Request feedback on the dither pattern and cosmic ray issue as discussed in the description of observations. ----- Attempting Submission 1 (Fri May 04 17:57:55 GMT 2007) ----- This is a resubmittal of ACS Proposal #413. The proposal has been modified to accomodate WPFC2. Assigned ID: 1214 ----- Attempting Submission 1 (Thu Feb 08 21:11:00 PST 2007) ----- Phase I Proposal 1214 (hst-WFPC2) successfully submitted. ----- Attempting Submission 2 (Fri Feb 09 12:31:59 PST 2007) ----- A Dark Core in Abell 520 We have recently disocovered that the rich cluster Abell 520 exhibits truly extreme multi-wavelength characteristics. The data indicate that the cluster is the site of a major merger. Our weak lensing analysis, based on a deep CFHT image, suggests the presence of a massive dark core that coincides with the central X-ray emission peak, while being largely devoid of galaxies. Although a displacement between the X-ray gas and the galaxy/dark matter distribution may be expected in a merger (e.g. as in the bullet cluster), the dark matter peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. A higher resolution mass map is required to make further progress, as it will enable us to examine the detailed structure of the dark matter distribution, as well as improve the significance of the dark peak. We propose a 3 x 3 WFPC2 mosaic of interlaced images, where each pointing consists of two sets of F814W exposures offset by 5.5 pixels. This will precisely pinpoint the locations of the highest lensing peaks, enhance the comparison with the Chandra X-ray data, and test physical and geometrical models for the spatial and thermal structure of this remarkable cluster derived from our suite of gas+dark matter simulations of head-on/off-axis cluster mergers.
A detailed map of the matter distribution is a crucial component in our analysis of Abell 520, as the dynamical tracers that are usually employed are no longer accurate. Based on our experience with HST WFPC2 observations we know that by constructing an interlaced image from four half-orbit exposures in the F814W filter (using a 45 degree parallelogram box pattern), we can reach an effective number density of $\sim 100$ galaxies arcmin$^{-2}$ (Hoekstra 2000). We therefore request for 2 orbits per pointing. We choose to observe using the F814W filter, because it is the broadest and most sensitive for detecting the high redshift galaxies needed in our lensing analysis. The layout of the mosaic was chosen to provide adequate coverage of the most interesting ground-based lensing features, as well as to provide a minimum 5" overlap among the various mosaic tiles, and a larger overlap in the region of the central dark peak. The chief goal of the parallelogram dither pattern is to produce images that can be interlaced in a 4x4 pattern without any interpolation. This means that if we start with four images, each pixel in each image becomes a pixel in a new interlaced image that has four times the number of pixels. We finally came up with a 4-point box (45 degree parallelogram) pattern spread over two orbits, which is shown in the a520-dither.gif file. From position 1-2 and 3-4 the dither is 2.5x2.5 (=3.54 linear) pixels, and from position 2-3 the dither is 0x3.5 pixels. If we have thought this through correctly, this pattern should allow us to produce the desired 4x4 interlace. In case the dither between orbits is not close to a half-integral value, we can treat each pair of images separately, producing two independent 2x2 interlaced images. Using this pattern, none of the images have an integral pixel offset from the others. This makes cosmic ray removal more complicated. Neither of us have used drizzle, but if the drizzle software can produce masks for each of its input images prior to combining them, we could use drizzle to reject cosmic rays. Alternatively, we could write a small software program which produces cosmic rays masks for the individual half-integer offset images by considering all four. In designing the layout of the mosaic, we have assumed that we cannot rely on WF4 because of the unstable bias levels. The fidelity of the mass reconstruction would suffer severely from holes in our mosaic. However, by positioning the mosaic along the major axis of the cluster mass distribution, all 4 peaks fit into the field, even if we can only use WF2 and WF3. COSMOLOGY 12 false 0.0 false false false false false false SUPPORTED CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 00 00 0.0000 +00 00 0.00 J2000 WFPC2 Imaging No F814W POS-TARG DITHER 0.35355 LINE 0.35 false false 135 4 45 BOX 1 false (Tic-Visit :proposal-id 11221 :visit-id "01" :proposal-version -1 :trans-version "20070325" :date "03-May-2007 00:46:18.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 537792 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11221 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 11221 :visit-id "01" :obset-id "A1" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11221 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2758 :al-overhead-at-end 122 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "01" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "01" :orbit-number 6 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) true 6 0 /Users/amahdavi/a520/OP-cache/hst-phaseii-v01.xml (Tic-Visit :proposal-id 11221 :visit-id "02" :proposal-version -1 :trans-version "20070325" :date "03-May-2007 00:46:26.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 537792 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11221 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 11221 :visit-id "02" :obset-id "A2" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11221 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "02" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "02" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "02" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2754 :al-overhead-at-end 126 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "02" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "02" :orbit-number 6 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) true 6 0 /Users/amahdavi/a520/OP-cache/hst-phaseii-v02.xml (Tic-Visit :proposal-id 11221 :visit-id "03" :proposal-version -1 :trans-version "20070325" :date "03-May-2007 00:46:34.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 537792 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11221 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 11221 :visit-id "03" :obset-id "A3" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11221 :visit-id "03" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "03" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2758 :al-overhead-at-end 122 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "03" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "03" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "03" :orbit-number 5 :pcs-time 355 :si-vis-utilization 2720 :al-overhead-at-end 160 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11221 :visit-id "03" :orbit-number 6 :pcs-time 355 :si-vis-utilization 2700 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) true 6 0 /Users/amahdavi/a520/OP-cache/hst-phaseii-v03.xml