I’m trying to get my head around what’s going on in the pipe_tasks/photoCal.py as the uncertainties we’re getting on our mags are too small (compared to their standard deviation from reference sources), so I’m putting out a request for a little help.
As I understand it from the top line description in
getZeroPoint, it should return the zeropoint, the width of the (src - ref) distribution, and the number of good points (after sigma clipping). Indeed, that does seem to what happens if
ngood == 0 at line 689. Otherwise, the sigma it returns is the inverse sqrt of sum(1/error**2), where error is, I believe, the error due to photon counting statistics. I’m struggling to work out how the latter gives a good estimate of the width of the (src-ref) distribution.
I can see how the latter would hold if photon statistics were the only source of errors, but that’s rarely (never?) the case in my experience!
We get far more appropriate errors if we instead adopt
sigma=sig on line 707, which would return an estimate of the width of the (src-ref) distribution (as on line 689).
What am I missing?