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  1. #1
    Dennis Woos's Avatar
    Dennis Woos Guest

    Default Occulting Eyepiece, observing Phobos and Deimos

    This morning a friend and I were observing Mars (with my friend's Takahashi
    TOA 130 - a wonderful scope!), and got talking about trying for either or
    both of Mars' moons, Phobos and Deimos. He said that he had once attempted
    to add an occulting bar to a UO orthoscopic, but that he wasn't successful
    in positioning the bar at the focal plane. After I got home this morning, I
    unscrewed the barrel from a 7mm UO ortho, and discovered that the focal
    plane is up inside the housing which holds the lenses, .100" from the bottom
    of two opposing slots in a retaining ring. These slots are each about .027"
    wide, and the ring is just over .400" in diameter. I cut a piece of credit
    card which is .030" thick into a strip .400" wide, and then cut a piece of
    that so that it protrudes .100" up into the housing. The thickness of the
    credit card material is such that it stays put when forced into the slightly
    smaller slots in the retaining ring. The edge of the card is nicely
    positioned at the focal plane, and seems to be of a thickness sufficient to
    block out Mars in any scope I have access to.

    Now, has anybody here ever observed either of Mars' moons? Does anybody have
    any tips on observing them?

    BTW, Mars was very nice with a 3mm Radian (333x) and a W85 salmon-colored
    filter. Not a tremendous amount of fine detail, but enough to keep us
    consulting Mars Previewer. Also, I wonder if the dust storms are not
    completely over. We briefly observed Venus and Saturn as well, with Saturn
    surprisingly nice for being as low in the east as it was.


  2. #2's Avatar Guest

    Default Occulting Eyepiece, observing Phobos and Deimos

    On Oct 5, 10:01 am, "Dennis Woos" <> wrote:

    I had good results by using thin, transparent plastic, about neutral
    density 2, to cover up one half of the eyepiece focal plane. I then
    placed Mars behind the plastic and turned the eyepiece so that the
    predicted location of a moon was in the uncovered part of the field.
    Since I could still see where Mars was in the field, it was that much
    easier to spot each of its moons.

    It would be much easier to do this with a longer FL eyepiece and a
    Barlow, instead of a 9mm FL eyepiece, though.

  3. #3
    River Pomes's Avatar
    River Pomes Guest

    Default Occulting Eyepiece, observing Phobos and Deimos wrote:

    Nice idea. Simple is better.

  4. #4
    Dennis Woos's Avatar
    Dennis Woos Guest

    Default Occulting Eyepiece, observing Phobos and Deimos

    > I had good results by using thin, transparent plastic, about neutral

    I am going to operate on a 12.5mm othro today, where the field stop is a
    metal disk in the barrel itself. This is much easier to access, and I am
    going to try your suggestion of occulting approximately one-half of the
    plane with some transparent but dimming plastic. I like the idea of being
    able to see Mars while looking for the moons.

    How difficult has it been for you to view the moons? What kind of scope did
    you use, and what powers? Doing a little web research has taught me a couple
    of things, including the facts that both Phobos and Deimos are captured
    asteroids, and that Phobos orbits Mars so rapidly that an observer on Mars
    would see it rise in the west and set in the east (i.e. it orbits faster
    than Mars rotates).


  5. #5
    Enyo's Avatar
    Enyo Guest

    Default Occulting Eyepiece, observing Phobos and Deimos

    "Dennis Woos" <> wrote in message

    Here is a post I made during the August 03 approach.

    "After the visitors and most members had left our Mars openhouse at clubs
    observatory two of us spent the early morning hours looking for the two
    moons rather than surface detail. The nearby 9.95 magnitude star
    (TYC6386-00334-1), Sky and Telescope's Mars moon java applet and TMB
    SuperMono's were critical to the hunt. All observations were made sitting
    down, one must be comfortable and patient to wait for the moment of good
    seeing. The sky conditions were mostly clear with some thin clouds, no wind
    and high humidity. After 5UT the seeing improved to being variable from a 7
    to nearly a 9. I had my AP130F6 and he a 13" Dob with a good galaxy

    After trying for a half hour to 45 minutes looking unsuccessfully for Demos
    I switched to the 13" Dob. I put in my 4mm TMB SuperMono and was able to
    find Demos after about 15minutes of looking at 6:10 UT. Had to keep Mars
    outside of the FOV though. It was not visible in the other observers 4.8
    Nagler due to excessive light scattering. The dob had not been set up on
    it's platform but smooth hand tracking keeping Mars on the edge of the field
    stop was possible or 30 to 40 seconds before more rapid motion was needed to
    reposition the planet. The nearby star made for a convenient estimator for
    the magnitude and position of Demos. With the visual field now know I went
    looking for it again with the AP130 and after another half hour of looking
    was able to get it using either a 4 or 5mm TMB SuperMono straight through
    (no barlow or diagonal). Demos was frequently not visible for periods of
    time as the transparency of sky conditions varied. However, when visible it
    was visible 100% of the time.

    The sighting in the AP130 was confirmed by the other observer and we bounced
    back and forth between the two scopes with our observations after that. The
    first observation was about 1 hour before greatest elongation (7:12UT per
    S&T). A driven scope sure makes it easier to keep Mars just outside and
    adjusted it north or south on the field stop. I was not able to get Demos
    with my binoviewer and TMB 8 or 10mm eyepieces with or without a barlow.
    After Demos we started looking for Phobos as it started to approach greatest
    elongation. I had one clear view at 6:50 UT for approximately 1 minute of
    the star, Demos and Phobos in their expected positions using the AP130 and
    TMB 5mm. This was approximate 1hr before maximum elongation (7:53 per S&T).
    The sighting was not confirmed by the second observer nor repeated by myself
    over the next hour, although Demos was frequently visible. I packed up at
    4am since sky transparency had fallen off and Demos became less frequently

    So did we see the moons in a 5.1" scope? Demos for sure, many times, and
    it's motion relative to Mars and Mar's motion relative to the star were
    obvious. I believe I saw Phobos as it was clear and steady for a short
    period of time."

  6. #6
    Dennis Woos's Avatar
    Dennis Woos Guest

    Default Occulting Eyepiece, observing Phobos and Deimos

    > So did we see the moons in a 5.1" scope? Demos for sure, many times, and

    Thanks for a great post. Our friend has a Losmandy G-11 mount, a Takahashi
    TOA 130, and a Celestron 9.25" SCT and we will try both scopes. We also have
    access to an AP SuperPlanetary 6", but that is a bit much for the G-11. Our
    10" dob has a great mirror, and we will try it as well though it will be a
    challenge to hand-track. We don't have any TMB Monos, but I hope that our
    occulting orthos will be sufficient. The orthos are our friend's, and I may
    try and add an occulting bar to one of our TV plossls, which for us are as
    optically clean as it gets.
    It is great that you describe how much time you spent before detecting the
    moons. So often I see people look for a minute or two, or even less, and
    then state that they don't see whatever it is we are looking at. I think
    some folks in our club doubt some of my claimed observations, like seeing
    Enceladus some years back in our 6" f/8 dob (with a flex mirror - it is
    great!) However, I spend a long time at the eyepiece, trying to relax my eye
    and be sensitive to the faintest points of light. I get a lot of false
    positives, but then there will be a "hit" that I can go back to repeatedly,
    and when I camp on it, it becomes more obvious as the moments of better
    seeing come and go. I think it is more a function of good technique than of
    great eyesight, though I am sure that good eyesight is also required.

    I think I will share your posting with our club membership at large, as I
    think it is a good description of what it takes to observe difficult




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