High spatial resolution performance of a triple Fabry–Pérot filtergraph

Kiepenheuer–Institut für Sonnenphysik, Schöneckstraße 6-7, D-79104 Freiburg i. Br., Germany e-mail: ovdluhe@kis.uni-freiburg.de

Send offprint request to: O. von der Lühe

Received: 8 June 2000
Accepted: 14 August 2000

Abstract


We investigate the consequences of monochromatic pupil apodisation in a filtergraph based on Fabry–Pérot etalons in a telecentric mount, an effect which was discovered by Beckers (1998a). The pupil apodisation gives rise to systematic changes of the point spread function in the presence of strong spectral gradients, like in the wings of a Fraunhofer line. These changes are likely to produce spurious velocity signals in Dopplergrams generated from high spatial resolution filtergrams of the solar surface. We include in our analysis for the first time the effects of optical phase changes on pupil apodisation which has the tendency to enhance velocity errors. 
We study the resolution and the velocity errors for the three–etalon filtergraph TESOS (Kentischer et al. 1998) at the Vacuum Tower Telescope on Tenerife. Peak–to–valley velocity errors for solar structure with sizes of 2 arcsec and less and a contrast of 15% amount to less than 50 m/s for the low resolution mode and to less than 10 m/s for the high resolution mode of TESOS in the worst case. We conclude that the performance of TESOS is not severely compromised by pupil apodisation.