This is a resubmission of the Phase II proposal for program GO-10569. Kailash Sahu contacted us to point out that the short (0.5sec) F606W PSF star exposures in Visit 05 would saturate the brightest pixels. On his recommendation, we have reduced the exposure time on those short exposures to 0.3sec. That is the only change we have made. ----- Submission 1 (Fri May 13 15:45:33 GMT 2005) ----- ----- Submission 2 (Mon Jul 25 19:59:25 GMT 2005) ----- [These comments for our resubmission of the Phase II proposal for GO-10569 are unchanged from our original submission.] We see a number of diagnostic warnings, which fall into three groups, and can be explained as follows: VISIBILITY OVERRUN: We use high-visibility (near-CVZ) orbits, so Orbit Planner gives warnings about Visibility Overruns. However, Visit Planner processes the visits without error, and finds that the near-CVZ orbits are schedulable. PATTERN POSITION OUTSIDE APERTURE, or TARGET AND SUBARRAY ON DIFFERENT WFC CHIPS: We seek a 2x2 mosaic of pointings that overlap in the center. We specify the target position as the center of the mosaic, and then use appropriate POS TARG values to specify each pointing. We use the WFC1 aperture (because WFCENTER cannot be used with custom subarrays). In two of the pointings, the center of the mosaic falls on the WFC2 chip by design. In other words, sometimes the target (and subarray if used) does lie outside the WFC1 chip, because it is supposed to. SUBARRAY OFF OF DETECTOR: Apparently our custom subarrays needed to have their XCORNER values adjusted from 0 to 1. Our reading of the diagnostic warning was that this adjustment was made automatically. Assigned ID: 140 ----- Submission 1 (Fri Jan 21 03:16:39 GMT 2005) ----- A Last Look at the First Gravitational Lens Strong gravitational lensing provides our best probe of the behavior of dark matter in galaxy halos and their substructure. We undertake ACS imaging to gain new strong and weak lensing data for the first-discovered lens system Q0957+561. We will make a weak lensing image spanning 6' to map the potential of the cluster around the lens galaxy, and simultaneously obtain a deep image of more than 10 multiply-imaged features in the strong lensing region. This will complete HST's legacy dataset for Q0957+561: a deep multicolor image of a rich multiple-image system, an extremely high-quality weak lensing map, and an existing deep NICMOS image of the Einstein ring. We will combine these three kinds of data into a single lens model--a first--to produce a sophisticated dark matter map that will provide a uniquely detailed view of the relation between visible and dark matter on galaxy scales, and the best available constraints on dark substructure. In addition, the best available $H_0$ measurement from lensing will potentially allow us to leverage the WMAP data into a more precise measurement of dark energy properties as well. Astrophysics with strong lensing depends crucially on HST's unique capability for precise, well-resolved observations of low-surface-brightness structures next to bright objects, so it is a high priority to address outstanding questions before the demise of HST.
The observations must produce two different images: (1) a "strong lensing" (SL) image, spanning the central 10 arcsec region where all multiply-imaged sources appear; and (2) a "weak lensing" (WL) image spanning the ~6 arcmin diameter over which the cluster convergence kappa is >~ 2% (to keep the uncertainty in H_0 from the cluster mass sheet degeneracy below ~2%). Both programs require highest S/N in a primary filter, which we choose to be F606W because its broad bandwidth and high efficiency yield the lowest noise. Both programs also require basic color information, so we choose F814W for its high efficiency and a significant wavelength baseline from F606W. Our general scheme is to obtain the SL and WL data simultaneously by constructing a 2x2 mosaic of ACS pointings that overlap by ~30 arcsec. Each of the four pointings is allocated 2 CVZ orbits in F606W and one CVZ orbit in F814W, with subpixel dithering at each pointing. Hence the WL image will span 6 arcmin with 7.7 ksec exposure in F606W plus 3.8 ksec in F814W. The central region will have 31 ksec in F606W plus 15 ksec in F814W, with extensive dithering allowing us to resolve aliasing without the need to use the HRC. Additional PSF-mapping exposures add 4 more orbits to the program, yielding a total of 16 orbits. We now describe the quantitative requirements imposed on the WL and SL imaging by the science, and how our program meets them. The richness and complexity of the strong lensing in the system make it impossible for us to predict quantitatively the accuracy in H_0 or substructure constraints that will result from any particular set of ACS observations. There is also an element of surprise in what will appear in better images. Thus, we cannot set precise specifications for the SL imaging, but we can make some educated guesses. (1) We must significantly reduce the surface brightness noise levels compared