calibration

[Ray Carlberg <carlberg@astro.utoronto.ca>]

Christian - is there an explicit calibration plan? This is easy for the ultra 
deep, feasible for the wide deep, but a real issue for the very wide. 

An idea is that the survey itself build an explicit calibration plan, largely 
based on the 15% of clear, but poor seeing time (some fraction of that will 
be photometric). Of  course that time wouldn't be added to the cost of the 
survey--if done efficiently it may well represent a time saving for CFHT. 

An interesting technical suggestion is that the calibrations might be done
with a "window pane" which would include panels of the 5 SDSS filters. 
Whether this would work depends on the projection of filter plane on to the 
detector plane. That would allow each field to be calibrated in a single 
exposure, instead of 5 of them. Of course we would need to zero point this 
calibration to the true filters, but that is a one time only operation. 

Calibration of 0.03-0.05 mag is fine for galaxies, but definitely not the 
standard for stellar objects, which will be much of the interest of the 
ultra-wide. 

In any case, some discussion of the calibration procedures and expected 
precision should be included in the plan. 

--Ray