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I generally don't consider 2arcsec as a "small systematic difference". I think you are working at high airmass since you get both evening and morning observations, so a more likely problem is differential refraction in your field, especially if you are taking unfiltered exposures. If these were unfiltered observations, you should consider using a filter or two to narrow your bandpass (they have other uses than just to provide standardized photometry!). Arne Berto Monard wrote: > The following astrometric positions for the new variable were derived on images taken during early evening and pre-dawn. The scattering is indicated by error limits: the actual error incl systematic off-sets of the Tycho reference stars are not included and are estimated to be much larger: > > 2 morning sets: 05 08 25.56 +/ 0.03 -68 26 22.3 +/- 0.0 > 1 evening set: 05 08 25.92 -68 26 22.7 > > There seems to be a small systematic difference in RA between the sets taken about 12h apart. > > Considering the overall astrometric uncertainty, the derived positions coincide well with that of the earlier identified star. > > I'll do one more evening measurement when clouds stay away. > > Regards, > > Berto Monard > Bronberg Observatory / CBA Pretoria > >
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