Implemented a few suggestions from contact scientist: Used BLADE=A for the 7s exposure, switched from UVIS1 to UVIS2 aperture, opened up ORIENT range slightly, and adjusted POS TARGs. 1101434541 ----- Attempting Submission 1 (Thu Jul 23 18:44:07 GMT 2015) ----- ----- Attempting Submission 2 (Wed Sep 23 15:52:20 GMT 2015) ----- Recently, deep HST images revealed that several massive intermediate-age star clusters (SCs) in the Magellanic Clouds exhibit extended main-sequence turn-offs (eMSTOs), and in some cases also dual red clumps. This poses serious questions regarding the mechanisms responsible for the formation of SCs and their well-known light-element abundance variations. While many recent studies indicate that the eMSTOs are caused by a range in stellar age of about 100-500 Myr among cluster stars, the obvious and important question is: Why has this not been observed yet in SCs in that age range? Our recent studies suggest that prolonged star formation in SCs requires a cluster escape velocity in excess of about 15 km/s, and young SCs with such high escape velocities are extremely rare in the Local Group. We recently identified a SC in the Large Magellanic Cloud (LMC) that has the right mass and size to be able to retain mass loss in slow winds of the first stellar generation while being young enough to still host some of its second stellar generation (if present) on the pre-main sequence (PMS) and showing emission-line signatures: the ~90 Myr old cluster NGC 1850 in the LMC. Since this SC has no adequate HST imaging to detect an eMSTO or PMS stars associated with the cluster, we propose to obtain deep WFC3/UVIS broad-band imaging with filters F275W and F814W to analyze the MSTO region and use F656N imaging to search for Balmer-line emission signatures from PMS stars of the second stellar generation. The detection of an eMSTO and/or PMS stars in this SC (or a lack thereof) will have a lasting impact on our understanding of the eMSTO phenomenon and of star cluster formation in general. The target star cluster is observed with WFC3 using the F275W and F814W broad-band filters and the F656N narrow-band Halpha filter. For the broad-band filters, we make sure we include one exposure that does not saturate the brightest stars expected from its age mentioned in the literature. On the other hand, we need color uncertainties less than 0.05 mag down to well beyond the estimated magnitude of the main sequence turnoff to achieve an accurate age determination and to enable a robust identification of multiple or broad turnoff structures if the target cluster in question is indeed found to host multiple stellar populations. Hence we also include at least two longer exposures per filter to achieve deep photometry. At least two dither positions are used for each filter to allow a robust elimination of hot pixels and cosmic ray hits during the analysis. ACS is used in parallel to the primary WFC3 exposures, using the filters F555W, F658N, and F814W. This will allow a robust determination of the field population contribution to the star cluster pointing. RESOLVED STELLAR POPULATIONS 12 false false false false 0.0 0.0 AVAILABLE STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F656N WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F658N ACS/WFC Imaging No No F814W 1443023207406 /private/var/folders/3h/23_s8wz16l1_fq1whjsqqmn000058r/T/OPVisit1642625618206945200.xml The scheduling data was generated by CASM 23.2. CASM was run in Phase II mode. The visit orientation requirements are set to (a) avoid bright stars in or near the ACS parallels, (b) avoid catching another star cluster by the ACS parallels (and hence not sampling enough "background field" area), and (c) to still make sure NGC 1850B stays within the field of view of the prime WFC3 images. (Tic-Visit :proposal-id 14174 :visit-id "01" :proposal-version -1 :trans-version "20150609" :date "23-Sep-2015 11:46:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 4588288 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14174 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14174 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3221 :al-overhead-at-end 123 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3556 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14174 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3330 :al-overhead-at-end 80 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3556 :pcs-gap 0 ) true 2 0