Return to Home Page
Hi all, I would like to recall my statement re the value of the systematic difference in astrometric results for the LMC variable/nova between my morning and evening observations. It has been pointed out to me from various sides that the actual difference is 2 arcsec and not 0.3 as I wrongly stated. To simply explain to those that may be puzzled, the latter value (0.36) represented the RA differential in timesecs on the 24 h RA scale, which has to be converted to the RA 360 degree scale (to be x15) in order to be in arcsec units. This value needs then to be multiplied with cosine of the DE. For deriving the total offset, that value then needs to be combined with the measured DE difference, Pythagoras way. I am sorry for the confusion caused by this and would like to apologise. The awful thing is that I was aware of this time-degree conversion from calculating SN off-sets in my SN reports... The exact consequence of all this is not clear yet. Due to the established large measuring imprecision the 'nova' could still be at the position of the indicated suspect. On the other hand, since the latter's position seems to fall outside the measured RA range, the new star is more likely different from this perceived suspect. Spectra will help, but I wonder if this will be done soon as the LMC is well below the South pole during the 12h (11.5h here) observing nights. As most of my LMC images were taken during twilight, I expect to get better resolving images from the faded object during pre-dawn within a week. Possibly a close double star will appear. I'll let you know. For those interested I have .fts images available of this LMC region taken on UT 20/6 (morning and evening), and 24/6 (evening). Coarse astrometry on the latter image, using two stars only, gave a position 05 08 25.88 -68 26 22.6 which is within 0.5arcsec from my 20/6 (evening) astrometry using the same parameters. Regards, Berto Monard Bronberg Observatory >>> "Berto Monard" <LAGMonar@csir.co.za> 06/24/03 08:53AM >>> Again.. where does the 2 arcsec come from? The systematic difference between the two sets is 0.3 arcsec, not 2. The derived mean position is not much larger than the generally accepted uncertainty of catalogued data, whom do differ amongst themselves at the same order of magnitude... I did use only two bright reference stars and as Taichi mentioned the software selection mechanism does contribute a fair amount. That .fits image should have been sent long ago. My neuron channels are more than 2 arcsec off. .. Regards, Berto >>> Taichi Kato <tkato@kusastro.kyoto-u.ac.jp> 06/24/03 03:18AM >>> >> I generally don't consider 2arcsec as a "small systematic difference". >> ...high airmass...a more likely problem is differential refraction... What would be the expected amount of differential refraction at the Berto's airmass? This problem could be avoided using reference stars with similar colors to the object. The "hidden" problem of the software could be a more significant cause, though... We (naturally including Yamaoka-san) would be happy to receive FITS files (as usual) from Berto to make further astrometric analysis. The image would be also useful for web presentation for helping other observers' identification. In any case, a 2-arcsec accuracy of astrometry is hardly sufficient to confirm the identification in the LMC field! Regards, Taichi Kato -- 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. -- 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.
Return to the Daisaku Nogami
vsnet-adm@kusastro.kyoto-u.ac.jp
