3.1 Optical Performance
Because the primary mirror has about one-half wave of spherical aberration, the Optical Telescope Assembly (OTA) did not achieve its design performance until after the first servicing mission in December 1993 when corrective optics were installed for the science instruments (SIs). From this time on, the detectors of all SIs (with the exception of the FGSs) have viewed a corrected beam, either via external corrective optics (COSTAR) or via internal optics (for the second and third-generation instruments). Table 3.1 gives a summary of general OTA characteristics.
Table 3.1: HST Optical Characteristics and Performance
|
Design
|
Ritchey-Chretien Cassegrain |
|
Aperture
|
2.4 m |
|
Wavelength Coverage
|
From 110 nm (MgF2 limited) to ~ 3 µm (self-emission limited) |
|
Focal Ratio
|
f /24 |
|
Plate Scale (on axis)
|
3.58 arcsec/mm |
|
PSF FWHM at 5000Å
|
0.043 arcsec |
|
Encircled Energy within 0.1" at 5000Å
|
87% (60%-80% at the detectors) |
Because each SI has unique characteristics, the actual encircled energy is instrument dependent and may also 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 TinyTim 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 TinyTim Web page. TinyTim simulated PSFs agree well with actual observations.
