WFC3 Instrument Science Report 2009-22 WFC3 SMOV Proposal 11421: Channel Select Mechanism Test H. Bushouse October 28, 2009 ABSTRACT The operation and positioning of the WFC3 Channel Select Mechanism (CSM) was verified in WFC3 SMOV program 11421. The extended target M81 was observed in the UVIS and IR channels, which determined that the field of view for both channels is not blocked or vignetted in any way, and provided “First Light” images for WFC3. Internal IR flat field images also verified the correct positioning of the CSM diffuser paddle, which is necessary for obtaining repeatable flat fields. Introduction SMOV program 11421 goals were to verify that the WFC3 Channel Select Mechanism (CSM) is properly positioned to provide unobstructed and vignetting free views of the sky in both the UVIS and IR channels, and the repeatability of the CSM position when used to obtain internal IR flat fields. The CSM should be fully retracted from the incoming WFC3 beam for a UVIS exposure and correctly redirect the incoming beam for an IR exposure, resulting in both cases in a completely filled field of view. Full-frame UVIS and IR exposures of galaxy M81 were obtained. M81 overfills the field of view of both WFC3 channels, enabling us to verify that the field of view is clear for both channels. Two IR internal flat field images, using the F098M and F160W filters, were obtained to verify the correct positioning and repeatability of the IR diffuser paddle. The exposures obtained for this program are listed in Table 1 along with pertinent exposure information. This program was executed on June 24, 2009 (day 175) and produced the first on- orbit WFC3 exposures of an external target (“first light” images). Two UVIS exposures of M81, bracketing the single IR exposure of M81, were obtained to verify the repeated operation of the Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration.