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  1. #1
    blaise's Avatar
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    Default Polar Alignment Question



    I started imaging last January, never used the GEM (just the alt/az with wedge) until about 4 months ago. It never occured to me that by using Ursa Major as an orientation constellation, that I would have an issue witih polar alignment due to UM dipping below the horizon.

    For instance, by 9:00PM, Ursa Major is well below the horizon and oriented in such a way that the diagram in the polar scope can no longer be oriented correctly to match up with UM so that I can use it as a reference when polar aligning. Does this make sense at all?

    My GEM has a hard stop, it cannot swing, or rotate if you prefer, to match the reference constellations in the sky so that I can accurately polar align. If I could begin the PA routine while UM was still high enough in the sky, no problem, but I can't do it ahead of time because obvioulsy there is no Polaris to get in the little circle in the diagram through the polar scope so I know where the NCP is (confusing,eh)?

    Thanks for the input.
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  2. #2
    Joe Lalumia's Avatar
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    Default

    Get POLARIS the North Star in the finder scope. Then rotate the scope in RA and get the constellations as close as possible lined up. Readjust Polaris.

    For visual observing this is close enough. For astrophotography you will need to do a drift alignment.

    Drift Alignment of an Equatorial Mount
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  3. #3
    blaise's Avatar
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    Default

    Thanks for the suggestion, Joe. I've never had to do drift alignment. Under normal circumstances, I can get up to 8 minute single lights without using it before star trailing becomes evident (if properly polar aligned).

    As mentioned, the position of Ursa Major and the hard stop of my GEM does not allow me to orient the polar scope as nomral when UM is higher in the sky.

    I'm uncertain if drift alignment is the solution, as I know there are plenty of astro imagers who do as I do, and use autoguiding on a GEM and no drift alignment...ever.

    The idea of getting the NCP in position relative to the polar scope position of Polaris and drift alignment seem at odds with eachother...
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  4. #4
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    Default

    but Polaris is in Ursa MINOR

  5. #5
    blaise's Avatar
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    Default

    It is in fact in Ursa Minor. However, the polar scope clearly has Ursa MAJOR as a reference point to help in the calculation to offset Polaris in consideration of the NCP.

    Learn something every day, eh?
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  6. #6
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    Default

    It also has Cass. I can't see UM either from my viewing position, so I use the Queen's Chair.


    perhaps I am mis-understanding.....
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    blaise's Avatar
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    Default

    I thought about doing that, Pierre, but in the polar scope Cass moves relative to UM, so the problem still exists, doesn't it?

    I also thought about using Kochab's Clock, but the instructions for that assume that you can move the counterweight bar in relation to Polairs and Kochab so that the bar lines up with both. Again, still face the hard stop problem. Maybe I should remove the gear box : )

    Instructions also insist that you are able to determine dec within the polar scope reticle so that you can line the polar scope reticle dec axis with the coutnerweight bar. Don't think I have a way to determine that since there is no predefined way of doing so that I know of.
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    Default

    I thought about doing that, Pierre, but in the polar scope Cass moves relative to UM, so the problem still exists, doesn't it?
    No, they both move relative to the center of the cross hairs, which defines the NCP. The diagrams of UM and Cass are just to define where Polaris happens to be on it's circut around the NCP at this point in time, as indicated by the little circle.
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    Default

    I thought that, in regards to the reticle in the polar scope, that if you could not use UM as a reference constellation in the reticle, than you could not use any constellation on the reticle to use for reference. Confusing, as neither one moves independent of the other when you turn the counterweight bar. The higher cass rises the lower UM dips below the horizon, so if the hard stop prevents me from using UM as a reference, it should prevent me from using Cass as one, too.
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  10. #10
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    Default

    blaise,

    I haven't used the polar scope in a while but If I remember correctly on the EQ5 pro if you rotate the DEC 180 degrees from where you have it now, the motor housing will clear the mount and let you rotate the RA 360 degrees......

    I never rotated mine. I just eyeballed the spot where Polaris should be in the large circle and put it close to that position. It got me close enough to begin and then I did a CCD drift alignment. (about 10 minutes max for setup.)
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