Fig. 1: WFC3 is moved from the clean room to the vacuum chamber. Credit: H. Bushouse
Fig. 1: WFC3 is moved from the clean room to the vacuum chamber. Credit: H. Bushouse
The Wide Field Camera 3 (WFC3) is a fourth-generation instrument that will be installed in the Hubble Space Telescope (HST) during Servicing Mission 4, currently scheduled for September 2008, and will replace the Wide Field Planetary Camera 2 (WFPC2). The camera includes two observation channels: a UV/Visible (UVIS) channel and an infrared (IR) channel. The UVIS detector consists of two butted 2kx4k CCDs (160″x160″ field-of-view, 3 e- rms noise) and the IR detector is a 1kx1k HgCdTe array (120″x140″) cooled thermoelectrically. The camera possesses a large complement of narrow-, intermediate-, and broad-band filters (62 in UVIS, 15 in IR, and 3 grisms) distributed among 12 separate wheels for the UVIS and a single wheel for the IR. With WFC3's unprecedented sensitivity, extensive wavelength coverage (200-1700 microns), and large discovery space, astronomers will tackle a wide range of astronomical problems and produce exciting new discoveries.
WFC3 recently began a ∼70 day thermal vacuum campaign at the Goddard Space Flight Center to validate its flight worthiness. In late May, WFC3 and the Calibrated Stimulus from Leftover Equipment (CASTLE) were moved into the large Space Environment Simulation Chamber (8.5mx12m). CASTLE delivers external light stimuli into WFC3, in particular monochromatic and broad-band point source and flat field illumination, by using a combination of fibers, lamps, and laser diodes while closely simulating the HST optical path and its aberrations. After system functional tests, alignment of the detectors, and throughput measurements were completed in ambient environment on June 10-14, the chamber underwent pumpdown to a ≈10-6 torr vacuum and calibration activities were initiated early in the week of June 18. For the entire campaign, all teams involved (calibration, thermal and CASTLE team support, control operations, etc...) rotate through three 8-hour shifts every day.
Fig. 2: The vacuum chamber where WFC3 resides for the summer 2007.
In the simulated space environment of the vacuum chamber, WFC3 will be subjected to a series of cold and hot cycles as in HST's aft shroud to verify its thermal control systems. At each temperature plateau and during transitions, the science team will acquire calibration data to fully characterize the instrument's optics and detectors. The data will permit the identification, diagnosis, and resolution of potential problems before launch. The vast majority of the images are acquired via a series of scripts written in the form of HST command sequences - some will be executed several times to verify stability and repeatability. The calibration activities can be divided into several broad categories. Some of these include: dark rate versus temperature, bias and super-bias, absolute and relative gains, linearity, PSF encircled energy, flat fields and stability, grism dispersion, readnoise, electronic crosstalk, image ghosts, shutter performance, throughput, internal lamp calibration, and charge transfer efficiency. These activities also offer an opportunity to verify the flight software as well as the transfer and processing of the data. After this campaign, WFC3 will undergo several more tests in ambient environment as well as a shorter thermal vacuum campaign in early 2008 for final check-out - it will then be fully certified for launch and installation in HST.
More information on WFC3 can be found at the Web sites of STScI/WFC3 and GSFC/WFC3.