  1. #1
    vinylanach@aol.com's Avatar
    vinylanach@aol.com Guest

    Default Eyepiece Question



    I bought a Meade ETX telescope last year for my kids and myself. A few
    weeks ago, I purchased a 9.7mm Super Plossl eyepiece on e-bay to
    replace the 26mm one supplied with the ETX. Basically, I just wanted a
    better view of the planets.

    I received it a few days ago, and while certain things are certainly
    larger, such as the moon, I find that some objects, such as Saturn and
    Mercury, are distinctly fuzzy and grainy. So I'm trying to figure out
    why I can't get the same clarity as I get with the 26mm eyepiece. Is
    the 9.7mm eyepiece too much for the ETX? Are atmospheric conditions
    the problem? Or is there a chance that the eyepiece is defective?

    Thanks in advance.


  2. #2
    Kickin' Ass and Takin' Names's Avatar
    Kickin' Ass and Takin' Names Guest

    Default Eyepiece Question


    vinylanach@aol.com wrote:

    There are several reasons why these objects are fuzzy and grainy.

    First, let's review some math.

    Maximum magnification
    The maximum magnification you can expect from a telescope equals
    (aperture in inches) X 60. Some folks use (aperture in inches) X 50.
    Whatever.

    You don't tell us which ETX you have so I'm just guessing. The ETX-80
    has an aperture of 3.15 inches; ETX-90 is 3.5 inches and ETX-125 is 5
    inches. Applying the factor of 60 would yield the following maximum
    magnification figures:
    ETX-80: 3.15 x 60 = 180X
    ETX-90 = 210X
    ETX-125 = 300X

    Eyepiece magnification
    The magnification provided by an eyepiece equals (telescope focal
    length) divided by (eyepiece focal length). The focal lengths for the
    ETX scopes are: ETX-80, 400MM; ETX-90, 1250MM. With a 9.7 mm
    eyepiece, your magnifications would be:
    ETX-80: 400mm divided by 9.7mm = 41X
    ETX-90: 1250mm divided by 9.7mm = 129X
    ETX-125: 1500mm divided by 9.7mm = 196X

    Now, what does this tell us? If the eyepiece is yielding a
    magnification that is close to or more than your scope's maximum
    magnification, that could cause fuzzy seeing because you are pushing
    the scope to its optical limits. HOWEVER -- we see that this is not
    the situation here -- for example, the ETX-90 has a maximum mag of 210X
    while the 9.7mm eyepiece gives a 129X magnification, well within the
    scope's capabilities.

    So, your problem is not caused by too much magnification. Here are
    some possiblities.

    1. Not focused correctly. I suspect when you put the eyepiece into
    the scope you insert the eyepiece barrel all the way into the eyepiece
    holder. It could be that the eyepiece needs to be pulled slightly out
    of the holder because the travel of the focuser is not quite enough to
    focus with that eyepiece. So -- insert the eyepiece into the scope so
    that about 1/3 of the silver barrel is sticking out of the eyepiece
    holder and try to focus the scope.

    In this same vein, are you certain you are focussing the scope? The
    ETX scopes use that tiny little silver knob that's impossible to grip
    and you may not be focussing the scope -- can you focus other objects
    such as stars, or, can you focus on terristreal objects -- that is,
    point the scope at a hilltop several miles away and make certain you
    can focus.

    As a former ETX-90 owner, I recommend you replace the stock focussing
    knob with a Flexi-Focus -- several dealers sell them -- do a google
    search for "ETX flexi-focus" or "ETX accessories." Here is an example:
    http://www.shutan.com/Merchant2/merc...ategory_Code=a

    2. Seeing conditions. Atmospheric conditions control what you see.
    The sky may appear to the naked eye to be clear when, in fact, it's
    anything but. It doesn't take much in the way of haze, light
    pollution, or humidity to cause objects to look fuzzy.

    If you are focussing correctly, try viewing over a period of several
    nights. Wait for a cold front to move through and clear out the air.

    3. You may have a bad eyepiece. Can you focus on other objects, both
    celestial and terrestrial?


  3. #3
    vinylanach@aol.com's Avatar
    vinylanach@aol.com Guest

    Default Eyepiece Question


    Kickin' Ass and Takin' Names wrote:

    This is something I've tried to find out on my own. The box, the
    owner's manual and the telescope itself say nothing other than ETX. I
    finally dug up the receipt just now, and it says ETX-90.


    I just tried this, and it didn't make a difference. In either
    position, the eyepiece is able to focus on the moon. It's when I tried
    to look at Mercury and Saturn last night that it just wouldn't focus
    well enough. My 26mm did just fine. Mercury and Saturn, of course,
    were closer to the horizon, which makes me think that it is an issue
    with the atmosphere. I do live in LA, which probably explains
    everything.


    Like I said, I could focus on the moon just as well with the 26mm
    eyepiece. Most of the brighter stars I focused on were just more
    indistinct and fuzzier no matter where they were in the sky.


    Since I've finally figured out that it is indeed an ETX-90, this sounds
    like a great idea. I've always been a little disappointed with the
    stock knob.


    I think this is it.


    I'll try this, too.


