The CCD imaging capability of STIS was designed primarily for target acquisitions, and therefore, only a small number of filters are available. Nevertheless, STIS CCD imaging has scientific utility of its own, due to the high throughput and relatively low read noise of the CCD detector. STIS CCD imaging can be obtained as prime pointings or in parallel with other instruments.
The optical performance of the CCD in image mode is good, and the plate scale of the CCD is 0.05071± 0.00007 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 to 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 x 5 pixel region. An image of a typical point source is shown in Figure 5.5. See Chapter 14 for encircled energies at the field center for the different imaging modes and information on the field dependence of the PSF.
Figure 5.5: A co-added image of SAO 255271 taken using 50CCD shows the structure in the STIS PSF. This figure is plotted with a logarithmic intensity scale and is about 10" across. 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 Optical Performance and Figure 7.4).
The throughputs used for the CCD imaging modes are for the most part based on measurements of on-orbit Cycle 7 calibration data and are accurate to within 10%.
The 50CCD aperture is a clear, unvignetted aperture which provides maximum sensitivity over the full 52 x 52 arcsecond field of view. The shape of the bandpass is governed by the detector, which has sensitivity from ~2000 to 10,300 Ċ. Figure 5.1 shows the throughput as a function of wavelength for this imaging mode (see also Chapter 14 for sensitivities, and signal-to-noise and saturation plots).
Figure 5.6 shows a plot of time to achieve a signal-to-noise ratio of 5 for this aperture, with results for the WFPC2 broadband filter modes superposed, assuming an A0 V star spectrum for the source. If color information and a wide field of view are not required, then there is a clear advantage of this imaging mode over the WFPC2.
Figure 5.6: Comparison of STIS 50CCD Imaging with WFPC2. The plot shows the limiting V magnitude for an A0 V star at a signal-to-noise ratio of 5 versus exposure time. STIS F28X50LP is nearly identical to WFPC2 F814W in this plot.
Figure 12.5 shows an example of a deep CCD image of a random field taken as part of the Archival Pure Parallel Program.
STIS's longpass filter cuts off at 
< 5500 Ċ. It images a 28 x 52 arcsecond field of view. The F28X50LP filter is the principal target-acquisition aperture (see Selecting the Imaging Aperture). The integrated system throughput for this filter is given in Figure 5.2 (see also page 379 for sensitivities, and signal-to-noise and saturation plots).
The combination of 50CCD and F28X50LP can provide deep imaging with sufficient color information for some types of color-magnitude diagrams.
This filter images a 28 x 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 5.7 (see also "F28X50OIII-CCD" on page 382 for sensitivities, and signal-to-noise and saturation plots). The STIS [O III] filter is very narrow: only 5 Ċ wide, compared to the WFPC2 [O III] filter which is roughly 30 Ċ wide. The STIS [O III] filter has a substantial red leak that begins at 10,600 Ċ and continues to at least 12,000 Ċ. 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 Exposure Time Calculator (ETC) and synphot. Observers are encouraged to use these tools to predict source and background count rates carefully.
The [O II] filter images a 28 x 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 5.8. See page 385 for sensitivities, signal-to-noise and saturation plots.
Figure 5.8: F28X50OII: (a) Integrated System Throughput and (b) Flux vs. Exposure Time to achieve a signal-to-noise=5 compared to WFPC2 for a FWHM=1 Ċ line, integrated over an area of one square arcsecond. There is no substantial red leak in this filter.
STIS has a single coronographic mask aperture for direct imaging. The aperture (50CORON) contains one occulting bar and two intersecting wedges and is shown in Figure 5.9. This illustration of the coronographic aperture is derived from an on-orbit lamp flat. The approximate positions of the predefined aperture locations are marked. The wedges vary in width from 0.5 to 3.0 arcsec over their 50 arcsec length, while the rectangular bar measures 10 by 3 arcsec. The small occulting finger on the right was damaged during the assembly of STIS and is not used. The entire coronographic aperture measures 50 by 50 arcsec, slightly smaller than the size of the unobstructed CCD aperture. The parallel readout of the CCD is along the AXIS2 direction, and heavily saturated images will bleed in this direction (vertically in this figure).
