Hello. I’m starting to use Siril for photometry and can’t seem to find any documentation with details about how the relative magnitude value is calculated that shows up in the “PSF and Quick Photometry Results” as well as elsewhere? And is it intended to be equivalent to instrument magnitude or differential magnitude or something else? Clearly it is not the “true” or standardized magnitude. In addition to understanding how the relative magnitudes are calculated, is it possible as a user to access all the underlying data in Siril that is used to make the calculation? Thanks all!
Hello.
Did you read this:
https://siril.readthedocs.io/en/stable/photometry/quickphotometry.html
Yes, indeed, I did read it. But it is not helpful. It doesn’t provide the equation they use much less the variables that go into the equation. In order to acceptable photometry and submit the results to AASVO I need to be able to explain exactly how Siril calculates relative magnitude and whether or not it is intended to be equivalent to what the photometry community calls instrument magnitude. Any info you have would be most welcome. Thanks!
But magnitude uses always same computation. All software should use same formula. If not … You are not computing magnitude.
In this case this is: m = -2.5 * log(flux)
And this is explained here: Principles — Siril 1.2.0 documentation
Where does Siril tell me what the flux is? And does Siril measure flux in electron counts, ADUs or something else? When I draw a box around a star and then click on statistics I see lots of numbers but no indication of the units. Same when I turn on photometry mode and click on a star. Are those numbers the flux data used to calculate relative magnitude and what are the units? I’m probably just showing my abject ignorance of photometry, so thanks in advance for your patience.
Flux is simply the sum of pixel values minus the background level. It’s in a unit proportional to ADU, not directly ADU because of image calibration and conversion to 32 bits.
Magnitude values do not have a unit.
What is that unit (that is proportional to ADU) and how is it calculated? Can you share the math behind the calculation of the unit? And is the intensity values shown in the photometry results box in those units? Thanks again!
Yes the intensity value is the same scale as the intensity, it’s based on the image pixel values, which are derived from ADU. ADU is itself just a representation of what the camera gives, not a linear unit, and depending on image calibration (subtracted by dark, divided by flat, multiplied by the mean of the flat) and conversion to 32 bits (divided by 65535), it has been scaled.