# AAVSO guide question

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• #573766
Matthew Jenkins
Participant

Morning All,

I was reading the AAVSO’s guide to CCD variable star photometry (Chapter 5, point 5) and something stood out. It says that to measure the standardized magnitude of a variable you take the differential magnitude of your image, measured-v – comparison-v and then use that delta to add to the standard mag of the comparison star. Surely this is wrong and you should add a scaled version of that, as the delta will be for an image not at a standard intensity, so presumably values using this formula are underestimating the variables magnitude?

Cheers

Matt

#578257
Participant

Hi Matt,

Magnitude is a log scale so v(target)- v(comparison) is actually the ratio of the intensities of the two stars, which is independent of the actual count/intensity scaling in the image.

the magnitude of the comparsion star is itself a ratio (To a standard star with vmag =0 eg Vega)

Cheers

Robin

#578259
Matthew Jenkins
Participant

… and the lightbulb turns on – thanks!

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