    Yes and yes.

    Thanks for the very thorough answer to my question!


  4. #4
    Matthew Ota's Avatar
    Matthew Ota Guest

    Default Eyepiece Question

    Viewing planets at high magnification is always a crapshoot. I looked
    at Saturn two nights ago with my 10 inch SCT, but the seeing was poor
    and I could not split the Cassini Division.

    There are many considerations when viewing planets with a telescope.
    They are best seen when nearest the zenith, where you are observing
    then through a smaller column of air.
    They are also best seen when at opposition, when they are closest to
    the Earth.

    A good book to read about this is "The Planet Observer's Handbook", by
    Fred W. Price, Cambridge press. ISBN 0 521 78981 8 paperback. There is
    an explanation about seeing conditions and how they effect planetary
    observing.

    Matthew Ota

    vinylanach@aol.com wrote:


  5. #5
    Brian Tung's Avatar
    Brian Tung Guest

    Default Eyepiece Question

    vinylanach wrote:

    There's a chance, but it's unlikely. The subjective impression is often
    that the view through the longer focal length (and therefore lower
    magnification) eyepiece is sharper. One thing you might try next time
    is to see if there's actually a detail you can see at 50x (with the
    26 mm eyepiece) that you can't see at 130x (with the 9.7 mm eyepiece).
    If there isn't, you can likely rule out eyepiece defect, though a defect
    is still not necessarily indicated even if there is such a detail. Your
    eye/brain is better able to see details at certain sizes.

    It may be useful to keep in mind that an eyepiece is really just a well
    corrected magnifying glass. The rest of the telescope forms an image in
    your focuser tube that's like a miniature of whatever it is you're
    looking at. The eyepiece simply serves to make that miniature more
    easily visible to your eye. So although a shorter focal length eyepiece
    makes the object look bigger, it also makes the blurriness from seeing
    (that is, atmospheric turbulence) look bigger, too.

    My guess is that your symptoms are rooted in atmospheric turbulence.
    This is especially the case with Mercury, which (unless you exhibited a
    certain degree of intrepidity) you probably observed while it was close
    to the horizon. Close to the horizon is just where turbulence is worst,
    because it is in exactly those cases that you are looking through the
    most atmosphere. Mercury is so small that the extent of the seeing is
    probably a good fraction of Mercury's apparent size. If it looks as if
    Mercury is bobbing, weaving, and bubbling, that's seeing.

    Saturn can be seen high in the sky these days, but you have to stay up
    kind of late--it is highest in the sky at around three in the morning.
    It rises around eight in the evening, so my guess is that you saw that
    while it was close to the horizon, too. Saturn isn't quite as low
    contrast as Mercury, but the details that one wants to see on Saturn,
    such as subtle shading in the cloud bands and the ring divisions, have
    the disadvantage that they are comparatively small and are therefore
    highly susceptible to atmospheric turbulence. You may have heard that
    the larger the telescope's objective (90 mm in your case), the finer the
    detail that can be seen through it. Well, the Cassini division in the
    rings of Saturn (the widest division) can be made out in a 60 mm scope
    under excellent conditions, but when Saturn is low to the horizon under
    poor conditions, it can be difficult to make out in a 10-inch (250 mm)
    scope. Atmospheric turbulence is the great equalizer: It doesn't quite
    bring a 10-inch telescope to its two-inch knees, but it makes things a
    heck of a lot closer than they would be otherwise.

    In short, I wouldn't worry too much about the eyepiece. It's probably
    just fine. You picked an extremely challenging target to begin with in
    Mercury--observers much more experienced than you have made scores of
    incorrect observations on it. Saturn is easier, but you might want to
    wait either later in the night or later in the year for it to get higher
    up in the sky. If you can stand to do it, try it at five in the morning
    these days. It will still be reasonably high in the sky, and seeing is
    often the best just before dawn.

    --
    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

  6. #6
    googleusenet3us@yahoo.com's Avatar
    googleusenet3us@yahoo.com Guest

    Default Eyepiece Question

    Hi,

    The planets are not a good choice for evaluating a new eyepiece when
    you are first starting out. They are a little fuzzy to begin with and
    if viewed when they are low in the sky it is hard to tell what is in
    the eyepiece and what is a problem with the viewing conditions. After
    you get to know your scope better you can try using planets to judge
    optics.

    I'd start with the moon when it is high in the sky. Try looking at the
    terminator (the imaginary line dividing the sunlit part from the part
    that is dark). The shadows there are sharp and full of contrast and
    will be easier to judge.

    You can also look at the Clear Sky Clock site to find a clock for your
    area
    http://cleardarksky.com/csk/index.html#clock_list You can look there
    to get a prediction on how good the seeing is going to be for your
    area. There is also a section that describes how to judge seeing
    conditions. If the seeing is bad, no eyepiece in the world can make
    everything look sharp.

    Let us know how it goes!

    Chuck Taylor
    Do you observe the moon?
    Try http://groups.yahoo.com/group/lunar-observing/
    ************************************************** **




    On Dec 31 2006, 8:05 pm, "Matthew Ota" <otake...@bigvalley.net> wrote:


 

 

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