# Ast-R2 and Ast-R3¶

Ast-R2. Repeated, large-‐dither observations of a given set of bright stars reproduces their relative sky positions with <100 mas median absolute error.

Ast-R3. Residuals to astrometric map do not show systematic anomalies with respect to array position above 50 mas (e.g. glowing edges are at size expected, residuals are smooth across each CCD, relative CCD positions remain stable, etc.)

Since both of these requirements have essentially the same procedure, I have combined them.

## Prerequisites¶

- Set of repeated observations in each band with high stellar density.
- Need set of standard star catalogs, potentially SDSS, USNOB, or 2MASS.
- Exposures have been cataloged, with initial WCS solution

## Procedure¶

- Derive refined WCS solution by requiring overlapping objects from different exposures to be consistent in dithered "star flat" data.
- With improved WCS solution calculate the mean, and rms of the residual error. Also bin the residuals with respect to position on the array, distance to the edge.
- Apply instrument WCS soln to MiniSurvey exposures (with refined exposure solutions) and measure RMS errors, binned residuals.
- Confirm that solution is stable over time and that MiniSurvey solution is consistent with StarFlat solution.

## Verdict¶

- Residual errors in the relative sky positions < 100 mas.
- No systematic anomalies above 50 mas.

## Consequences¶

If we do not meet the requirements we will need to determine the underlying cause and whether it can be corrected or improve the WCS model.