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Thread: Collimation of Apertura Dobsonian Telescope

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    Default Collimation of Apertura Dobsonian Telescope



    Thanks for reading this.
    Last week I received my new AD12 and assembled it. However it is not very well collimated. I read the instructions and tried to follow it, but the instructions repeated here are not clear.

    Step1: Remove any eyepiece from the focuser. (no problem here)

    Step2: Look into the focuser and confirm that you see the secondary mirror centered in
    the hole. Reflected in the secondary mirror, you should be able to see the primary
    mirror and the three small clips around the edge of the primary mirror. If not, you
    will need to rotate the secondary mirror assembly with your hand (be careful not
    to TOUCH the reflective surface of the secondary mirror itself) until the reflection
    of the primary mirror is centered in the secondary mirror. In addition, although
    the shape of the secondary mirror is actually oval, when properly aligned it will
    appear to be circular as viewed through the focuser.

    Question: How does one rotate the secondary mirror? I don't see any adjustment screw for it. Is this even advisable as I have read elsewhere that this is not usually necessary.

    Step 3. Insert the laser collimator into the 2” adapter sleeve and then insert that into the
    focuser. Secure the laser collimator with the thumbscrew. Turn on the laser
    collimator and rotate its target face toward the back end of the scope (the
    primary mirror end). (no problem here)


    Step 4. Hold up a piece of paper in front of the telescope to see if the laser is missing the
    secondary mirror on reflection. If the laser isn’t reflected onto the paper, then
    most likely it is hitting the secondary mirror as it should. (In my scope, the laser reflection was not hitting the secondary mirror at all.)


    Step 5. Carefully peer into the tube to see where the laser is hitting the primary mirror
    below. CAUTION: Do NOT allow the laser to shine directly into your eye from the
    telescope’s mirrors. The laser should be shining in the middle of the small donut
    ring in the middle of the primary mirror. If it is not, adjust the tilt of the secondary
    mirror using the three small screws on the top of the secondary mirror assembly
    until the laser is centered in the donut ring on the primary.
    This is where I am having problems: no matter how I adjust the 3 screws, the laser spot moves around in one small section of the primary mirror (~45 degree sector and 1" to 3" away from the center of the mirror). I cannot get the laser spot to move out of this limited zone (to the opposite side for example).
    Also am I supposed to loosen two knobs and tighten one simultaneously?
    How many turns does it typically take? I am worried that if i turn more than 2-3 turns, the secondary mirror will fall off.
    After spending 1/2 hour trying to get the spot to move out of the narrow sector it was stuck in, I started playing with the center screw. I unscrewed it about 2 turns. Now I am able to get the laser spot move to the opposite side. But it is still quite difficult to center the spot on the mirror. Closest I can get it is right on the outer edge or the Donut marker.
    Are the 3 screws supposed to be tight when finished?
    All in all, i feel it is not intuitive and each adjustment of screw seems to be like spinning the Roulette wheel (in terms of outcome :-)).

    Thank you again for reading this! Any comments are very much appreciated. I could call High Point Scientific for help, but it is the weekend now.

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    Default Re: Collimation of Apertura Dobsonian Telescope

    The secondary mirror is held in place by one screw that has three smaller screws around it.
    The center screw adjusts the in/ out distance and rotation and the other three screws adjust the tilt of the mirror.

    Try these instructions instead, Astro Babys Guide to Collimation - Newtonian Telescope - Reflector

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