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    Re: Big fat sunspot and a solar eclipse
    From: Paul Hirose
    Date: 2014 Oct 27, 15:22 -0700

    On 2014-10-24 21:41, Francis Upchurch wrote:
    > Does anyone know of modern equivalents for amateur astronomers to measure 
    small angular differences (eg double stars) via a telescope?
    
    Filiar micrometers are still manufactured, but a good new one like the
    NEEDLEYE is about $2500. It's a beautiful device, though.
    
    http://www.observatory.org/bfm.htm
    
    At the other end of the complexity spectrum is the ring micrometer,
    which has no moving parts. In the old days it was a precisely circular
    metal ring cemented to a slip of glass at the focal plane or perhaps in
    a special eyepiece. I don't think any such thing is available anymore,
    so nowadays the edge of the field of view is the "ring".
    
    With a ring micrometer Earth rotation moves the objects through the
    field. The appearance and disappearance of both bodies is timed with a
    stopwatch. Chauvenet vol. 2 chapter X explains the principle and the
    formulas for reducing the observations:
    
    http://books.google.com/books?id=eTs5AQAAMAAJ&pg=PA3
    
    For a much more recent book, see "Observing and Measuring Visual Double
    Stars," edited by Argyle:
    
    http://link.springer.com/chapter/10.1007/978-1-4614-3945-5_12
    
    Here a beginner at double star measurement reports results with the ring
    micrometer method:
    
    http://www.cloudynights.com/page/articles/cat/articles/how-to/learning-to-measure-double-stars-r1173
    

       
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