Re: OT: algorithm > Pixel X Y to RA Dec

From: Greg Roberts (
Date: Thu Mar 19 2009 - 21:56:06 UTC

  • Next message: Bjoern Gimle@GlocalNet: "Possible obs of USA 161"

    Evening again - mainly for John
    Ive done a little thinking about Johns request and sent him some code directly 
    but some was rejected by his server. As I said in my previous message I did 
    Astrometry for about 10-15 years but this was about 20 years ago and Ive 
    forgotten most of it. The observatory was very active in stellar parallax work 
    and I wrote the fortran software used to reduce the photographic plates taken 
    with a 24 inch refractor telescope. I MAY still have the code somewhere but 
    basically five or six reference stars were chosen whose positions were 
    accurately known and these were measured on a Zeiss measuring machine as well as 
    the parallax star so one ended with the X,Y coordinates and RA/Dec of the 
    reference stars. This was then solved using the METHOD OF LEAST SQUARES and 
    using the star DEPENDENCIES and a PLATE SOLUTION derived.
    There were other things involved like aberration and the parallax factor 
    ,precession etc - anyway quite a procedure. We ended with an RA/Dec and proper 
    motion for the parallax star.
    We also did astrometry on fast moving asteroids and comets and in fact provided 
    ground support for the GIOTTO Halley probe during its close fly-by to Comet 
    Halley. In those days computers were scarce and the observatory had a NOVA 
    computer with 16k memory(I think?) which was much much in demand so most of the 
    reductions were done by hand ! Had to write the fortran progam in about 16 
    pieces and run it them in succession. Also used punched cards to input the data- 
    those were the days!
    Possibly a web search on the capitalized items will help as well as ASTROMETRIC 
    PLATE SOLUTION. There are other ways of tackling the task - there was for 
    example the CHRISTIE-DYSON method and I think also one by FRANK SCHLESINGER and 
    several others which escape me at the moment.
    I think I also still have PASCAL code for using the method of least dependencies 
    which I will try and locate tomorrow.
    Again using ancient memory I think the standard text book decades ago was 
    SPHERICAL ASTRONOMY by Smart. I MAY have an E-book version of the book which I 
    will also try and locate.
    Hope this helps
    Cheers- my bedtime now
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