The maximum pixel resolution of the prism is 5.3 Å at 1800 Å. At 3500 Å, the dispersion drops to 105 Å/pix and is 563 Å/pix at 5000 Å. The result is a bunching up of the spectrum to long wavelengths with about 8 pixels spanning 1500 Å. For bright objects, this effect can lead to blooming of the HRC CCD from filled wells; the overfilled pixels bleed in the detector Y direction, and would thus affect other spectra. Figure 6.21 shows the sensitivity versus wavelength for PR200L and the wavelength extent of the pixels is indicated. The variation of the dispersion across the detector for PR200L amounts to about ±4% (corner to corner) at 2000 Å. The angle of the prism causes a large deviation between the position of the direct object and the region of the dispersed spectrum. The numbers in Figure 6.21 indicate the resolving power (R) and the offset from the direct image in pixels (x) as functions of wavelength. On account of the size of this offset, special apertures have been defined in the observation scheduling system so that the spectrum of the target centered on the direct image occurs near the center of the field in the prism image, and a small angle maneuver occurs between the direct and prism images.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |