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Thread: GEM Questions

  1. #1
    Davoud's Avatar
    Davoud Guest

    Default GEM Questions



    What are the disadvantages of using a German Equatorial Mount?

    - Are portions of the sky unreachable?

    - Can a GEM track across the meridian?

    - GEM owners seem to fuss a lot over polar alignment 'scopes. How
    (briefly) does one align a GEM with sufficient precision for
    astrophotography? Does drift alignment work?

    - What else don't I know?

    TIA

    Davoud

    --
    usenet *at* davidillig dawt com

  2. #2
    Tom's Avatar
    Tom Guest

    Default GEM Questions

    "Davoud" <star@sky.net> wrote in message
    news:071020052155192500%star@sky.net...

    While I am too much of an amateur to answer the first question, I do have
    experience with the latter 2. Drift alignment is an option. In fact, it is
    what a lot of people use who observe deep sky objects. It allows the scopes'
    drives to be accurate for hours at a time. If you are observing planets, on
    the other hand, a quick alignment with Polaris is almost always good enough.
    I have heard of people getting as much as 15 or 20 minutes of tracking
    ability with a simple Polaris alignment. I would say 10 minutes is more
    easily achieved.

    Have fun!



  3. #3
    William R. Mattil's Avatar
    William R. Mattil Guest

    Default GEM Questions

    Davoud wrote:

    David,


    Too many to list here :^)


    No, and it doesn't matter how much equipment you have hanging off the
    focuser either. No fork arms to clear.



    Most can easily go 2-3 hours past the meridian. Surprisingly enough I
    hear that the Millinium won't. But in a CCD only world that's probably
    not a big deal.


    Not me. I have two GEM's and one has the PAS and I never use it.


    Drift alignment always works. And *even* if you use a PAS to get close
    Drift Aligning can polish it up and get very close.
    It's a simple process that takes roughly an hour once you get the hang
    of it. And you can start long before it's dark enough to open a shutter
    too.



    You dare to ask that on s.a.a. ???? wow .... courage !

    You'd probably get better info on the Losmandy list since I seem to
    recall you are thinking about a G-11 ?


    http://www.celestial-images.com/testing.html

    G-11 with 75 pounds of junk hanging off of it and a few sample images.

    Regards

    Bill

    --

    William R. Mattil : http://www.celestial-images.com

  4. #4
    Brian Tung's Avatar
    Brian Tung Guest

    Default GEM Questions

    Tom wrote:

    Drift alignment is used by a lot of people who *image* deep sky objects.
    I assume that is what you meant (in any event, it should be what you
    mean <g>), but it was unclear.

    For visual use, you need no better alignment with a deep sky object than
    you do with the planets. In fact, if anything, since planets are often
    observed with a smaller true field of view than DSOs, you need better
    tracking for observing planets than you need for observing DSOs, not the
    other way around.

    --
    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.txt

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

    Default GEM Questions

    Davoud wrote:

    They're a bit ungainly for some beginners, and I've seen a couple of
    them whack themselves in the arm due to an incomplete understanding of
    the balancing system, but probably you're far along enough that you'll
    be able to figure the main issues out.

    Otherwise, they have no significant *inherent* liabilities. (Of course,
    you can always make a poor one.) They wouldn't be as common as they are
    if they did have.


    Not intrinsically. If the mount is made poorly, you may be unable to
    point directly at the pole, or in some small region around the pole.
    But the same problem exists with respect to the zenith in a Dobsonian.


    They might not be stopped right at the meridian, but they won't make
    it all the way across the sky.


    I personally never found much use for a polar alignment scope. Either
    you're using the scope visually, in which case a couple of degrees from
    the pole is entirely acceptable, or else you're imaging, and a polar
    alignment scope won't be reliable enough. (At least, I wouldn't risk a
    four-hour exposure on a polar alignment scope alone. If I were an
    imager. Which I'm not.)