the previous STIS 50CCD image of the system (GO-7332, PI Bernstein), in order to confidently detect and measure the lensed structures that are marginally visible in the STIS image. (2) We must execute Poisson-limited subtraction of the two quasar images over the maximum possible fraction of the image. (3) We must obtain color information on the lensed features in order to distinguish substructure anomalies from differential extinction. The noise requirement (1) can be evaluated by ETC estimations, or by measuring the STIS image itself and comparing to ACS F606W from a separate program (GO-9433, PI Bernstein). In either case we find that ~30 ksec of F606W time will meet that stated goal. Note that the surface brightness at the locations of the arcs has contributions from G1 and from scattered quasar light that are as bright as the zodiacal background; also, we do not know the color of the arcs, and we are hoping to discover new features about which we presently know nothing. So it is not possible to provide a more exact S/N calculation than this rough goal of lowering the noise by 0.5 AB mag. The 15 ksec of F814W exposure in the central region will give us good colors for all but the faintest objects detected in F606W, meeting requirement (2). Our experience with STIS indicates that proper PSF subtraction requires contemporaneous, dithered exposures of a bright template star at each CCD location of a quasar. We use a star of g-i color similar to the quasars', and schedule a series of short dithered exposures at each of the eight CCD locations where quasars will be imaged, plus additional exposures in F814W and of varying exposure lengths to improve the dynamic range. That requires 128 short exposures. While the exposure time can be made short by selecting a sufficiently bright star, we will accumulate a lot of overhead. By choosing a star accessible in the CVZ, and by using WFC subarrays to avoid crushing serial-dump overheads, we can acquire the PSF stars in 3 orbits. (Note that our mosaic naturally places the quasars near an output amplifier of the WFC in each exposure.) The figure of merit for a WL image is the density of measured background galaxy shapes. The existing ground-based image has 30 galaxies per square arcmin, and yielded the mean surface density kappa_{30} in the central 30 arcsec region to be 0.26+/-0.08. Since H_0 \propto (1-kappa_{30}), we can reduce the H_0 uncertainty due to the mass-sheet degeneracy to <5% if we can increase the lensing S/N ratio by at least a factor two, which requires quadrupling the background galaxy density. Our latest WL techniques, when applied to the GOODS 5.2 ksec F606W images, yield 140 galaxies per square arcmin, meeting our criterion. The 7.7 ksec F606W images we seek will allow for some periods of high earthshine background in the CVZ and some safety factor in the galaxy counts. The F814W time allocated should suffice for identifying "red-sequence" galaxies at low redshift, which can then be eliminated from the lensing analysis. We note that in the central 30 arcsec we should find ~300 galaxies per square arcmin, which will allow a higher-resolution WL map in this critical central area. We allocate one additional orbit to exposures of a field peripheral to 47 Tuc in order to generate a PSF map contemporaneous to the Q0957+561 exposures. This is necessary for an accurate WL analysis. CVZ orbits tend to have higher earthshine levels than regular orbits. For the SL image, this does not matter because the background near the arcs is dominated by scattered quasar light rather than zodiacal light anyway. The CVZ is efficient for the WL image, too: we divide the orbit into many exposures, and we can discard or downweight those few taken in the brightest portions of the orbit. We still end up well ahead of a regular orbit. Our program was approved for CVZ orbits in Phase I. Unfortunately, APT now says that orbits with truly continuous visibility are not available. There is at least one observing window in Cycle 14 with nearly continuous visibility (science time as high as 88.9 minutes per orbit), and we have successfully constructed our program using these near-CVZ orbits. We emphasize that near-CVZ orbits are crucial for meeting our science goals. (Note that Orbit Planner gives warnings about visibility overruns because our orbits exceed the nominal 48 minute standard orbit, but Visit Planner processes the program without error.) COSMOLOGY 12 false 0.0 false false false false false AVAILABLE GALAXY GRAVITATIONAL LENS This object was generated by the targetselector and retrieved from the NED database. We then adjusted the coordinates by hand to match the position of quasar A. J2000 10 01 20.8000 +55 53 49.40 J2000 STELLAR CLUSTER GLOBULAR CLUSTER Magnitude of main sequence turn-off stars. Field 6' W of cluster center. J2000 00 22 38.5000 -72 04 4.00 J2000 STAR F0-F2 J2000 10 02 23.0000 +55 34 31.00 J2000 STAR F0-F2 10 02 23.0000 +55 34 31.00 J2000 POS-TARG DITHER 0.171 UDF dither pattern. 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