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  1. #1
    alternatively, my mind's a blank's Avatar
    alternatively, my mind's a blank Guest

    Default query on 4.5" newt...



    hi all.

    i have a 4.5" celestron scope, and it's been banged about quite a lot
    (it lived in the car trunk for a couple of thousand miles of fairly bad
    roads), and i had the opportunity to set it up again yesterday.

    this is what i observed: stars are point-sources with *no* coma at all,
    nor is there any sort of color fringing around them. and saturn's an
    amazingly real jewel too. but it was with jupiter that i was a little
    disappointed. with a 20mm ep it shows up as a very good disc (with 4
    jovian satellites), but the disc is not sharp at all, and i could
    detect some color fringes too... red on top and blue at bottom. using a
    10mm ep only made the matter worse, in addition to not allowing me to
    focus properly (moons were visible with the 10mm too, and they were
    point sources again).

    one point to note is that while saturn was pretty high up (almost near
    the zenith), jupiter had risen only for an hour or so and there was a
    large amount of light pollution too... but i could see 4 moons very
    clearly.

    since i bought this 1.5+yrs ago, i've never collimated it even once, as
    i never observed any sort of coma around stars. plus, i believe
    celestron supplied plossl eyepieces with this model (114eq firstscope
    short).

    the fact that point sources are *very* clear means that it doesn't need
    collimation, despite rolling around my car's trunk... is this
    assumption correct? if this is the case, how come jupiter didn't
    resolve well? is this only due to its low position or is there
    something i'm entirely missing?

    thanks for all the help, in advance!
    alt.blank


  2. #2
    Brian Tung's Avatar
    Brian Tung Guest

    Default query on 4.5" newt...

    alternatively, my mind's a blank wrote:

    Those fringers are telltale signs of atmospheric dispersion, which is
    to be expected when Jupiter is low in the sky. The fact that you can
    see the four Galilean satellites is not very indicative, since they
    are as good as point sources to a 4-1/2-inch telescope and you should
    see them as well as you can see any star.

    Atmospheric dispersion means, in a sense, that the color registration
    of your view is not very good, and therefore the details will get all
    washed out. This is especially deleterious with Jupiter, which has
    lots of detail to see when conditions are right.


    Is this an f/8 scope? If so, the focal length is 900 mm and your 10 mm
    eyepiece "only" yields 90x. That's not very high power, and you won't
    see the effects of miscollimation unless it's pretty poor. Those
    effects are, unfortunately, most easily seen on Jupiter as a loss in
    detail. But it could just be dispersion. Get up early in the morning
    and try Jupiter again, and you'll have a better idea as to what's going
    on.

    --
    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

  3. #3
    alternatively, my mind's a blank's Avatar
    alternatively, my mind's a blank Guest

    Default query on 4.5" newt...

    @brian: thank you very much for the detailed reply! all my questions
    answered, and in one shot!

    yep, this is a f/8 scope with focal length of 900mm, and the two
    supplied eyepieces give only 45x and 90x magnifications.

    i'll try jupiter again tonight (tending toward early morning, that is).

    cheers,
    alt.blank


 

 

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