Status: To Do
Fix Version/s: None
Currently lsst.verify_metrics defines AF1,AD1; AF2,AD2; and AF3, AD3 metrics for the three specification thresholds: minimum, design and stretch, see:
The pair AFx, ADx characterize the distribution of the astrometric distance for stellar pairs with separation of D=5, 20 and 200 arcmin. Thus, strictly, there is a dependency on AFx when you compute ADx. For instance if you compute AD1 assuming the minimum specification for AF1 you have AF1_minimum and AD1_minimum, and so on.
As a result, and adding AM1, AM2 and AM3 to the list we end up with 21 metrics to monitor the astrometric precision, which seems unnecessary.
Talking with Keith Bechtol he suggested that we should look at AFx, ADx as a pair since they characterize the same distribution for a fixed separation D.
LPM-17 says "No more than AFx % of the sample will deviate by more than ADx marcsec from the median."
In that sense, AFx or ADx could be primary or secondary, depending on whether we want to quote a percentage or a value.
Keith also suggested a nice visualization. If you think that AF1 and AD1 form a plane with axes of value and percent, the actual data makes a curve in that plane (that represents the distribution, basically a CDF) and the design, minimum, and stretch specifications represent points in that plane. See attachement.
In summary, we could reduce table Table 18 of LPM-17 to 6 metrics: AMx and "AFx+ADx" for x=1,2,3.