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Thread: Question about Double Star Postion Angle

  1. #1
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    Default Question about Double Star Postion Angle



    Hello All:

    I've been delving further and further into the study, observation and recording of Double Stars. Doing much research and I've come across a question that I'm sure is a basic question, but I just want to make sure I have it right before I go any further. It's about determining a Double Stars position angle. I get the concept, but I'm unsure about the depiction shown on different graphs.


    Sirius_PA.jpg

    As you see here, North is up (0 degrees) and South is down (180 degrees). East and West are reversed and I assume that it is depicting the view from a SCT w/diagonal, which is what I use (102mm f/13). I am using the Astro League Double Star Observing list to make my observations. On that list, they list the position angles, but I'm curious to know the positions that their directions are on their graphs, I.e, could south be at the top and north at the bottom, etc...?


    Is there a standard that is used when making lists? Am I wrong completely in understanding this? Thanks in advance of the help.

    Vince
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  2. #2
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    Default Re: Question about Double Star Postion Angle

    The image is not reversed. What is shown is how the directions appear to the naked eye. If your scope inverts the image due to an odd number of reflections, one of the axes would be reversed. Hint: you are not looking down at a map. You are looking up. That makes a difference. Imagine holding a properly-oriented map over your head, and looking up at it from the back side.

    Position angle is measured counter-clockwise relative to an uninverted image, or clockwise on an inverted image, from north.
    https://en.wikipedia.org/wiki/Position_angle

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    Default Re: Question about Double Star Postion Angle

    When you do the sketch you include north and east/ west as depicted in your view.
    As long as your direction match the view it is good.
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    Default Re: Question about Double Star Postion Angle

    Chart is correct, west is the direction that the star drifts when the telescope drive is stopped. Actually it is the definition of west and can be used to calibrate the reticle or a CCD image of the pair. A fun pursuit that I did for many years when living in the light dome of the city, measured a couple thousand pairs with video camera 'lucky imaging' technique, didn't know that it was called that back in the '90s.

    Clear skies,
    Steve
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    Default Re: Question about Double Star Postion Angle

    Quote Originally Posted by KathyNS View Post
    The image is not reversed. What is shown is how the directions appear to the naked eye. If your scope inverts the image due to an odd number of reflections, one of the axes would be reversed. Hint: you are not looking down at a map. You are looking up. That makes a difference. Imagine holding a properly-oriented map over your head, and looking up at it from the back side.

    Position angle is measured counter-clockwise relative to an uninverted image, or clockwise on an inverted image, from north.
    https://en.wikipedia.org/wiki/Position_angle

    Fantastic Kathy! Thank you. I see what you mean. It makes sense to me. I appreciate the reply and the Wikipedia link (I need to check Wikipedia more often for this type of stuff)
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    Default Re: Question about Double Star Postion Angle

    Quote Originally Posted by Gabby76 View Post
    When you do the sketch you include north and east/ west as depicted in your view.
    As long as your direction match the view it is good.
    Thank you Gabby. This was going to be my next question and you answered it.
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    Default Re: Question about Double Star Postion Angle

    Glad to see a fellow double spliter here! Join our Double stars group.

    I find drift method most useful. I made the dial from the link below for my type of scope (SCT/refractor), point "W" where the stars drift in my EP, and you get the PA.

    https://bestdoubles.wordpress.com/tag/position-angle/
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    Default Re: Question about Double Star Postion Angle

    Quote Originally Posted by Bigzmey View Post
    Glad to see a fellow double spliter here! Join our Double stars group.

    I find drift method most useful. I made the dial from the link below for my type of scope (SCT/refractor), point "W" where the stars drift in my EP, and you get the PA.

    https://bestdoubles.wordpress.com/tag/position-angle/
    Perfect! I just joined the group and bookmarked the website! Thanks for the info!
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    Default Re: Question about Double Star Postion Angle

    Good luck on your double star observations.
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