Appendix A: about eventual residual astigmatism on the corrected images
The wavefront analysis sensor (WFAS) is divided into several microlenses. Each microlens is coupled with 88 pixels of an intensified CCD working in the visible. The WFAS takes the light gravity center over the 64 pixels, for each microlens. The microlenses are squared, while the pixels are rectangular. Because of this slight mismatch in shape, the microlens area is smaller than the corresponding pixels area.
The reference position for the WFAS is given by observing an optical fiber source. Since the microlens area and the pixel area are different, the reference position is not the pixel area center. During the observation, any offset between the reference and the current position is supposed to be due to atmospheric turbulence effects. This assumption is correct when one is looking at a point-like object. But if the wavefront analysis is performed on an extended object, the situation is more complex.
With an extended object like a galaxy, the pixels are fully illuminated, and the light gravity center is shifted in regard with a point-like source. This induces an astigmatism effect. This effect is not present on the PSF reference star, which is a point-like source for the WFAS. So, residual astigmatism cannot be removed while applying the deconvolution by the PSF. We have estimated the residual elongation due to this effect on NGC 7469: the images are not elongated at FWHM, but we find an elongation factor of 1.3 in the north-south axis, at 25% of maximum peak intensity. This residual effect is quite small and its impact on our measurements was minimized by avoiding to perform a PSF deconvolution and by deriving diaphragm-like fluxes.
However, a warning is given that one might have to take care of residual astigmatism when observing faint extended objects with ADONIS. This effect should be detectable on the WFAS control monitor during data acquisition. For a complete description of this effect, see Gendron et al. (1998).
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998