device rather than a science instrument, but occupied one of the four axial instrument bays. Since the final servicing mission in 2009, the four active instruments have been ACS, COS, STIS and WFC3. NICMOS is kept in hibernation, but may be revived if WFC3 were to fail in the future. Advanced Camera for Surveys (ACS; 2002–present) Cosmic Origins Spectrograph (COS; 2009–present) Corrective Optics Space Telescope Axial Replacement (COSTAR; 1993–2009) Faint Object Camera (FOC; 1990–2002) Faint Object Spectrograph (FOS; 1990–1997) Fine Guidance Sensor (FGS; 1990–present) Goddard High Resolution Spectrograph (GHRS/HRS; 1990–1997) High Speed Photometer (HSP; 1990–1993) Near Infrared Camera and Multi-Object Spectrometer (NICMOS; 1997–present, hibernating since 2008) Space Telescope Imaging Spectrograph (STIS; 1997–present (non-operative 2004–2009)) Wide Field and Planetary Camera (WFPC; 1990–1993) Wide Field and Planetary Camera 2 (WFPC2; 1993–2009) Wide Field Camera 3 (WFC3; 2009–present) Of the former instruments, three (COSTAR, FOS and WFPC2) are displayed in the Smithsonian National Air and Space Museum. The FOC is in the Dornier museum, Germany. The HSP is in the Space Place at the University of Wisconsin–Madison. The first WFPC was dismantled, and some components were then re-used in WFC3. Flawed mirror Within weeks of the launch of the telescope, the returned images indicated a serious problem with the optical system. Although the first images appeared to be sharper than those of ground-based telescopes, Hubble failed to achieve a final sharp focus and the best image quality obtained was drastically lower than expected. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function (PSF) concentrated within a circle 0.1 arcseconds (485 nrad) in diameter, as had been specified in the design criteria. Analysis of the flawed images revealed that the primary mirror had been polished to the wrong shape. Although it was believed to be one of the most precisely figured optical mirrors ever made, smooth to about 10 nanometers, the outer perimeter was too flat by about 2200 nanometers (about mm or inch). This difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its center. The effect of the mirror flaw on scientific observations depended on the particular observation—the core of the aberrated PSF was sharp enough to permit high-resolution observations of bright objects, and spectroscopy of point sources was affected