FLEXURES OF CONVENTIONAL CASSEGRAIN-FED SPECTROGRAPHS

In this work we have derived the flexure patterns of two classical Cassegrain-fed spectrographs, by cross-correlating CCD spectra. After a detailed description of the method used to derive the flexure models, emphasis is given to the fact that extreme care must be used in compensating for those flexures when high precision is sought in wavelength-calibration and radial-velocity work involving instrumentation similar to that investigated here. If not fully accounted for, the flexures can introduce spurious shifts up to one hundred times the typical error of the cross-correlation technique usually used to compare the spectra. With the aid of our models it is now possible to remove most of the flexure biases from spectra taken with both spectrographs of the Asiago Astrophysical Observatory over a large fraction of the sky accessible to the telescope. This will ensure high consistency of the radial-velocity measurements derived with the cross-correlation technique. A brief discussion on possible ways to monitor in real time the flexure pattern of a telescope + spectrograph combination is also given.