The Extreme Ultraviolet Telescopes (EUT) Operates at wavelengths between 100-1000 Å. As an important parameter
of the telescope system, before the launch, angular resolution is necessary to be calibrated for testing the imaging
performance of EUT. However, the difficulty and expense of fabricating optical testing systems capable of imaging the
characteristic EUV wavelengths, has precluded in working wavelength resolution testing. This article taken a
Ritchey–Chrétien normal incidence optical system as sample and resolution tests were carried out at visible wavelength.
Based on this measurement, the angular resolution error budget at visible wavelength was calculated. At working
wavelength, we added the squares of the pointing jitter error, the resolution focusing error and the scattering error, to the
theoretical Rayleigh diffraction limit at the wavelength of operation, and then take the square root of this sum, an upper
limit estimate of telescope’s resolution was obtained about 0.4705 arcsec. This result proved that the EUT worked at
diffraction-limited level and the resolution performance has met the demand of design.