’s Optical Telescope Assembly (OTA) did not perform as designed because the primary mirror was manufactured with about one-half wave of spherical aberration. This was remedied by corrective optics installed for the science instruments (SIs) during the first servicing mission in December 1993. Since then, all SI detectors (with the exception of the FGSs) have viewed a corrected beam, either via external corrective optics (COSTAR) or via internal optics (for second and third-generation instruments). COSTAR was removed from the telescope during Servicing Mission 4 in May 2009; currently, all onboard instruments have internal corrective optics. Table 3.1
gives a summary of general OTA characteristics.
Because each SI has unique characteristics, the actual encircled energy is instrument-dependent and may vary with observing strategy. The HST Instrument Handbooks
(see Section 1.3
) should be consulted for instrument-specific point spread function (PSF) characteristics over various wavelength ranges. The Tiny Tim software tool, developed at STScI by John Krist, with support from Richard Hook at the ST-ECF, can be used to simulate the PSFs of several HST
instruments. It is available for download from the Tiny Tim webpage
. PSFs generated by Tiny Tim are coarse matches to observed PSFs because of transient focus changes in HST
and other effects. As a result, for many types of data analysis, PSF subtraction using Tiny Tim models is inadequate.