The aperture cannot be combined with a filter and so, when used with the CCD, yields a bandpass of ~2000-10,300 Ċ. See Unfiltered (Clear) CCD Imaging-50CCD for the spectral properties of the images obtained. A number of locations on the occulting masks have been specified, to correspond to widths of 2.75, 2.5, 2.0, 1.75 and 1.0 arcseconds on each wedge. The mask is not available for use with the MAMA detectors due to concerns about bright-object protection of the MAMAs.
Figure 5.9: Design of the STIS Coronographic Mask
In combination with the option of a coronographic mask, there is a limited amount of apodization via a Lyot stop which masks the outer perimeter of the re-imaged exit pupil. Consequently, diffraction from the secondary mirror assembly and the telescope spider is not apodized. The STIS coronographic imaging facility is well suited to imaging problems involving faint material surrounding a relatively bright source. Typical examples include circumstellar disks, such as 
Pictoris, and the host galaxies of bright QSOs.
In Figure 5.10 we provide a comparison of the PSF suppression provided by STIS coronographic imaging relative to WFPC2 imaging and the Optical Telescope Assembly scatter. It had been hoped that the optical performance of the STIS CCD clear aperture without the coronograph would be comparable to that with the coronograph, although, without the coronograph, the CCD long wavelength halo from the central source and the window reflection ghosts are present. In practice the coronograph does provide substantial additional suppression of the PSF wings, especially at wavelengths >8000 Ċ, where the halo of light scattered within the CCD itself dominates the far wings of the PSF.
Since the coronographic PSF shape is strongly dependent on the target's spectral energy distribution, when subtracting PSF images or estimating coronographic exposure times it is especially important to match the color of the target and comparison star as closely as possible. It is also essential to compare stars at the same location on the coronographic mask. Breathing and focus differences will also affect the quality of such a subtraction. Coronographic images of stars of various colors have been obtained as parts of calibration programs 7151, 7088, 8419, 8842, and 8844 and are available from the archive. A searchable STIS PSF Library containing both direct and coronographic images is planned to be released by early summer 2001. This tool, similar to that already available for WFPC2, will allow the user to easily find and retrieve PSF images for particular detector, filter, and stellar color combinations.
If an observer wishes to image the full 360 degree region around a target, the simplest approach is to obtain two observations, one each on the vertical and horizontal wedges. However, a much better strategy is to obtain two or preferably three sets of images at the same wedge position, but with each set taken at a different roll angle. This is especially useful if the observer is attempting to detect faint point sources or asymmetric structures as such images can be subtracted from each other, greatly reducing the contribution of the central source's PSF. As the best alignment of STIS PSF images occurs when comparing images taken in the same part of adjacent orbits, the visits at each roll angle should be constrained to be adjacent. However, remember the total length of such a combined block of visits is usually subject to the same operational limits as length of a single visit. Taking a number of identical images at each roll position will allow those that align best to be chosen for the subtraction. Similarly when subtracting a PSF comparison star to detect circumstellar material, the best subtraction will be obtained if it is possible to observe a star of very similar color in the same part of the sky in an adjacent orbit, as this will minimize differences due to focus and breathing changes.
In planning any observing program with the 50CORON aperture, observers should carefully consider the required orientation of the target. The telescope's V2 and V3 axes are at 45° to the STIS AXIS1/AXIS2 coordinate system (see Figure 11.1 on page 213) and so diffraction spikes further reduce the unocculted field of view.
A series of apertures has been defined for the coronographic mask so that targets can be placed on the 3" wide bar and 5 locations on each of the two wedges. These apertures are summarized in Table 5.3 below. We defined a special coronographic acquisition technique for placing stars at these predefined locations. This involves performing a bright-target acquisition with a filtered aperture, followed by a slew to the chosen location on the coronographic mask. An example of an acquisition into one of the bars on the 50CORON aperture is provided in Coronographic Imaging-50CORON).
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