[vsnet-chat 2717] Re: Supernova Magnitude Observations

[Hitoshi YAMAOKA <yamaoka@rc.kyushu-u.ac.jp>]

Dear Prof. Skiff,

>The preferred astrometric reference catalogues are Tycho-2 and
>USNO-A2.0 in that order.  If your CCD field is large enough to
>include at least 9 or 10 Tycho-2 stars that are not much saturated,
>then these should give better results than USNO-A2.0.

>     All this sounds fine.  Be aware that if the original GSC version
>1.1 positions are being used as reference for astrometry, then the
>results are subject to systematic errors of sometimes over an
>arcsecond (even if internal errors are small) due to the way in which
>the GSC v1.1 was reduced.  It is much preferable to use either GSC
>v1.2 or Bill Gray's GSC-ACT.   

Could you give the comparison between USNO-A2.0 and GSC 1.2 or ACT as
positional reference ?

Sincerely Yours,
Hitoshi Yamaoka, Kyushu Univ., Japan
yamaoka@rc.kyushu-u.ac.jp

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