Resolve 20 mas astrometry bias in the DC1 datasets

[cwwalter]

There is some discussion of this issue in #39. But, I wanted to put it into it's own issue and understand where we are on this and what our plan going forward is.

Using the dithered imSim catalog which have been matched with the reference catalog we see a 20 mas shift between the measured and true value. Stars:

and

Galaxies:

More detail on how these plots were made is here:

https://github.com/cwwalter/DC1_Analysis/blob/master/Notebooks/Star-Galaxy.ipynb

@fjaviersanchez has shown that similar results are seen in the PhoSim files.

Our hypothesis (from @ctslater) is that this is due to the fact that although we accounted for proper motion in our catalogs currently DM doesn't in the stack. See:

https://jira.lsstcorp.org/browse/DM-8828

@danielsf Made some plots of the proper velocity of the MW stars in CatSim (perhaps he can repost them below) but it still isn't exactly clear why we see what we do.

Another question is what model is used for the MW stars. Apparently their motion comes from a model from @mjuric. If he could add a reference that would be good.

If this is in fact the problem the solution is likely to wait for reprocessing with a version of the stack that properly includes proper motion. But, we should actually confirm we understand this and there are no extra issues.

The consensus seems to be that if we do understand it, it is not worth trying to "correct" for the effect before reprocessing. Please do comment if this is not the case.

[danielsf]

Here are my plots.

Each set of 4 panels represents one of the undithered DC1 fields of view. Each of the sub-panels represents a different cut in magnitude. The plots show the distribution in (proper motion RA, proper motion Dec) space of the stars in the CatSim database (i.e. the data used to generate the InstanceCatalogs which were used to generate the images). Units of the axes are milliarcseconds per year. "stars per pixel" refers to the number of sources in one particular (muRA, muDec) bin (a pixel of these plots).

[fjaviersanchez]

I think this is solved. Please @cwwalter feel free to re-open it if you think this is still relevant