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Instrument Science Report WFC3 2008-45

CSM Mapping of the UVIS Field-of-View
H. Bushouse and J. Kalirai September 23, 2008

ABS

TRACT

UVIS channel flat field images obtained as a function of CSM position have been used to determine the fraction of the WFC3/UVIS field-of-view that is obscured by the CSM when it is in an off-nominal position. The total range of CSM travel between the nominal UVIS and IR channel positions is 330 resolver steps. The UVIS field-of-view is 50% obscured when the CSM reaches 45% of its total travel from the UVIS to IR positions and becomes completely obscured with the CSM at and beyond 75-80% of the distance to the IR position.

Introduction
During WFC3 testing in July 2008, a test procedure was executed that obtained a series of external UVIS channel flat field images in order to determine the fraction of the UVIS field-of-view that is obscured by the Channel Select Mechanism (CSM) when it is in off-nominal positions. For this test, WFC3 was located in the Goddard Space Flight Center (GSFC) clean room, in an ambient environment. The UVIS detector was cooled to -50° C and WFC3 was operating on the Side 2 electronics (MEB2). The test images were obtained using one run of the Science Mission Specification (SMS) UV36S01, which was executed on July 18, 2008. The WFC3 "CASTLE" optical stimulus was used to provide external flat field illumination. The UV36S01 SMS obtains a series of 11 flat field exposures using the F673N filter and 2x2 on-chip CCD binning. The first exposure was obtained with the CSM at the nominal UVIS position, with a resolver value of 382. In between each exposure, the
Operated by the A ssociation of Univ ersities for Research in A stron omy, Inc., for the National A eron autics and Space Adm inistration


WFC3 Instrument Science Report 2008-45 WFC3 operator manually commanded the CSM to move 33 steps, so that when the final exposure occurred the CSM had traveled a total of 330 steps, placing it at the nominal IR channel position of 52. The illumination produced a median signal level of 12,000 e- per binned pixel.

Data Analysis and Results
The raw images from the test were processed through the standard OPUS WFC3 pipeline, which included calwf3 processing to subtract bias level and trim the overscan regions. The resulting _flt FITS files were analyzed to determine the fraction of the area of each CCD that was not obscured by the CSM shadow. This was done by simply finding the fraction of pixels in each chip that were above the nominal flat field signal level. The results are shown in Table 1. The images themselves are shown in Figure 1. The UVIS field-of-view is 50% obscured at a CSM position of ~230. The UVIS field is completely obscured for all CSM positions less than ~150. Table 1. Test information, including the CSM position and percentage of the visible field for each exposure. OPUS file name TVNUM CSM position (steps) 382 (UVIS) 349 316 283 250 217 184 151 118 85 52 (IR) CCD1 visible area (%) 100 99 84 57 24 7 0 0 0 0 0 CCD2 visible area (%) 100 100 100 100 97 74 39 8 0 0 0 Total visible area (%) 100 99 92 79 61 41 20 4 0 0 0

i i i i i i i i i i i

u360101r_08200205250_flt.fits u360103r_08200205637_flt.fits u360105r_08200210024_flt.fits u360107r_08200210411_flt.fits u360109r_08200210758_flt.fits u36010br_08200211145_flt.fits u36010dr_08200211532_flt.fits u36010fr_08200211919_flt.fits u36010hr_08200212306_flt.fits u36010jr_08200212653_flt.fits u36010lr_08200213040_flt.fits

59391 59392 59393 59394 59395 59396 59397 59398 59399 59400 59401

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WFC3 Instrument Science Report 2008-45

Figure 1. The 11 flat field images obtained in this test, with image ID numbers and visible field percentages for each chip overlaid.

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