The native resolution of The ASAS Photometric Catalog is 15 arcsec (which corresponds to 14.8 arcsec/pixel resolution on the CCD). Typical astrometric accuracy is around 3-5 arcsec for bright stars, but could increase even above 15 arcsec for faintest ones. Also, due to the strong winds or mechanical flaws PSF of the stellar images may degrade substantially leading to the lowered astrometric accuracy even for the bright stars.
The unavoidable consequence of the fully automated process of creating the catalog is therefore a chance for having multiple catalog entries for the same star (even within the same observed field) separated by more then that native catalog resolution.
Trying to retrive all possible photometric data from the catalog one has to query catalog for the entries around the requested position. Our default search radius is 30 arcesc.
This however has another drawback: photometry of two different stars (separated by less then 30 arcsec) could be returned as one item.
To avoid possible problems of this type we have decided to divide our data into 'datasets' separating data coming from different fields and individual catalog entries.
#---------------------Header lines ------------------- # # Number of points in all datasets: # #ndata= xxx # # # Each 'dataset' starts with the following line: # #dataset= 1 ; 1 F1845+48_614 # # In this example the first number (1) identifies sequential dataset # number, the second one (1) counts catalog entries within the same # field (F1845+48_614). # # ASAS designation derrived from the equatorial coordinates # in the format hhmmss+ddmm.m: # #desig= 183952+4323.1 # # Catalog coordinates. For brigth stars they were taken from Tycho catalog: # #cra= 18.664485 18:39:52.1 #cdec= 43.385547 43:23:08.0 # # Average reference magnitude & dispersion: # #cmag_0= xx.xxx #cmer_0= x.xxx # #cmag_*= ,... for each aperture. #cmer_*= ,... for each aperture. # # Number of data points skipped when calculating 'cmag' & 'cmer': # #nskip_0= xx #nskip_*= ,... for each aperture. # #Averaged coordinates of all measurements: # #ra= 18.664485 18:39:52.1 #dec= 43.385547 43:23:08.0 # #--------------------Rows-------------------------- # # Each data row consists of the following fields: # - HJD-2450000 # - magnitudes (one for each aperture) # - frame errors describing average photometric quality of the frame # (one for each aperture). These ARE NOT photometric errors of # each measurement, but average dispersion of the differences between # catalog and frame. They are used to determine 'grade' of the frame. # - frame number # - grade : # A - Data coming from the best data # B - Data coming from the average quality frame # C - Star was too faint to be measured on this frame (29.999 indication) # D - Data coming from the low quality frame # - frame: sequential frame number (not always uniq) # # 99.999 means, that star was detected, but it's flux measured through # respective aperture was negative. # 29.999 (grade C) means, that star was below detection threshold. #