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    Re: Sextant index error and distance
    From: Peter Monta
    Date: 2014 Apr 6, 21:02 -0700
    Hi Debra,


    Hi, I've been trying to correct errors on my astra 111b.When i got it new i found a mirror screw very loose so i thought that was why i it had so many correctable errors ,as the certificate said it had none.

    Do you happen to remember which screw exactly?  I also have an Astra IIIB, though one made about ten years ago.  Glancing at it, there seem to be three types of screws on the mirror mounts:  one type passes through the coils of the wire springs, another clamps a small metal bar (something like a machinist's "toe clamp") against the mount on one end and the adjustment screw threads on the other, and finally the adjustment screw itself is the large one with the cylindrical shroud and square adjustment head.

    If any of the first two types of screws were loosened in shipping, just snug them back in and things should be fine.  They serve to keep the wire springs in place and to provide enough force to lock the main adjustment screws at their intended settings.  Just make sure that turning the main adjustment screw with the provided socket wrench is a smooth process and that there is no backlash or "lost motion".  Little torque is needed.  It's important not to run these screws to their limits---nominally adjusted mirrors will have them somewhere around their midpoints.

    Incidentally, the certificate refers to variations along the arc---it's understood that index error and the other small alignment errors (side error and frame-parallelism and telescope collimation) are part of normal adjustment.

     
    Anyway since then i've been using the tangent screw going back and forwards to both stars and horizon for micro adjustments.

    Ok.  I'm guessing you're using both stars and horizon to assess your index error.  They should indeed give similar results.  Try also the Sun by touching lower to upper limb and then vice versa.
     
    Now what i need to know is, when you are looking at an object which isnt the horizon,example something closer like a rock ,a tree, i get a step when i look at them ,but my horizon is fine even after rocking .The arc is zero      .Do i have more errors if this is happening?

    This is normal and doesn't represent an adjustment error.  The horizon mirror and index mirror are displaced vertically by 50 mm or so, and the resulting parallax can be large for nearby objects.  Think of the two mirrors as looking at the rock from two different points of view---the index mirror, being higher, must "look down" a little bit at the rock.  For a horizon, though, which will be at least 3 km distant and probably more, this parallax is at most 4 arcseconds, which will be swamped by the uncertainty in dip.  If you measure index error using a celestial body, then even this tiny error goes away.

    Cheers,
    Peter

       
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