The comparsion star and check star mags do not match in the the AAVSO report format
Last updated on:
17 Nov 2009
According to the Official AAVSO Extended Format document (http:////www.aavso.org//observing//submit//extended.shtml), CMAG and KMAG report *instrumental* magnitude for the comparison and check stars, not the standardized or catalog values. Instrumental magnitude is simply
M[ins] = -2.5 log10 Intensity
where Intensity is the integrated value of the light within the aperture in excess of the background. Since this measurement depends on exposure duration, aperture and F-ratio, camera sensitivity, etc. in addition to the star's apparent magnitude, instrumental magnitude bears no relationship to
published (catalog) magnitudes. So it's perfectly normal, if at first surprising to the unsuspecting, that for example, a report shows ~12 where you were expecting ~8. The *difference* between CMAG and KMAG should however reflect the actual difference in published magnitudes for these two stars.
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