WFC3: SMOV and Cycle 17 Calibration Programs

The newly installed Wide Field Camera 3 on the Hubble Space Telescope will be extensively used by GOs during Cycle 17 for imaging astrophysical phenomena in the UV, optical, and IR. During SMOV4 and Cycle 17, the WFC3 Instrument Team will measure the photometric performance of both the UVIS and IR cameras. These observations will target a set of bright spectrophotometric standard stars and two nearby star clusters. We describe the observational strategy to measure the photometric zeropoints of all filters and the transformation equations to relate the fluxes of objects measured in these filters to other popular ground and space based systems. A detailed discussion of the rationale for the choice of targets, and the analysis plan, are also presented. Introduction – The Hubble Space Telescope and Servicing Mission 4 The Hubble Space Telescope (HST) has served as the world’s benchmark for astronomical research for almost two decades. The first of its kind, the UV/optical/NIR telescope has successfully been serviced through four manned missions in December 1993, February 1997, December 1999, and March 2002, which continously provided it with cutting edge new technology to meet the rapidly changing needs of professional astronomy. As a direct result of these servicing missions, Hubble’s ability to answer the most fundamental questions of the Universe, from our understanding of planetary science to cosmology, has always been at the competitive forefront of astronomical missions. Copyright c © 2008 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. WFC3 was installed during the May 2009 servicing mission, replacing the extraordinarily successful WFPC2. The new WFC3 instrument will improve HST’s sensitivity by over an order of magnitude, enabling the undertaking of exciting new scientific research projects. The versatility of the WFC3 instrument is particularly exciting as it provides sensitive, wide-field imaging in the UV, optical, and NIR within a single orbit. The instrument will immediately become the workhorse of the HST, carrying half of the total Cycle 17 approved orbit allocation, more than a factor of two higher than the next busiest instrument (ACS). WFC3 will be used as the primary instrument in a diverse range of studies from the characterization of low mass brown dwarfs and planets, to understanding the formation of stars and galaxies, to measuring the expansion rate of the Universe. In this document, we describe the procedures that will be undertaken to understand the photometric performance of WFC3. The Wide Field Camera 3 WFC3 is a new instrument containing both optical/UV CCDs and a near-infrared HgCdTe detector array, providing high-resolution imaging over the wavelength range extending from 200 nm to 1700 nm. The UVIS channel contains two CCDs that subtend an angular field of view of 162 × 162 arcseconds and pixels that are 0.04 arcseconds. The IR channel contains a single detector with a field of view of 123 × 136 arcseconds and pixels that subtend 0.13 arcseconds. The complement of filters on WFC3 is substantial. There are 48 filter slots on the UVIS side that include 6 long pass and very wide filters, 12 broadband filters, 8 medium-band filters, and 36 narrow-band filters (20 of which are arranged on five quad slots). In addition to these 62 imaging filters, a UV grism is also included. On the IR side, there are 5 broadband filters, 4 medium-band filters, 6 narrow-band filters and two grisms. The primary use of WFC3 as an HST instrument will involve imaging observations of astrophysical phenomena in these filters. Linking these observations to scientific results requires an understanding of the photometric performance of the instrument. To accomplish this global task the specific goals for the SMOV4 and Cycle 17 observing program are: • To measure the photometric zeropoints for the 53 WFC3/UVIS filters requested by observers in Cycle 17, and for all 15 of the WFC3/IR filters. The zeropoints will be measured on both CCDs of the UVIS camera. We aim to achieve 1% precision in the 18 broadband filters, and 2-3% accuracy in the medium and narrow-band filters on the UVIS channel. Our goal is also to reach the same level of accuracy in the IR filters, although, it remains to be seen if this can be achieved within the limits of the count and count-rate linearity corrections. The photometric zeropoint of a filter is defined to be the magnitude of a star-like object that produces one count per second within the photometric aperture.