Voor de record: Vandaag nog even meer verdiept in extinction. De reden dat de Unihedron en ASTAP waarden zo gelijk zijn is dat Unihedron calibratie circa 0.35 magnitudes gecorriseerd is. Dit is gelijk aan de extinction in het Zenith.
http://www.lightpollution.it/download/sqmreport.pdf
The different zero point is likely due to the fact that the relation has been obtained with outside-the-atmosphere magnitudes V’ and B’. As an example, assuming an extinction of 0.35 mag in V and 0.15 mag in B and replacing V’=V-0.35 and B’=B-0.15, where V and B are the apparent magnitude below the atmosphere, we obtain SQM−V= 0.2(B−V)−0.31 in agreement with my previous results. The zero point of the current SQM calibration made by Unihedron gives SQM≈V’ for stars with B’=V’, above the atmosphere, and consequently the below-the-atmosphere SQM-V correction factor for the alpha Lyrae spectrum come out different from zero. Fig.18 shows that for alpha Lyr is SQM-V=-0.35 mag arcsec−2.