Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 05/06/2021 - 15:07
Yes, I know ASAS is by now 'old stuff'! But recently I have had recourse to it. When choosing 'get data' there is a whole load of results for one star in several lists. I can see no help on their website (which is very dated now of course) as to what the difference between each of those lists is. I know it's something to do with the various apertures used in the survey but no further information as to which list to use. Any help from someone in the know?
Good data never become old ;)
The same star may have several datasets (coming from adjacent fields) and each one has its own zero point (there are different stars used in the calibration) so they have to be corrected individually.
You can usually see if there are differences by checking the ASAS light curve, where every dataset is plotted with a different color.
As a whole, ASAS-3 zero points in Southern fields are fainter than V up to 0.05 mag.
Since the star will be placed at different distances from the image center, some datasets may show increased scatter because the star will lie near the edge. Those datasets will also have a much lower number of observations so they are easy to identify. Their designations may also show that the astrometry was poor. The datasets with the larger number of points and the ASAS name matching the actual star's position are the ones to be used for the main analysis. The other ones have to be checked for problems.
The ASAS-3 results show 5 magnitude columns, each one corresponding to a different photometric aperture, from smallest (MAG_0) to largest (MAG_4). The first column displayed usually shows the correct aperture result but you have to check.
The best apertures for uncrowded stars are:
MAG_0 = <12.0
MAG_1 = 11.0-12.0
MAG_2 = 10.0-11.0
MAG_3 = 9.0-10.0
MAG_4 = >9.0
For crowded fields you may want to select smaller apertures, but scatter will also increase.
It may be better to use the blended magnitude and then correct the results subtracting the companion's light. Each case should be treated individually, depending on the degree of contamination.
For stars brighter than 8th magnitude, earlier data (up to 2452950) may have saturation problems (especially before 2452300). Because of defocusing, some nights may be useful though.
Afterwards, observations as bright as 6th mag. may be okay (especially between 2452950 and 2453300).
To detect saturation, compare the results between MAG_0 and MAG_4. If the difference is larger than 0.05 mag., then the object is probably saturated.
I keep finding new variable stars using ASAS-3 data when I have some free time. It is a very useful resource.
Cheers,
Sebastian