[vsnet-chat 753] Position of V17-NGC5128

[Mati Morel <morel@ozemail.com.au>]

Hi all,
The recent postings on V17-NGC5128 have prompted me to have a closer look at
the coordinates and ids of V17 and its companion. Both of these stars would
present problems for astrometrists and photometrists, being superimposed on one
of the brighest parts of the galaxy. The brighter star (the "companion") is
missing in the GSC (no surprise here), but is listed in both the southern DM
catalogues.  It is CoD-42 8389 (m=9.7), 13h18m8.6s  -42 23.0';  also known
as CPD-42 6250 (m=9.8), 13h18m9.3s -42 23.0'.  Coords are for Epoch 1875.
In view of the well known scale error of the DM catalogues, a visual mag. of
about 10.5  is more realistic (confirmed by my own visual observation some
years ago).

Adopting the CPD coordinates, and precessed forward to 1950.0 gives
       13h33m31.4s  -42 46'32"  (final figures in both ordinates unreliable).

[It may be useful if observers were to refer to the "companion" by one of the
above designations, rather than some vague or non-specific descriptor.]

Now, looking up the position of V17 in the GCVS yields the following:
(1950)  13h22m26.0s -42 45'16"
This does not tally at all with the position shown on the Bryant/Thompson
chart for NGC 5128.  By measuring the offsets from CoD-42 8389, I find the
position of V17 is approximately (1950) 13h22m35s -42 46'12".

It would be a real challenge if some enterprising astrometrist could get
an accurate position for V17.  A REAL challenge. All of the photos I
have looked at are grossly overexposed.

Regards,



 

/Mati

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