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    Re: Index mirror adjustment with offset mirror
    From: Luc Van den Borre
    Date: 2014 Jan 15, 21:41 +0100

    On 15/01/2014 20:48, Frank Reed wrote:
    > Yes, and as I mentioned in a post last night, a really useful trick is
    > to use objects (could still be dice) that are different colors. Marker
    > caps, legos, and other small toys work great for this.
    
    LEGO bricks, that's a great idea! They have a nice sharp edge (so the
    top face is clearly defined) and I hear they have very precise dimensions.
    
    > You also wrote:
    > "* It's not correct to use the top _edge_ of the dice. One must be
    > looking along the top plane."
    >
    > I'm not sure I understand the distinction you're drawing here. You sight
    > in line with the top edge of the two objects. Do you mean that one
    > should not be sighting from "above" trying to get the top faces of the
    > two dice to appear aligned? If that's what you meant, then yes, that's
    > an important distinction. The goal is to get the two top edges to align
    > in a straight line when viewed from exactly the height of the dice above
    > the arc.
    
    Yes, I meant 'edge' as the line segment for the intersection between 2
    faces of the dice. Rather than 'top plane' I should have said 'top
    face'. This is one of the things I did wrong - I was matching up the
    edges without checking the height from which I was looking.
    
    > Yeah, I don't understand those peculiar shapes. They may have been
    > modeled on a bit of testing gear created for the optics lab. Clearly
    > simpler objects work just fine.
    
    I found several posts in the archive, made by Alexandre Eremenko in 2004
    about this same situation, e.g.
    http://fer3.com/arc/m2.aspx/Adjusting-perpedicularity-Index-Mirror-Eremenko-oct-2004-w18186
    http://fer3.com/arc/m2.aspx/Perpendicularity-SNOT-Eremenko-oct-2004-w18298
    Since the SNO-T has a very offset index mirror, it is great that they
    put the correct procedure in the manual, but sad that they then shipped
    with these 'diopters' that don't have a top surface. Strange.
    
    Today I have mocked up a sextant arc and an index mirror in POV-Ray, a
    ray tracer, and it has confirmed what I described in my earlier post.
    It's a really fun tool to play with and surprisingly easy to use (the
    objects are defined in a text file with a simple syntax). I can
    introduce errors in alignment and increase the mirror offset and see the
    result on the reflection.
    
    > By the way, it's far better to call that Russian sextant a "SNO-T".
    
    Will do. I copied it from the PDF posted earlier today. I guess the
    importer preferred a less snotty sounding acronym for this noble instrument.
    
    Thank you very much for your reply,
    
    Luc
    
    

       
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