The optical quality of the ESO 1m telescope was measured and improved by R. Moliton, using a curvature sensing software (EF, by Roddier). Results follow:

• Before telescope alignment (April 27, 1997):

Total aberration: 1.2", due to secondary decentering (3.69 mm), spherical aberration at the DENIS focus (0.6"), and main mirror moving in its cell.

• After telescope alignment (May 1997):

Remaining secondary decentering: 0.86 mm (0.61 mm NS and 0.61 mm EW); triangular aberration due to slight constraint on main mirror to prevent it from moving.

Best total aberration, excluding spherical: 0.56" at 8 deg. from zenith toward east: 0.26" astigmatism, 0.32" coma, 0.38" triangular.

This increases to 0.75" at 15 deg. from zenith toward west.

• After second telescope technical period ( November 1997):

The problem of the moving primary has been solved by a systematic check of the astatic levers and some work on the mirror cell. Most of the residual astigmatism has vanished giving a total aberration contribution at zenith of 0.35" at the best focus:

Astigmatism=0.15" / Coma=0.24" / Triangular=0.21" / Spherical=0.002" Quadratic=0.02" => TOTAL=0.35"

However, some thermic effects which are probably producing mirror seeing varying between zero and 0.7" at maximum amplitude have been reported.

The image quality has been measured at the best focus, using gaussian 2Dfit on images taken with a direct CCD in good seeing conditions (0.5"). The average measured FWHM is: 0.63"which is close to the theoretical optimum in such seeing conditions (optimum=0.62").