Demonstration of an imaging technique for the measurement of PSF elongation caused by Atmospheric Dispersion

Elongation of the point spread function (PSF) due to atmospheric dispersion becomes a severe problem for high-resolution imaging instruments, if an atmospheric dispersion corrector is not present. In this work, we report on a novel technique to measure this elongation, corrected or uncorrected, from imaging data. By employing a simple diffraction mask, it is possible to magnify the chromatic elongation caused by the atmosphere and thus make it easier to measure. We discuss the theory and design of such a mask and report on two proof of concept observations using the 40-cm Gratama telescope at the University of Groningen. We evaluate the acquired images using a geometric approach, a forward modelling approach and from a direct measurement of the length of the PSF. For the first two methods, we report measurements consistent with atmospheric dispersion models to within 0.5 arcsec. Direct measurements of the elongation do not prove suitable for the characterization of atmospheric dispersion. We conclude that the addition of this type of diffraction mask can be valuable for measurements of PSF elongation. This can enable high-precision correction of atmospheric dispersion on future instruments