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The Solar TErrestrial RElations Observatory (STEREO), the third mission in NASA's Solar Terrestrial Probes program,
was launched in 2006 on a two year mission to study solar phenomena. STEREO consists of two nearly identical
satellites, each carrying an Extreme Ultraviolet Imager (EUVI) telescope as part of the Sun Earth Connection Coronal
and Heliospheric Investigation instrument suite. EUVI is a normal incidence, 98mm diameter, Ritchey-Chrétien
telescope designed to obtain wide field of view images of the Sun at short wavelengths (17.1-30.4nm) using a CCD
detector. The telescope entrance aperture is divided into four quadrants by a mask near the secondary mirror spider
veins. A mechanism that rotates another mask allows only one of these sub-apertures to accept light over an exposure.
The EUVI contains no focus mechanism. Mechanical models predict a difference in telescope focus between ambient
integration conditions and on-orbit operation. We describe an independent check of the ambient, ultraviolet, absolute
focus setting of the EUVI telescopes after they were integrated with their respective spacecraft. A scanning Hartmann-like
test design resulted from constraints imposed by the EUVI aperture select mechanism. This inexpensive test was
simultaneously coordinated with other integration and test activities in a high-vibration, clean room environment. The
total focus test error was required to be better than ±0.05mm. We cover the alignment and test procedure, sources of
statistical and systematic error, data reduction and analysis, and results using various algorithms for determining focus.
The results are consistent with other tests of instrument focus alignment and indicate that the EUVI telescopes meet the
ambient focus offset requirements. STEREO and the EUVI telescopes are functioning well on-orbit.
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