When I carry out a photometry measurement using MAxim DL I select the target object and a number of reference stars. I have to enter the catalogue magnitude of each reference star. Maxim DL outputs the instrumental and calculated magnitude of the object star. But it also outputs the instrumental and calculated magnitude of all the reference stars. The catalogue magnitudes of the reference stars are used to calculate the magnitude of the target star. I understand how that is done. But I can't see how or why it would calculate the magnitude of the reference stars. The figures are generally close to the catalogue values.
I've asked this question on diffraction limited's forum but haven't had a useful reply yet. I asked the question 'how is the magnitude of the reference stars calculated?" and was told "The same way as it calculates the magnitude of the target star". But that doesn't make sense. - Chicken and egg problem.
Maybe this will help
https://cdn.diffractionlimited.com/help/maximdl/Photometry_Calculations.htm
Thanks, I have read that. My problem is this. It says -
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If I is the intensity of an object in an image, and t is the exposure time of that image, then
Let
, where
is the instrumental magnitude of reference star 
, and
is the catalog magnitude of 
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But it's talking about the calculation for the target star which is fine. But I don't see how you can apply it to a reference star. If you do, then in that case v is the instrumental magnitude of the reference star. But in the second equation it says ri is the instrumental magnitude of the reference star which we just calculated anyway. So that means v is the same as ri and the second equation simply becomes Vi = Rcat.
I think in the instances where it is calculating mag of Ref star, it makes that particular ref star the target
So I guess it takes out the original object star, makes the reference star the target star and then uses the remaining reference stars to calculate a magnitude for the new target star.
The calculation is a bit more involved than I said. Because you can have multiple reference stars it works out a magnitude for the target star relative to the first reference star. So that would be V1. If there are say 4 reference stars you end up with V1, V2, V3 and V4. Then it works out a weighted average of those magnitudes to get the final magnitude of the target star. The weighting is based on the SNR of each reference star.
I can see how that might work for calculating the magnitude of a reference star if you measure each reference star against each of the other reference stars and do the same weighted average for each one but I'm not sure what the point is. Also I suppose it would be possible to use the calculated magnitude of the target star in the calculation for each reference star. So the target star swaps places with the relevant target star in the calculation.