3. Wavefront analysis sensing
To better interpret the NGC 1068 observations with ADONIS, we need to track precisely which visible image of NGC 1068 is seen by the wavefront analysis sensor (WFAS) in its evaluation of the AO correction.
In order to recover this information, we have used HST images with 45 mas pixel size both in the continuum and in the lines (F791W, F547M F658N, and F502N), properly aligned and flux scaled to a same 1 sec exposure at the entrance of the telescope, to construct a composite image which mimics the one seen by the WFAS. The image alignment was performed using point-like sources in the field of view, which allows a registration within 10 mas (Tsetanov, private communication). The flux scaling has included corrections for the exposure time of each image and the mean HST camera efficiency over the corresponding filter. Then, each of these corrected images has been weighted by the corresponding mean WFAS wavelength response, before addition. We find indeed that the light received from NGC 1068 on the WFAS is largely dominated by continuum light and is not much weighted by the [O III] - line light distribution.
And finally, to mimic the image seen on the WFAS CCD, from which the AO corrections are computed, we have degraded the HST composite image by a "seeing" effect of 1": this includes both an atmospheric effect ( 0.6" according to the mean seeing value -FWHM- measured on the selected nights of the observing run) and the instrumental spread function (0.85"), which is rather large because of the photocathode -CCD spacing. As the WFAS takes into account, to compute the AO correction, the light centroid over a 6" circular diaphragm, we have derived the light centroid within a 6" aperture on the degraded composite HST image. This position is the reference for the WFAS.
We need also to locate the centroid of the degraded HST composite image with respect to the Lynds visible peak, as measured from the HST image F547M. We find a slight offset between the visible centroid and the Lynds visible peak: the visible centroid (using NGC 1068 for the correction) is located 99 mas to the N and 86 mas to the E of the Lynds peak. This offset is taken into account in our estimate of the NGC 1068 infrared sources location (Sect. 4.1). The offset may differ from one AO system to another. In the case of ADONIS, the offset is largely due to the 6" entrance to the WFAS.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999