For processing data which already has a good WCS (such as CFHT and SDSS) it is unnecessary to perform an initial fit of astrometry before computing the PSF. Furthermore it is risky to do so because the astrometry fitter may fail.

At present CFHT handles this by overriding the CalibrateTask with its own variant task and a variant run method. However, the amount of code duplication required is unacceptable.

To avoid this I suggest we add the ability to disable the initial astrometric fit.

I also think we should Refactor CalibrateTask so its run method is much simpler and easier to override. But even if we do that (and perhaps it should be a separate ticket) it is such a common need to not run the initial astrometry that I think one should not have to override the run method to do it.