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  1. #1
    Andy Grove's Avatar
    Andy Grove Guest

    Default Saturn - Why can't I see Cassini division?



    Hi,

    I was observing Saturn last night between 8:30 and 9:30 GMT using my 8"
    SCT (LX90) and I was hoping to see the Cassini division. According to
    several articles I've read this is supposed to be visible even in a
    much smaller scope. However, Saturn's disc and rings are both very
    bright and I could make out no detail.

    Do I need to use a filter to be able to see this sort of detail?

    Also, which eyepiece is best for observing Saturn? Everything looked
    very well defined using a 20mm eyepiece but when using a 2xbarlow with
    that eyepiece or using a 9mm eyepiece on its own the image wasn't so
    well defined. I tried to collimate the scope last night as well and I
    don't think I've got the hang of that yet so maybe that is the problem.
    Or perhaps conditions last night weren't ideal?

    Any advice appreciated!

    Thanks,

    Andy.


  2. #2
    Norbert's Avatar
    Norbert Guest

    Default Saturn - Why can't I see Cassini division?

    Andy Grove wrote:

    No filter is necessary for the planets. But the rings of Saturn are
    'closing', so the Cassini division is not really visible all around the
    planet this year, only on the edges.
    With an 8" SCT, you can use a 4mm eyepiece if the conditions are good, ie
    turbulence is low. But even with turbulence, you should be able to use a
    9mm.
    If the view with a 9mm is not good, I think that your scope is not well
    collimated. Do the collimation again when the OTA is at the outside
    temperature (wait for at least 2 hours).
    If you think that the collimation is OK, may be there was a thin layer of
    high altitude clouds, and to be sure you will have to observe again

    --
    Norbert. (no X for the answer)
    ======================================
    knowing the universe - stellar and galaxies evolution
    http://nrumiano.free.fr
    images of the sky http://images.ciel.free.fr
    ======================================



  3. #3
    Adam's Avatar
    Adam Guest

    Default Saturn - Why can't I see Cassini division?

    Andy you have probably identified the two of the three main reasons why your
    views may not be too good. I was observing Saturn last night at about
    midnight with a 12inch Dob at powers up to 300x. The Cassini division was
    visible in moments of good seeing but nowhere near as clear as it can be.
    My scope was cooled and collimated so I put it down to mediocre seeing down
    here in Dorset. So what is the third reason why your views may not be so
    good? Your scope needs to be close to ambient temperature. In the UK this
    means that it needs to be outside cooling down for at least an hour before
    it will perform at its best. Keeping it cool and collimated will allow you
    to see Cassini as best as the seeing will allow. There are no need for
    filters for this.

    Adam





  4. #4
    davehowes's Avatar
    davehowes Guest

    Default Saturn - Why can't I see Cassini division?

    Hi Andy. You will be able to see the Cassini division in your scope - I
    saw it in my TV Pronto (70mm semi-apo refractor) earlier this year at
    100x. Three things you need to do to improve your chances are:
    1) Make sure your scope is perfectly collimated. Get someone who has
    experience with SCT's to do your scope and show you how it is done.
    Don't rely on laser collimators - the ONLY way with an SCT is star
    testing.
    2) Use a bog standard plossl or super plossl eyepiece - you won't need
    a huge wide field eyepiece like naglers and stuff for seeing fine
    planetary detail. The extra glass elements in wide field designs
    contribute to increased loss of contrast. That's why you suffered image
    breakdown using the barlow. There comes a point when you can
    over-magnify things. The secret is in finding out just how much
    magnification the seeing will permit.
    3) Wait until later in the night. At 9.30 p.m. Saturn is not really
    high enough in the sky to give the best image. When objects are near
    the horizon, you will be viewing them through a thick air mass, a bit
    like the heat haze you see along a road on a warm day. Try again around
    midnight when the planet is at it's highest point in the sky.
    Don't be discouraged if you still have trouble - it takes practice and
    experience to pick out fine details and low contrast features. It may
    be that you're actually seeing it, but expecting something more
    dramatic. Good luck, Dave.


 

 

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