LBC PSF Variation & image quality
The data taken during the Nov commissioning run have been often plagued by poor guiding, and are not very useful to analyze the PSF stability across the field.
A first hint of the quality can be obtained by looking at the 20 minutes exposure obtained in Dec in the U band, with excellent seeing (0.7") and good guiding. The image below shows the observed FWHM (as measured by SExtractor on stellar-like objects) as a function of the radial distance (top panel) and of the XY position in the focal plane (central and lower panel), all measured from the nominal central position of the optical design. Colors correspond to different chips: black for the right chip (#1), red for the central one (#2), blue for the left one (#3) and green for the upper, "horizontal" one.
The data show a 10% increase of the FWHM from the center to the edge of field.
The data also show that the region with better image quality is not centered on the nominal central position of the optical design. The increase of the FWHM is consistent with the design specs. The de-centering is within the alignment specs as well, but can also likely be improved by a better active optics control.
We note that, since the images have not been registered to a constant pixelsize, a 3% increase of the FWHM is due to the varying pixelscale.
A more refined analysis of the image quality is undergoing using the new data of the dec run and will be released as soon as possible.