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STIS Instrument Handbook for Cycle 24 |
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The optical performance of the CCD in imaging mode is good, and the plate scale of the CCD is 0.05078 arcsecond per pixel, providing a good compromise between sampling of the PSF and field of view. There is some degradation of the image quality towards the edge of the field. Observers can assume that 15-20% of the light from a point source falls in a single STIS CCD pixel and that ~80% of the light from a point source is contained within a 5 в 5 pixel region. An image of a typical point source is shown in Figure 5.4. See Chapter 14 for encircled energies at the field center for the different imaging modes and information on the field dependence of the PSF. For a discussion of the coronagraphic mask, see Section 12.11.Figure 5.4: A Co-added Image of SAO 255271 taken Using 50CCDThis figure is plotted with a logarithmic intensity scale and is about 10” across. It shows the structure in the STIS PSF. The ring seen below the center of the PSF is a ghost image. The position angle of this ghost varies as a function of location on the CCD (see Section 7.2.8 and Figure 7.6).The 50CCD aperture is a clear, unvignetted aperture which provides maximum sensitivity over the full 52 в 52 arcsecond field of view. The shape of the bandpass is governed by the detector, which has sensitivity from ~2000 to 10,300 е. Figure 5.1 shows the throughput as a function of wavelength for this imaging mode (see also Chapter 14 for sensitivities, signal-to-noise plots, and saturation plots).The coronagraphic aperture 50CORON contains two wedges and a bar which can be used to occult a bright object. The unocculted parts of the aperture are clear, and so the throughput as a function of wavelength is the same as for the 50CCD aperture. Further details about this aperture as well as advice for conducting STIS coronagraphic observations are given in “Coronagraphic Imaging—50CORON”STIS’ longpass filter cuts off at λ < 5500 е. It images a 28 в 52 arcsecond field of view. The F28X50LP filter is the principal target acquisition aperture (see “Selecting the Imaging Aperture” on page 175). The integrated system throughput for this filter is given in Figure 5.1 (see also page 393 for sensitivities, signal-to-noise plots, and saturation plots).The combination of 50CCD and F28X50LP can provide deep imaging with sufficient color information for some types of color-magnitude diagrams.5.2.4 [O III]: F28X50OIIIThis filter images a 28 в 50 arcsecond field of view and can be used in target acquisitions or for direct imaging in the light of [O III]. The [O III] filter integrated system throughput and a signal-to-noise comparison with the WFPC2 [O III] filter are shown in Figure 5.5 (see also “F28X50OIII—CCD” for sensitivities, signal-to-noise plots, and saturation plots). The STIS [O III] filter is very narrow: only 5 е wide, compared to the WFC3 [O III] filter which is roughly 58 е wide. The STIS [O III] filter has a substantial red leak that begins at 10,600 е and continues to at least 12,000 е. In the case of a very red star (K0 spectral type), the red leak will contribute approximately one third of the detected counts. The red leak for this filter is included in the passbands used by the STIS ETC, pysynphot, and synphot. Observers are encouraged to use these tools to predict source and background count rates carefully.Figure 5.5: F28X50OIII Throughput and Flux vs. Exposure Time5.2.5 [O II]: F28X50OIIThe [O II] filter images a 28 в 52 arcsecond field of view and can be used in target acquisitions or for direct imaging in the light of [O II]. The [O II] filter integrated system throughput and a signal-to-noise ratio comparison with WFPC2’s [O II] filter are shown in Figure 5.6. See page 399 for sensitivities, signal-to-noise and saturation plots.Figure 5.6: F28X50OII:Throughput and Flux vs. Exposure Time