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7.3 Observing Faint Targets Near Bright Objects

If the bright companion will saturate and bloom, it will be necessary to rotate the CCD so that blooming along the CCD columns does not obliterate the faint target. See Figure 7.8 on page 160 for an illustration of the bloom directions. It may also be useful to orient the field so that the OTA diffraction spikes from the bright companion (along diagonal lines on the CCDs) avoid the faint target. Table 7.1 summarizes ORIENTs which can be used to avoid CCD blooming tracks and OTA diffraction spikes caused by bright objects. For example, if a faint companion is at PA 60 degrees on the sky relative to a bright companion, it would be advantageous to observe on PC1 with ORIENT= PA + 45 degrees = 105 degrees . Ideally, some range in ORIENT would be specified to ease scheduling, hence "ORIENT=90D TO 120D" might be specified on the Phase II proposal. Note that "ORIENT=270D TO 300D" is also feasible, and should be indicated in the visit level comments.

Table 7.1: ORIENTs for Avoiding Bloom Tracks and Diffraction Spikes.

If instead of observing a known companion, one is searching for companions, it is advisable to observe at several ORIENTs so that the CCD bloom track and OTA diffraction spikes will not hide possible companions. For example, three ORIENTs, each separated by 60 degrees , would give good data at all possible companion position angles.

If PSF subtraction will be needed during data analysis, then the PC CCD may have some advantage, since it provides better sampling of fine undulations in the PSF. It may also be useful to obtain observations of a second bright star for PSF calibration, though these may be of limited utility since thermal effects and OTA "breathing" can modify the telescope focus, and hence the PSF, on timescales of less than one hour. Any such PSF star should be similar in color to the target, and should be observed at the same CCD position (within 1" ) and with the same filter. Sub-pixel dithering may also be useful, so as to improve sampling of the PSF (see "Dither Strategies" on page 154).

Figure 7.2 illustrates the effect of OTA breathing, and periodic focus adjustments, on PSF subtraction. It shows the difference between an "in focus" PSF and one where the OTA secondary mirror has been moved by 5mm. This amount of focus change is comparable to the range of OTA "breathing" effects (timescale <1 hour), and the periodic (semi-annual) focus adjustments of the OTA. Each panel shows a different contrast setting; the percentages indicate the energy per pixel which is plotted as white, expressed as a fraction of the total (un-subtracted) PSF energy. For example, features which are just white in the "0.003%" panel contain 0.003% of the total PSF energy in each pixel. In other words, the feature labeled "a" is, in effect, ~10 magnitudes fainter than the PSF of the bright object, so that it may be very difficult to detect a "real" companion object ~10 magnitudes fainter than the bright object, at this distance from the bright object. In a real PSF subtraction situation, other effects including PSF sampling, noise, and pointing instability would further degrade the subtraction. (The elongated appearance of the residuals in the PSF core is due to astigmatism in PC1.)

Figure 7.2: Impact of OTA Focus Shift on PSF Subtraction.

Table 7.2 gives some quantitative indication of the performance expected for PSF subtractions in the high signal-to-noise limit. It gives the magnitude of "star-like" artifacts remaining in the subtracted image, as a function of distance from the bright object, and magnitude m_bright for the bright object. The right-most column gives an effective magnitude limit imposed by artifacts from the PSF subtraction. These results are derived for the 5 micron focus shift described above, and are for PC1 and filter F555W. It may be possible to do somewhat better than these limits by subtracting accurate model PSFs, or by finding an observed PSF with matching focus.

Table 7.2: Approx. PSF Subtraction Artifact Magnitudes and Magnitude Limits.

Large angle scattering may also impact identification of very faint objects near very bright ones. This scattering appears to occur primarily in the camera relay optics, or in the CCD. Hence, if a faint target is more than ~10" from a bright object (i.e. very highly saturated object), it would be advisable to place the bright object on a different CCD, so as to minimize large angle scattering in the camera containing the faint target. See section on "Large Angle Scattering" on page 99. Note also that highly saturated PSFs exist for PC1 in filters F439W, F555W, F675W, and F814W, and for F606W on WF3; these may be useful when attempting to subtract the large-angle scattered light. As of this writing TinyTIM does not accurately model the large angle scattering, and should be used with caution when analyzing highly saturated images (Krist 1996).

It is generally unwise to place bright companions or other bright objects just outside the area imaged by the CCDs. The region of the focal plane just outside the CCDs (within about 6" of the CCDs) contains a number of surfaces which can reflect light back onto the CCDs, hence placing bright targets there can have undesired results. Also, the un-imaged "L" shaped region surrounding PC1 should be avoided, since incomplete baffling of the relay optics allows out-of-focus images of objects in this region to fall on the CCDs. Figure 7.3 illustrates various bright object avoidance regions near the WFPC2 field-of-view; the indicated avoidance magnitudes will produce 0.0016 e- s-1 pixel-1 in the stray light pattern for F555W. Figures 7.4 and 7.5 show examples of artifacts which can result from bright stars near the PC1 CCD. The report "A Field Guide to WFPC2 Image Anomalies" (WFPC2 ISR 95-06, available on the WFPC2 WWW pages and from help@stsci.edu) gives more detailed discussions of artifacts associated with bright objects, and their avoidance.

Figure 7.3: Bright Object Avoidance Regions Near WFPC2 FOV.

Figure 7.4: Example of PC1 "Direct" Stray Light Ghost.

Figure 7.5: Example of PC1 "Diffraction" Stray Light Ghost.

Figure 7.3: - Bright Object Avoidance Regions Near WFPC2 FOV.