PA1Measurement¶

class lsst.validate.drp.calcsrd.PA1Measurement(matchedDataset, bandpass, numRandomShuffles=50, verbose=False, job=None, linkedBlobs=None, metricYamlDoc=None, metricYamlPath=None)[source]¶

Bases: lsst.validate.drp.base.MeasurementBase

Measurement of the PA1 metric: photometric repeatability of measurements across a set of observations.

Parameters:

matchedDataset : lsst.validate.drp.matchreduce.MatchedMultiVisitDataset

bandpass : str

Bandpass (filter name) used in this measurement (e.g., ‘r’)

numRandomShuffles : int

Number of times to draw random pairs from the different observations.

verbose : bool, optional

Output additional information on the analysis steps.

job : lsst.validate.drp.base.Job, optional

If provided, the measurement will register itself with the Job object.

linkedBlobs : dict, optional

A dict of additional blobs (subclasses of BlobBase) that can provide additional context to the measurement, though aren’t direct dependencies of the computation (e.g., `matchedDataset).

Notes

We calculate differences for 50 different random realizations of the measurement pairs, to provide some estimate of the uncertainty on our RMS estimates due to the random shuffling. This estimate could be stated and calculated from a more formally derived motivation but in practice 50 should be sufficient.

The LSST Science Requirements Document (LPM-17), or SRD, characterizes the photometric repeatability by putting a requirement on the median RMS of measurements of non-variable bright stars. This quantity is PA1, with a design, minimum, and stretch goals of (5, 8, 3) millimag following LPM-17 as of 2011-07-06, available at http://ls.st/LPM-17.

This present routine calculates this quantity in two different ways:

RMS
interquartile range (IQR)

and also returns additional quantities of interest:

the pair differences of observations of stars,
the mean magnitude of each star

While the SRD specifies that we should just compute the RMS directly, the current filter doesn’t screen out variable stars as carefully as the SRD specifies, so using a more robust estimator like the IQR allows us to reject some outliers. However, the IRQ is also less sensitive some realistic sources of scatter such as bad zero points, that the metric should include.

Attributes

rms	(ndarray) Photometric repeatability RMS of stellar pairs for each random sampling.
iqr	(ndarray) Photometric repeatability IQR of stellar pairs for each random sample.
magDiff	(ndarray) Magnitude differences of stars between visits, for each random sample.
magMean	(ndarray) Mean magnitude of stars seen across visits, for each random sample.

Attributes Summary

`label`
`metric`
`units`
`value`

Attributes Documentation

label = 'PA1'¶

metric = None¶

units = 'mmag'¶

value = None¶