Dome Vs Sky Flats with Y4KCam

The ability to do photometry depends directly on the flats matching the sky. SMARTS provides only dome flats by default. After the first month of observatins, good sky flats were obtained, and these revealed that the dome flats only succeed in flattening to 1% the data only for a 2000x2000 region offset from the center (around pixel 1500,1500):

Sky dividied by dome:

This plot shows a cut through the middle (left to right):

This shows a plot through the middle (up to down):

Things are off by 7% at the top and on the right:

Images of T Phe were obtained by David James (Vanderbilt) on 2005-09-25, centered both at the "sweet spot" (1500,1500) and then offset to the upper right (3500,3200). Indeed, the images reduced with the dome flats results in a significant error in the photometry:


                            Instrumental magnitudes
                             Reduced using     Reduced using
                              Dome Flat         Sky Flat
Sweet spot (1500,1500)        13.888            13.883
Upper right (3500,3200)       13.949            13.888
diferences                    -0.061mag         -0.005mag