PSF far wings and red halo in the photometry of galaxies - One more source of errors

PSF far wings have been measured in UBVRi at the 120 cm Newtonian telescope of Observatoire de Haute Provence, during 3 observing runs in 2000-1. The choice of appropriate star fields allowed us to extend the measurements up to a radius of nearly 3 arcmin, and down to a level of about 0.5 x 10(-6) of the central peak. It was found that these wings, farther than a radius of 15 arcsec, do not change with atmospheric seeing, but are dependent on the spectral passband and the time elapsed since the coating of the mirrors. The most prominent spectral effect is the red halo occurring with thinned CCDs, but PSF wings in U, B, and R may also stand above the V light wings. All PSF wings were greatly reinforced after 10 months of mirror ageing. The consequences of the far PSF wings for galaxy surface photometry and colorimetry have been studied by convolution of models with the measured PSF. Gray models of E-type objects acquire spurious colour gradients, large enough in V - i to reverse the classically measured ones, and sufficient in U B or U V to significantly bias the results. The colours along the major and minor axis are unequally affected for flattened objects. Experiments with a model of the lenticular NGC 3115 show that spurious disk colours may also be introduced. It has been verified that the effects of the red halo on V - i colour gradients may be corrected by convolving the frame i with the V PSF and conversely, before measuring the colour distribution. The same is true for the lesser effects in other colours. These corrections are made at the expense of resolution and cause further uncertainties in the results.