[vsnet-alert 3689] Re: NGC 1097 BVRI sequence

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

>The stars below can also be used to make a local zero-point
>adjustment to USNO-A magnitudes for fainter stars.

Identifications are:
              A1.0 ID        rmag bmag    A2.0 ID       rmag bmag
NGC 1097 1 = U0525_00752843  10.9 11.8 = U0525_00943089 11.2 11.9
NGC 1097 2 = U0525_00752954  11.0 12.0 = U0525_00943218 11.1 11.7
NGC 1097 3 = U0525_00751104  12.4 14.0 = U0525_00941004 12.2 14.0
NGC 1097 4 = U0525_00751705  12.8 13.6 = U0525-00941729 13.2 13.8
NGC 1097 5 = U0525_00751172  13.1 14.7 = U0525-00941087 13.7 14.4

From Skiff's sequence, B, V and R are given as:
             B     V     R
NGC 1097 1  12.10 11.45 11.09
NGC 1097 2  12.24 11.51 11.10
NGC 1097 3  14.27 13.04 12.39
NGC 1097 4  13.91 13.27 12.89
NGC 1097 5  14.39 13.71 13.31

Between R and USNO rmag (both A1.0 and A2.0), there seems no
systematic difference.  The dispersion (|R-rmag|), however, is larger
with USNO-A2.0, especially for fainter stars.  I guess The CCD
observers would be able to use USNO-A1.0 (or A2.0) rmag directly for
comparison. 

The VSNET sequence, which is expected to be provided shortly by Dr.
Kato, is based on A1.0 mag with the transration 
v = rmag + 3/8*(bmag-rmag).  For these five stars,
             v
NGC 1097 1  11.24
NGC 1097 2  11.38
NGC 1097 3  13.00
NGC 1097 4  13.10
NGC 1097 5  13.70

are calculated.  Compareing with V, there is small (0.1 mag or so)
systematic difference, and dispersion is larger for brighter star.
I guess, however, it is enough agreement for visual observer (though
17 mag star is a touch target).

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

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