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RE : http://www.fer3.com/arc/m2.aspx?i=116534
[NavList] Re: Any "on line" Star Position Computation available ?
From: cfuhb-acdgw---net
Date: 14 Jun 2011 13:54

Dear Paul,

Thnk you very much for your explanation involving a Proper motion correction between 1991.25 and 2000.0 performed in ICRS 2000.0 Reference System.

However, and for both Stars earlier quoted, I still have the following "reconciling" difficulties.

Therefore, please be so kind as to consider the following (hopefully no typos since I used copy/past for almost all quoted figures) :

*******

1 - For the star DIPHDA / DENEB KAITOS (Beta Cet)

1.1 - From the FK6 (Ref : 2000.0 / Epoch 2000.0 ) we get the following values for FK6 Star Number 22 :

1.1.1 - QUOTE RA = 0 43 35.371451 and Dec = -17 59 11.77811 UNQUOTE, which is in full agreement with the data you just indicated.

1.2 - For the same DIPHDA / DENEB KAITOS (Beta Cet) HIP Star 3419 from SIMBAD

(http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HIP3419&submit=SIMBAD+search)

we get the following values :

1.2.1 - QUOTE ICRS coord. (ep=J2000) : 00 43 35.22827 -17 59 12.0627 ( Optical ) [ 0.13 0.12 0 ] A 2007A&A...474..653V UNQUOTE

Note : I think that these " ICRS coord. (ep=J2000) " are HIPNEWCAT coordinates derived from the 1991.25 HIPNEWCAT coordinates into Epoch 2000.0 since the very end of the just previously quoted line directs us to HIPNEWCAT.

and also get for DIPHDA / DENEB KAITOS from SIMBAD for FK5 Star Number 22 :

1.2.2 - QUOTE FK5 coord. (ep=J2000 eq=2000) : 00 43 35.228 -17 59 12.06 ( Optical ) UNQUOTE

We observe that Coordinates 1.2.1 (HIPNEWCAT ?) and 1.2.2 (FK5) are (almost) exactly the same, which would seem to fully confirm (because of the presence of the FK5 System which IS DEFINITELY in Eq. 2000.0 for Epoch 2000.0) that they are BOTH derived for Epoch 2000.0 in a 2000.0 reference Sytem (actually ICRS here).

AS A CONCLUSION, there is a definite 2 arc second difference between the coordinates of DIPHDA / DENEB KAITOS between FK6 Epoch 2000.0 coordinates and HIPNEWCAT / FK5 same Epoch 2000.0 coordinates.

*******

2 - And also as regards the star HAMAL (alfa Ari)

2.1 - From the FK6 (Ref : 2000.0 / Epoch 2000.0 ) you get the following values for FK6 Star Number 74 :

2.1.1 - QUOTE RA = 2 7 10.407240 and Dec = +23 27 44.71195 UNQUOTE

2.2 - For the same HAMAL (Star Number HIP9884 ) from SIMBAD

(http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HIP9884&submit=SIMBAD+search)

get :

2.2.1 - QUOTE 02 07 10.28580 +23 27 45.9989 ( Optical ) [ 0.22 0.14 0 ] A 2007A&A...474..653V UNQUOTE

Note : I think that these " ICRS coord. (ep=J2000) " are HIPNEWCAT coordinates derived from the 1991.25 HIPNEWCAT coordinates into Epoch 2000.0 since the very end of the just previously quoted line directs us to HIPNEWCAT.

and also get for HAMAL from SIMBAD for FK5 Star Number 74 :

2.2.2 - QUOTE FK5 coord. (ep=J2000 eq=2000) : 02 07 10.286 +23 27 46.00 ( Optical ) UNQUOTE

We observe that Coordinates 2.2.1 (HIPNEWCAT ?) and 2.2.2 (FK5) are (almost) exactly the same, which would seem to fully confirm (because of the presence of the FK5 System which IS DEFINITELY in Eq. 2000.0 for Epoch 2000.0) that they are BOTH derived for Epoch 2000.0 in a 2000.0 reference Sytem (actually ICRS here).

AS A CONCLUSION, there is a definite 2 arc second difference between the coordinates of HAMAL between FK6 Epoch 2000.0 coordinates and HIPNEWCAT same Epoch 2000.0 coordinates.

*******

So, in spite of your very interesting explanation, I am still "a bit in the dark" since the SIMBAD tool give informations which do not permit (at least to me) reconciling HIPNEWCAT data for Epoch 2000.0 in ICRS 2000.0 Reference System with FK6 data for the same Epoch and in the same Reference System.

I will continue investigating, and will also perform proper motion corrections, exactly as you did (I am assuming in 3 dimensions), but ... meanwhile if you can "destroy" my both counter-examples given here-above (i.e. show me where I am wrong in these 2 counter-examples), I will be the happiest of men on Earth.

Thanks again for your interest here.

Best Regards

Kermit

Antoine M. "Couëtte"

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