Wide Field Camera 3 Update

John W. MacKenty, mackenty@stsci.edu


The Wide Field Camera 3 (WFC3) continues to perform well. This article highlights several recent operational and calibration improvements.

Of interest to many observers is the major release of updated ultraviolet-visible (UVIS) flat fields and photometric zero points. The culmination of several years of work, these flat fields update the pre-launch flat-field calibrations with significantly smaller low-spatial-frequency residuals. Although the pixel-to-pixel (i.e., high-spatial-frequency) flat fields are well calibrated by the ground measurements (and have proven to be very stable in flight), they contained 4–6% gradients across the field of view. These gradients are due to mismatch between the ground calibration illumination source and the Hubble’s actual optical system. Using the techniques previously employed by the teams of the Wide Field Planetary Camera 2 and Advanced Camera for Surveys (ACS), WFC3 obtained multiple offset observations in ten broad-band filters of a rich star-field in the globular cluster Omega Centauri. Fitting smoothed surfaces to the differences between these observations provides a correction to the original flat-field calibration. Since the corrections vary slowly with wavelength, flat fields for the remaining filters were corrected based upon the ten measured filters.

Creating these new WFC3 flat fields required the resolution of two major complications. First, the UVIS detector package has two optical windows and charge-coupled device (CCD) detectors whose surfaces are significantly more reflective than their ACS counterparts. These increases are a consequence of the greater wavelength coverage of the WFC3 detector. This higher reflectivity results in a more pronounced “flare” feature, which significantly contributes to the gradient in the pre-launch flats. Second, thermally induced shifts in Hubble’s focus (a.k.a., “breathing”) have a disproportionate impact on the encircled energy profile of stars near the A amplifier of the CCD detector array. Given the fairly crowded star field of Omega Cen, it is necessary to establish a satisfactory breathing correction for small apertures as a function of position on the detector, wavelength, and breathing state. These two effects—flaring and breathing—are intertwined and their resolution is discussed in the previous STScI Newsletter article by Elena Sabbi and John MacKenty). These calibrations were installed into the pipeline and made available for download in December 2011. (Seven of the UVIS flat fields and all of the infrared (IR) flat fields were previously installed into the pipeline. Please see the online documentation here).