SPACE TELESCOPE SCIENCE

We present the results and measurement of Charge Transfer Efficiency (CTE) of the WFC3 UVIS detector, based on data acquired during the monthly internal Extended Pixel Edge Response (EPER) observations over a 2 year period. We present an algorithm for CTE assessment and fit a power-law to CTE measures versus signal level. We find that at each signal level, CTE declines linearly over time and CTE losses are worst at the lowest signal levels. 1

UVIS Flat Field Uniformity

The quality of the six ground-based TV3 WFC3/UVIS flat-fields F225W, F275W, F336W, F438W, F606W and F814W, used in the standard calibration of the WFC3/UVIS data, has been analyzed by comparing the magnitude of several stars on different positions of the detector. We find a difference of 1.5 % to 4.5 % between the ground-based flat-fields and the on-orbit data, with higher uncertainties at shorter wavelengths

CTE External Monitoring- NGC6791

We present preliminary results of the Charge Transfer Efficiency (CTE) evaluation based on external calibration observations from the new instrument Wide Field Camera 3 (WFC3), a fourth- generation imaging instrument which was installed in HST during Servicing Mission 4 in May 2009. Observations of the rich open cluster NGC6791 have been used to provide an initial assessment of the WFC3/UVIS CTE and its dependence with time. The observations with low-sky background ( ∼ 0.1- ∼ 1.0 e − ) for the flux range 500 to 2000 e − exhibit a clear trend with Charge Transfer Inefficiency increasing from 2 % in October 2009 to 7 % in the fall of 2010. The CTE degradation will continue to be monitored with both internal and external observations. Additional observations of NGC6791 and 47Tuc (CAL-12379, PI K. Noeske) planned for the fall of 2010 and spring of 2011, will be used to develop formal photometric and astrometric corrections for WFC3 science data. 1

Multi-Wavelength Geometric Distortion

Standard astrometric catalog based on ACS/WFC observations of the globular cluster ω Cen field has been utilized to examine the geometric distortion of WFC3/UVIS and IR as a function of wavelength. The observations of this field were taken with large dither patterns and a wide range of HST roll-angles exposed through 13 UVIS and 8 IR band-passes. A 4 th order polynomial model was used to derive the UVIS and IR geometric distortion coefficients relative to the distortion free coordinates of the astrometric field. The main result of these calibrations are: 1) geometric distortion can be successfully corrected at the 0.1 pixels (2 and 7 mas) precision level in UVIS and IR channels, respectively; 2) the UVIS relative scale changes from filter to filter by ∼0.02%; 3) the IR relative scale is nearly constant from filter to filter; 4) the unique polynomial coefficients of the geometric distortion in the 13 UVIS and 8 IR filters are used in the STScI reprocessing pipeline to correct WFC3 images for distortion

Distortion Calibration

The goal of the WFC3 SMOV4 astrometric calibration program –11444 is to obtain a coordinate system free of distortion to a precision level of 0.2 pixels or ∼ 8 mas. The astrometric calibration of WFC3/UVIS is based on two astrometric standard fields: a reference frame in the globular cluster 47 Tuc and in the Large Magellanic Cloud. Both 47 Tuc and the Large Magellanic Cloud, observed with the F606W filter and with different dither patterns, have been used to determine the geometric distortion in WFC3/UVIS. We used a 4th-order polynomial model to derive the geometric distortion in the UVIS channel relative to the distortion–free coordinates of our astrometric fields. As a result, the geometric distortion can be successfully corrected down to the precision level of 2 mas, which is four times better than the required precision