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Collimation. Have I done it right?

Discussion in 'Astronomy Gear' started by Grant, Jul 3, 2017.

  1. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Here is my first attempt at collimation. The dot is in the circle but the outer oval isn't quite central.
    Have i done it correctly?

    LRM_EXPORT_20170703_163244.jpg
     
  2. Dale kershaw

    Dale kershaw CAS Editor CAS Member CAS Committee

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    Graeme says It should all be concentric.
     
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  3. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Do you know if it's the back or the front i need to move?
     
  4. Graeme Kershaw

    Graeme Kershaw New Member CAS Member

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    mmmmm.... ummmm..... errrrrr.......pass.... (sorry) trial and error required :)
     
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  5. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Haha. OK no worries. Serves me right for getting a big beast (more assembly required).
     
  6. Euan Mason

    Euan Mason Active Member CAS Member

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    Do you have a mark in the centre of the primary?
     
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  7. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Yes. The dot and the circle line up perfectly
     
  8. Euan Mason

    Euan Mason Active Member CAS Member

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    So you need to adjust your secondary and then readjust your primary.
     
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  9. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Ah ok, thank you. I'll start again tomorrow. Lol
     
  10. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    what are you using? is this a camera view through a colimation cap?

    if the camera is not perfectly centred the image will not be much use.

    once the camera/eye is perfectly centred using a cap with a hole in the centre, then look at the position of the secondary mirror itself.

    From the photo the secondary mirror appears to be in the correct position, ie equal distance to the focuser tube all around. The exact position of the secondary is not super critical, so long as the entire primary can be seen in the secondary when the primary reflection is centred under the focuser. moving the focuser in or out helps to frame the secondary in the focuser tube.

    the tilt of the secondary mirror appears to be out. tilt it up/down and rotate it if required to centre the reflection of the whole primary mirror in the secondary. ignore the reflection in the primary for now. the side to side adjustment screws will shift the secondary away from centre, rotate the secondary to move the reflection side to side. you need to loosen a screw to rotate the mirror. hold the mirror in place when you tighten the screw again. you need to get the primary reflection pretty close to centred in the focuser tube. If the secondary mirror is already centred this is a bit easier to judge. getting the reflection of the primary directly under the eyepiece is a bit more critical. If it's off the light will come into the eyepiece at an angle, well more of an angle since light from the edge of the mirror comes from an angle anyway. you want to be at least within 10% of the mirror diameter, and more like 5%. a laser colimator makes this easier, just remember to rotate the secondary rather than shift it side to side. (fine side to side adjustments are ok, but you can end up really far out if you don't keep a close eye on it)

    Once the secondary is right, ie mirror centred with focuser tube, reflection of primary centred in secondary mirror, then start on the primary. think in 60 degrees not 90! Also take care that the screws are not too far in or out (loose or binding up)

    adjust the primary so the reflection of the hole in the colimation cap (the very centre dot) is in the centre of the ring on the primary. This is the most critical part of colimation. this means that the centre of the mirror is exactly 90 degrees to the line of sight from the centre of the eyepiece.

    keep checking as you tighten the lock screws bit by bit

    Finally you'll need to re-align the finder.

    In reality, the image above may not be prefect, but it's not that far out, probably hard to tell when observing. The whole primary is visible, the primary is within 5% of the centre of the focuser tube, and the hole in the cap is in the centre of the ring.
     
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