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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 -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. Mailscanner thanks transtec Computers for their support.
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