At z~1.6 the main progenitors of present-day massive clusters are undergoing rapid collapse, and have the highest rates of galaxy merging and assembly. Recent observational studies have hinted at accelerated galaxy evolution in dense environments at this epoch, including increased merger rates and rapid growth in galaxy size relative to the field. We propose WFC3 G102 spectroscopy and F125W (Broad J) imaging of a sample of four massive spectroscopically-confirmed clusters at z = 1.6. Our primary scientific goal is to leverage the CANDELS Wide Legacy dataset to carry out a head-to-head comparison of the sizes of cluster members relative to the field (as a function of stellar mass and Sersic index), and quantify the role of environment in the observed rapid evolution in galaxy sizes since z = 2. These clusters are four of the highest significance overdensities in the 50 square degree SWIRE fields, and will evolve over time to have present-day masses similar to Coma. They were detected using IRAC [3.6]-[4.5] color, which identifies galaxy overdensities regardless of optically red or blue color. A heroic ground-based spectroscopic campaign has resulted in 44 spectroscopically-confirmed members. However this sample is heavily biased toward star-forming (SF) galaxies, and WFC3 spectroscopy is essential to definitively determine cluster membership for 200 members, without bias with respect to quiescent or SF type. The F125W (rest-frame V-band) imaging is necessary to measure the sizes and morphologies of cluster members. 17-passband broadband imaging spanning UV, optical, near-IR, Spitzer IR and Herschel far-IR is already in hand. This program observes four galaxy clusters at z~1.6. We aim to confirm cluster members via the OII and 4000Ang break spectral feature and to use the OII emission line strength as an estimate of star formation rates. Each pointing is roughly centered on the the galaxy cluster BCG with small offsets applied to remove bright foreground stars from pointings where possible. Each cluster will be observed in two seperate visits with each visit consisting of two orbits. For a given target, the roll angle of the second visit is set to differ from the roll angle of the first visit by at least 60D except in the case of target J022546-035517 where the roll angle difference is set to be at least 45D. The purpose of scheduling two visit with significantly different roll angles is to reduce/eliminate contaimination of overlapping spectra. As these targets are galaxy clusters, the objects are packed quite close together and a key aim of the program is to detect large numbers of faint objects. Thus having two sets of data at different roll angles will greatly improve the success of confirming redshifts of faint objects. Unfortunately, given the FOV of WFC3 compared to the average size of the galaxy clusters at this redshift, restriction of the roll angles of the first visit to each target is necessary. Each first visit roll angle restriction is implemented such that, at minimum, the first spectral order will land on the detector for as many confirmed cluster membes and plausible cluster member candidates as possible. Within each two orbit visit to a target there are four dithered exposures of both WFC2/G102 and direct imaging with F160W (necessary for detection identification and science goals). Note, we opt for the F160W filter rather than the F125W filter, which usually accompanies G102 observations and falls within the G102 wavelength range, for three reasons. First, this filter allows us to better achieve one of our science goals, ie measuring morphology and sizes of cluster galaxies, since the redder F160W filter will allow us to better sample the continuum light and be less bias towards star forming regions. Second, using the F160W filter will allow direct comparisons to previous studies of field galaxies, ie CANDLES, without making assumptions involving a morphological k-correction. Third, by choosing the F160W filter we still allow for reliable modeling of spectra and facilitation of their extraction from the G102 frames. The observations of each visit follows the pattern Orbit 1 : direct(A), grism(A), offset, grism(B), direct(B), offset Orbit 2 : direct(C), grism(C), offset, grism(D), direct(D) where ABCD are a combination of interger and interger+1/2 pixel shifts. This is very similar to the strategy adopted in program 12590. We choose to dither to enable rejection of hot/bad pixels and use 1/2 pixel shifts to improve sampling of the PSF. As the standard WFC3-IR-DITHER-BOX pattern uses only 1-4 pixel offsets, we adopt a modified version of the standard WFC3-IR-DITHER-BOX pattern that utilizes a slightly larger pattern to avoid bad pixels that come in clumps. This is similar to the dither pattern adopted in program 12177. We set the dither pattern explicity using the POS TARG keyword rather than a preset dither pattern so that the dither can be spread out over two full orbits and overheads associated with pointing maneuvers are reduced. The dither pattern is as follows: POS TARG : (0,0) (1.355,0.424) (0.881,1.212) (-0.474,0.788) pixels : (0,0) (10.0,3.5) (6.5,10.0) (-3.5,6.5) No attempt is made to dither over the "Death Star" as larger dithers introduce larger differential distortions. The same dither pattern is used for both visits to a target since the differing orientation will ensure that the spectra from same objects will fall on different pixels. COSMOLOGY 12 false false 0.0 0.0 false false false SUPPORTED CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER CDFS44 J2000 J2000 J(AB)=21-27 CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER XMM113 J2000 J2000 J(AB)=21-27 CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER XMM105 J2000 J2000 J(AB)=21-27 CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER LOK200 J2000 J2000 J(AB)=21-27 WFC3/IR Spectroscopic No No G102 WFC3/IR Spectroscopic No No G102 WFC3/IR Spectroscopic No No G102 WFC3/IR Spectroscopic No No G102 The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. Comments go here. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. (Tic-Visit :proposal-id 13306 :visit-id "08" :proposal-version -1 :trans-version "20130722" :date "19-Aug-2013 15:30:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1967200 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13306 :visit-id "08" :obset-id "08" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13306 :visit-id "08" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3140 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3491 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13306 :visit-id "08" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3265 :al-overhead-at-end 0 :internal-dur-at-end 443 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3491 :pcs-gap 0 ) false 2 0