from_archive() uses an average distortion

[altuson]

The function from_archive() is not simply loading a WCS from archival data. It retrieves the SPOC WCS, but replaces the SIP distortion with the mission lifetime average. This is resulting in a better overall fit, as illustrated by the example below.

I compared the predicted and measured positions for a sample of bright (V<18) asteroids in sector 1, cameras 1/2 using tess-asteroids:

• Measured position is the flux-weighted centroid inside the target aperture.
• Predicted position is the RA,Dec from JPL horizons converted to row,col using i) true SPOC WCS and ii) tesswcs from_archive() WCS.
• Offset = predicted - measured.

The results differ at the corners/edges of the CCDs (see figure). The solution from SPOC shows a gradient towards larger offset at the corners/edges, whereas the tesswcs solution shows a significantly smaller offset. This illustrates that using the mission lifetime average distortion has actually improved the overall WCS solution.

[christinahedges]

Thanks for reporting this Amy, the from archive function is indeed providing the mission-lifetime-averaged SIP polynomials rather than the as measured ones from SPOC. While this is behavior I meant to put into tesswcs, it's not actually documented anywhere. I'll add this as a documentation issue.

Thanks!