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 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 1100 Å (MgF2 limited) To ~3 microns (self-emission limited) |
|
Focal Ratio
|
f /24 |
|
Plate Scale (on axis)
|
3.58 arcsec/mm |
|
PSF FWHM at 5000 Å
|
0.043 arcsec |
|
Encircled Energy within 0.1" at 5000 Å
|
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 techniques. For instrument specific Point Spread Function (PSF) characteristics over various wavelength ranges, please consult the Instrument Handbooks (see Section 1.2). The TinyTim software, developed at STScI and the ST-ECF, is available on the TinyTim Web Page for detailed HST/PSF simulations, which agree well with actual observations.
