How do I know if aperture correction has been applied to aperture fluxes in a `src` catalog?

Data Management
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#1

If I load a src catalog from a data repo, how do I determine if aperture correction has been applied to the aperture fluxes.

The PSF fluxes list a flag and a value applied for the aperture correction. E.g.,

    (Field['D'](name="base_PsfFlux_apCorr", doc="aperture correction applied to base_PsfFlux"), Key<D>(offset=432, nElements=1)),
    (Field['D'](name="base_PsfFlux_apCorrSigma", doc="aperture correction applied to base_PsfFlux"), Key<D>(offset=440, nElements=1)),
    (Field['Flag'](name="base_PsfFlux_flag_apCorr", doc="set if unable to aperture correct base_PsfFlux"), Key['Flag'](offset=416, bit=2)),

but there is no equivalent for aperture fluxes.

Is the answer that an aperture correction is never applied to aperture fluxes?

Which flux measurement should be used for the photometric reference catalogue
#2

Currently, the circular apertures (base_CircularApertureFlux) are not aperture corrected. Of the base measurements, only the PsfFlux and GaussianFlux are aperture corrected by default.

I believe the omission of the circular apertures is deliberate. Historically, it was because of technical difficulties, but now I believe it’s not aperture corrected to maintain consistency with the historical practise and because it allows users to make their own curves of growth using measurements that haven’t been tortured by the pipeline.

#3

Thanks @price!

That completely makes sense and is consistent with the documentation. One shouldn’t have to document all of the things one didn’t do in a measurement.

#4

Hi,

I realise this conversation is a little old now but I thought I would try commenting here instead of starting a new thread. I am confused about aperture correction with regard to calibration. How is it possible to calibrate against a different instrument (HSC to PanSTARRS) if the aperture magnitudes used to calibrate are not aperture corrected? As far as I am aware the science pipelines use the 12 pixel diameter aperture magnitudes as the reference and target for calibrating but I would presume that both would need to be aperture corrected in order to be comparable.

Further I don’t understand why PSF magnitudes can be aperture corrected because I though they are not measured in an aperture but based on a fit of a given image size to the PSF model.

Apologies if these are basic questions but I cannot find the answer in the Bosch et al. 2018 paper which talks in general about aperture correction but not to which measurements it is applied.

Many thanks,

Raphael.

#5

All flux calibrations in the pipeline are relative. Raw measurements of fluxes aren’t useful by themselves for accurate photometry over a field because the PSF varies over the field. Aperture corrections are an attempt to correct for these effects. We choose an aperture that is large enough to contain all (most) of the PSF variation as the reference. This reference aperture is too large for good signal-to-noise measurements of faint objects (or doesn’t have the right shape, weighting, etc.), so what we do is apply an “aperture correction”. This is a model of the magnitude difference between the aperture of interest and the reference aperture for point sources as a function of position.

For LSST, we calculate and apply such aperture corrections for all flux measurement algorithms (even the model-based ones: e.g., PSF magnitudes use an aperture with a weighting function equal to the PSF), and the reference aperture is (by default) a 12 pixel radius aperture. The only exceptions I’m aware of are the circular apertures that are intended to be used for profile studies (and therefore applying aperture corrections would not be helpful). For this reason, the difference of magnitudes from any two flux measurement algorithms should be a distribution centered around zero for point sources. This is what is used for the base_ClassificationExtendedness algorithm which does simple star/galaxy separation: it thresholds on psfMag - cModelMag.

Some other surveys attempt to do an aperture correction to infinity. I don’t believe this is necessary, because as long as the reference aperture is chosen so that it contains all the PSF variation as a function of position, the correction from the reference aperture to infinity is constant. So maybe you don’t count every single photon from your sources, but you count a consistent fraction of them, which is what matters for most of the applications our users care about.

So calibrated magnitudes reported by the pipeline are relative to what we would measure for the magnitude of a point source in the reference aperture, scaled to what the reference catalog would report for that source: it’s all relative.

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#6

UPDATE: given this question drifted from the original topic I have asked it as again as a separate question:

Hi,

I have one further question. Given that all the flux measurements are relative to the PanSTARRS calibration fluxes, what is the recommended flux measurement to use for the calibrators? Are they PSF fluxes and limited to point sources (e.g. ’ gMeanPSFMag’)? We are adding our VISTA calibration fluxes using a positional cross match to the PanSTARRS reference objects. Currently we are using ‘curve of growth corrected’ aperture fluxes from the public VISTA surveys. It occurs to me that this choice is not optimal. We also have Kron fluxes available. I suppose it is somewhat arbitrary given the relative nature of the flux measurements but I wonder if different measurements were tested in terms of their ability to predict HSC fluxes and colour term performance.

Thanks again for your original answer which was very instructive.

Best,

Raphael.

#7

I think this question about the details of the Pan-STARRS catalog and/or the LSST reference catalog format is getting far away from the original intent of this topic. Would you mind posting it under a new topic?

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