Hello, The output flux of ‘getFlux’ corresponds to the field base_CircularApertureFlux_12_0_instFlux of the ‘src’ catalog of a calexp. I would like to know what is 12_0? and why it is used for flux among other aperture options ApertureFlux_4_5, ApertureFlux_70_0, …? Thank you in advance
I’m not sure what getFlux you’re actually referring to in the original post, but I hope this at least explains why that particular flux is special.
The 12_0 refers to “12.0” pixels (radius), which is the size of the aperture we use for calibration by default. Because we map that flux to the flux in the reference catalog, it’s effectively what we declare to be “total flux”. This may seem a bit strange - it implies that larger apertures measure something larger than the total flux, but note that essentially all photometry is relative, and computing an aperture correction to infinity is extremely hard, especially in the presence of blending.
Most other flux measurements we perform are also corrected to this calibration flux via our aperture corrections, which are essentially just the ratios of different photometry measures on bright stars, spatially interpolated over the images. This has some serious drawbacks (it relies on background subtraction being quite good, especially at the faint end, which is often not the case in our current processing), but something like this is necessary to put all photometry on the same system.
You can find further discussion in the HSC Pipeline paper (https://arxiv.org/pdf/1705.06766.pdf; there this measurement is referred to as a 4" diameter aperture), and a related discussion about how this interacts with fake source injection here.
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Thank you for your answer. So this 12 pixel radius is used for both extended and point sources. I thought that extended source fluxes are calculated with larger aperture radius