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Subject: [vsnet-chat 6349] Re: RS Ophiuchi ASAS-3 chart > You can, of course, test this. You can use either the Landolt or > Cousins E-region standards to test the errors as a function of magnitude, > color, and consistency across the sky. Although it's trickier, you could > use a strictly periodic variable like an RR Lyrae or W UMa star to > look at consistency as a function of phase (i.e. how well the ASAS3 data > reproduce a recently-studied "good" variable). Done. I randomly chose an EW and a RRab stars and test ASAS-3 data quality. I used Hipparcos Epoch photometry transformed to V to compare. The results are at: http://ar.geocities.com/varsao/Curvas_UX_Ret_y_V338_Pup.htm The stars are UX Reticuli (EW) and V338 Puppis (RRab) Doing this I stumbled upon a discrepance that I had stumbled upon before: When I transformed Hp mag. to V magnitudes I realized there was a 0.015 mag. difference with the V values in the GCVS. Since it had happened to me before [V468 Pup (1999A&A...341..469C) and AZ Dor (1999A&A...346..134B)] I suspected they were Geneva V magnitudes not transformed to Johnson's V. All the stars that entered the GCVS by means of a paper with Geneva data suffer from this lack of transformation. (In this case: HD 47147 - A small-amplitude extreme metal-poor RRab pulsating variable, Grenon, M.; Waelkens, C. 1986A&A...155...24G) When one reads "V" in the GCVS assumes Johnson's V but it's not always the rule... It turns out that this 0.015 mag. difference is explained by the difference of the two photometric systems, where Geneva's V can be even 0.02 mag. brighter for the bluest stars. It also turns out that ASAS-3 data confirm this, which means its external accuracy is higher than 0.015 mag. for this star. Results: ASAS-3 data is comparable -or even better in the case of faint stars- with Hipparcos' accuracy. Comparing both sets of data is even possible a more accurate period determination for both stars: UX Reticuli - EW - V= 8.19-8.92 - HJD 2452721.546 + 0.490104 x E V338 Puppis - RRab - V= 9.09-9.27 - HJD 2451952.583 + 0.797324 x E (Hipparcos = 0.79733; Grenon and Waelkens = 0.79732) Best wishes, Sebastian.
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