The HST is a collaborative ESA/NASA mission that was launched on April 25, 1990 by the space shuttle Discovery (STS-31) into a low-Earth 600 km altitude orbit. The telescope is 13.2m long, 4.2 m in overall diameter, and weighs 11,110 kg (Figure 7.22). With a primary mirror of diameter 2.5 m, the theoretical angular resolution of HST is approximately 0.1" in the visible and near-infrared, which is ten times better than can be achieved with most ground-based observations without adaptive optics. Unfortunately, soon after launch, the primary mirror was discovered to have been ground incorrectly and thus the telescope initially suffered from spherical aberration, which significantly impaired its performance. Some of these defects were dealt with by corrective optics installed by the December 1993 shuttle servicing mission (STS-61, Endeavour), and subsequent servicing missions have now completely replaced the original instruments launched on HST such that they all now correct for the aberration of the primary. The instruments currently onboard HST, at the time of writing, prior to Servicing Mission 4, will now be reviewed.
Wide-Field/Planetary Camera 2 (WFPC2)
WFPC2 was a replacement for the original Wide-Field/Planetary Camera (WFPC1) launched on HST and was installed by the December 1993 shuttle servicing mission to
negate the aberration in the primary mirror. WFPC2 records high-resolution images of astronomical objects over a relatively wide FOV and a broad range of wavelengths (115 to 1,100 nm), which are defined by a large selection of filters. WFPC2 is composed of four cameras, each of which has an 800 x 800 element silicon CCD array. Three of the cameras make up the wide-field camera (WFC) and operate at f/ 12.9 giving a pixel size of 0.1" and a FOV of 150 x 150". The pointing of these cameras is arranged to form a projected "L" shape in the sky. The fourth camera, known as the planetary camera (PC), operates at f/28.3 and thus covers a 35 x 35" area with a pixel size of 0.046". This camera points in the gap left by the WFC.
Space Telescope Imaging Spectrograph (STIS)
STIS covers a similar spectral range to WFPC2 (115-1,000 nm) and uses three detector arrays. The spectral regions 115nm-170nm and 165nm-310nm each use a Multi-Anode Microchannel Array (MAMA), while a CCD array covers the 305 nm to 1,000 nm range. All three arrays have 1,024 x 1,024 elements and the field of view is 25 x 25" for each MAMA (0.024"/pixel), and 50 x 50" for the CCD (0.05"/pixel). Although STIS covers a smaller region of the sky than WFPC2, it has higher angular resolution and other advantages at UV wavelengths.
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