    Loads. But that's true of all of us.

    --
    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.txt

  6. #6
    Tom's Avatar
    Tom Guest

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    "Brian Tung" <brian@isi.edu> wrote in message
    news:di7dmb$v02$1@zot.isi.edu...
    have
    is
    scopes'
    on
    enough.

    Actually, I meant both observe and image. People at star parties oft times
    use their drives to keep something in alignment, so their friends can take
    turns looking without losing the object.


    I stand corrected on the visual vs image usage. You are right. But if
    imaging was the objective, then DSOs would definitely require better
    alignment.




  7. #7
    Brian Tung's Avatar
    Brian Tung Guest

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    Tom wrote:

    It's unusual for a social observing session to involve keeping the
    object in view for much longer than 10 or 15 minutes, but it does
    happen, and when it does, yes--you do need better alignment. I
    suppose a polar-alignment scope could come in handy in that case.


    Yes, though for a different reason--hardly anyone needs to do long-
    exposure imaging of a planet. Even the new tenth planet won't need
    anything like an hour of tracking.

    --
    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.txt

  8. #8
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default GEM Questions

    On Fri, 07 Oct 2005 21:55:19 -0400, Davoud <star@sky.net> wrote:


    Theoretically, GEMs are less stable than forks because of their two
    cantilevered axes. In practice, of course, stability is a function of
    engineering quality, and aftermarket GEMs are usually of higher quality
    than mass-produced forks from Meade and Celestron. That is why good GEMs
    are frequently acquired by serious imagers.


    Not unless something is wrong with the design.


    Yes, but in most cases you can't track from horizon to horizon. At some
    point you reach a position that requires flipping the scope. This is a
    big disadvantage if you are an imager, since it means interrupting your
    exposure and probably finding a new guide star.


    Aligning a GEM is identical to aligning a fork. The techniques are the
    same, the level of difficulty is the same.


    While forks are better for imaging because they don't require an axis
    flip, they also can be problematic since there is limited distance
    behind the OTA that can be used before your equipment won't clear the
    base. GEMs don't have this problem.

    IMO the ideal fixed mount for an imager with an observatory is an
    aftermarket fork, such as those made by Mountain Instruments. But you
    will pay more for one of those than for a good GEM.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  9. #9
    Mark D's Avatar
    Mark D Guest

    Default GEM Questions

    Howdy Davoud,
    I'd like to comment here, and make a point, in which others here have
    said otherwise about the inherit qualities of a GEM.

    Yes, depending on what type of OTA you are using, there can be the
    possibility of a portion of the sky that cannot be easily accessed, and
    that might be in the instance with a long FL Refractor, and trying to
    point to the Zenith.

    The long OTA could possibly interfere with the GEM's Tripod Legs. With
    SCTs, this shouldn't be much of a problem.

    I have this problem myself with one of my Refractors on a Byers 58
    GEM/Meade GFT Tripod.

    If you are considering a G-11, and then getting the Losmandy Polar
    Scope, be ready for a 3-5 month wait for the scope. Evidently, these
    items come, and go from staying in available stock, and there currently
    is a waiting list for them I understand.

    None the less, for decent polar alignment for visual use, and getting
    fairly accurate use from the setting circles, one can easily sight
    through the Polarscope Bore on the Mount, center Polaris as best one
    can, and get reasonably good results. Mark


  10. #10
    Tom's Avatar
    Tom Guest

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    "Brian Tung" <brian@isi.edu> wrote in message
    news:di7fgh$v3a$1@zot.isi.edu...
    times
    take

    I didn't mean a viewing session of more than 10 or 15 minutes. But to keep
    something in the FOV for more than a matter of seconds requires decent
    alignment. Certainly the drift method wouldn't be needed, just aligning with
    Polaris real quick would do the job fine.


    Oh, I agree. Many people imaging Pluto make exposures from 1 to 5 minutes.




 

 
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