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    Re: Any "on line" Star Position Computation available ?
    From: Antoine Couëtte
    Date: 2011 Jun 14, 10:14 -0700

    Re : http://www.fer3.com/arc/m2.aspx?i=116528
    [NavList] Re: Any "on line" Star Position Computation available ?
    From: navigationalalgorithms---com
    Date: 14 Jun 2011 11:12


    Dear Andr�s,

    In further reply to your last post hereabove referenced :

    - All the details you indicate can be found in the FK6 publications N�35 & N�37 of the Heidelberg Institute dated 1999.

    At that time, i.e. in 1999, there was still some (hot) debate about the "best" precession models. The FK6 "Catalog Builders" having worked for a number of years well before 1999 did not benefit the subsequent Precession refinements which came out just a few years later.

    By the time of the FK6 Publication the P01/P02 models from Bureau des Longitudes had started to get definite IAU attention and interest, eventually up to the point that IAU therefater endorsed the "P03" model as its official(IAU 2006) Precession Model. P03 was subsequently further challenged against very accurate last available VLBI results (accuracy up to or better than 2 * 10-4 arc seconds) and it was considered that - for the time being - no further refinement to P03 is necessary yet - see A&A 432,355-367 (2005) - .

    What makes me to question the accuracy of FK6 is simply a number a quite significant differences between FK6 and FK5 / IERS values :


    A few examples ?

    Just to name a few SINGLE stars (not double or triple stars obviously)

    -The 2000.0 coordinates for Beta Cet DIPHDA / DENEB KAITOS) - FK6 N� 22 , HIP 3419 - differ by over 2 arc seconds if you pick them up in FK6, or if you pick them up in the HIPNEWCAT / ICRS reference. There is a very significant 0.14 s difference in RA (equivalent to a 2 arc second difference ) and also an already significant 0.3 arc second difference for its Declination, and

    - The 2000.0 coordinates for Alfa Ari "HAMAL" - FK6 N� 74 , HIP 9884 - also differ by over 2 arc seconds if you pick them up in FK6, or if you pick them up in the HIPNEWCAT / ICRS reference, with a very significant 1.3 arc second difference just for its declination.
    If you compute the apparent coordinates of HAMAL for Jan 01, 2000 and with both Catalogs and will get results differing by the same 1.3 arc second difference just for Declination, and over 2 arc seonds in total. Here, since computation is performed for the Catalog Epoch, neither Precession, nor Equator "zero point " come to play.

    Comparison between FK6 and HIPNEWCAT stars published in the ICRS frame shows here and there differences well above the claimed accuracies for either FK6 or HIPNEWCAT. In other words, with a 2.0 arc second difference including a 1.3 arc second declination difference totally ourside either Catalog claimed accuracy, I can see no way of reconciling data between both Catalogs.

    This is why I am personnally very curious here.

    From the their publication dates (FK6 1999) and (HIPNEWCAT 2007) I would favor HIPNEWCAT for plain RA and Dec 2000.0 coordinates, while the FK6 Proper motions, observed over up to 2 Centuries, might still be better than the HIPNEWCAT Proper motions.

    It is possible that for FK6 Computations, there are specific computations rules to be followed, but nowhere could I read about such "FK6 only" computations.

    This is why I am seeking advice from any NavList Member familiar with the demonstrated/proven accuracy of the current Star Catalogs, who could clarify and explain the Catalog differences we can observe here and there between HIPNEWCAT and FK6.


    Best Regards

    Antoine

    PS : If this conversation becomes too complex, and of limited interest to NavList members, then we can continue it privately.


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