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[vsnet-chat 6233] V4641 Sgr, Discrepance between Visual and CCDV mags

Mystery solved.  Ron Stubbings kindly sent me the Henden/Sumner photometry for the field.  Comparing the AAVSO chart 1813-25e for V4641 Sgr with the Henden photometry there is approximately a 0.4 mag difference in zero point.  As a result of this, my measurements based on the AAVSO sequence were about 0.4 magnitude fainter than observations based on Henden photometry.  I will start using the Henden sequence for comparison stars.

Regards,
Doug West (WJD)


In a message dated 4/14/2003 8:05:46 AM Central Standard Time, LAGMonar@csir.co.za writes:


Doug,

I didn't check up on the 'position' you gave, but the problem might be
that you use the AAVSO sequence and not the Henden calibrated sequence.

For targets with no history before 1997, there is no reason to use
preliminary and inaccurate AAVSO sequences, possibly derived from
USNO-A2.0 . Most observations, visual and otherwise, reported on V4641
Sgr use the Henden sequence.

I must admit that V4641 Sgr looks about 0.5 fainter than the usually
reported 13.5 +/-0.2 V based on Henden stars, but the reason for this
could be the bright MW background and fair reddening of this region,
diminishing the 'visual' contrast.

I don't think it to be a not up-to-usual-standard field calibration
from the master, but field standards might not have been very good or
possibly the background subtraction not adequate.

Berto Monard / MLF


>>> <DWest61506@aol.com> 04/12/03 11:31PM >>>
In a message dated 4/11/2003 7:19:46 PM Central Standard Time,
tkato@kusastro.kyoto-u.ac.jp writes:


> Re: [vsnet-campaign-v4641sgr 168] Re: V4641 Sgr, recent data
>
> > > # I wonder what is the reason for a 1-mag difference between
visual and
> > > # CCV V-band observations.
> > >
> >
> > I am guessing the difference is due to emission lines.  This is
similar
> to
> > what is observed in novae during the nebular phase.
>
>     Less likely.  V4641 Sgr lacks strong emission lines in
quiescence,
> which was the reason why the object was disregarded by most of CV
> observers.
> The spectrum strongly resembles that of an A0 star, indicating that
> there is little chance for an error arising from
transformation/response
> function problems.
>
> Regards,
> Taichi Kato
>

Taichi:
To make sure I was measuring the correct star, I measured the position
on one
of my images to be 18 19 21.6 -25 24 24.9 +/- 0.4 arcsec (based on GSC
1.1). 
IBVS 5068 gives a position of 18 19 21.7 -25 24 25.  I also checked
some
other images on the web and IBVS and I feel I am measuring the correct
star. 
There is a star at 18 19 22.66 -25 24 10.0 (my astrometry).  I measured
this
star at V=13.8 using AAVSO chart photometry.  This star is 20.7"
(PA=316
degree) from V4641 Sgr.  Maybe this is the star others are observing??
Regards,
Doug West

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