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

Elongation of the point spread function due to atmospheric dispersion becomes a severe problem for high resolution imaging instruments, if an atmospheric dispersion corrector is not present. This work reports 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 of 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 point spread function. 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 characterisation of atmospheric dispersion. We conclude that the addition of this type of diffraction mask can be valuable for measurements of atmospheric dispersion enabling high precision correction of this atmospheric effect on future instruments.