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WFC3 Instrument Handbook for Cycle 24 |
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Both WFC3 channels are equipped with a broad selection of spectral elements. These elements were chosen on recommendation of the WFC3 SOC, following a lengthy process with wide scientific and community input. Initial community suggestions and advice were considered at the WFC3 Filter Selection Workshop, held at STScI on July 14, 1999. Other input came from the WFC3 Science White Paper (see Section 1.3), from a suite of SOC-developed test proposals representing a broad range of current astronomical investigations, and from statistics of historical filter use in previous HST imaging instruments. The filter sets were chosen to fully cover the wavelength regimes of both WFC3 channels with a range of bandwidths, while complementing the filter sets available in ACS and NICMOS.Based upon the SOC recommendations, the WFC3 IPT (see Section 1.2) developed detailed specifications for the vendors who designed and manufactured the filters. The final flight spectral elements were fully characterized, evaluated by the IPT and SOC, approved for flight, and installed into the filter wheels.The IR channel has a single filter wheel (FSM, or Filter Select Mechanism) housing 17 elements: 15 filters and 2 grisms; an 18th slot contains an opaque element (or BLANK). For IR observations, the requested single element is rotated into the light beam. The FSM is a bidirectional wheel and always takes the shortest path to a new filter position. The filter wheel and all of its filters are housed, along with the HgCdTe detector package, in a cold shroud maintained at –30°C, a thermally-isolated enclosure which reduces the thermal loads and background emission onto the detector.The filter sets in both channels include wide-, medium-, and narrow-band filters, as well as low-dispersion grisms (one in the UVIS channel, two in the IR channel) for slitless spectroscopy. The wide- and medium-band filters include popular passbands used in extragalactic, stellar, and solar-system astronomy, as well as passbands similar to those already used in other HST instruments for photometric consistency and continuity. The classical UBVRIJH, StrЖmgren, and Washington systems are reproduced, along with the filters of the Sloan Digital Sky Survey (SDSS). In addition, several extremely wide-band filters have been included in both channels, for ultra-deep imaging.There are also a total of 36 different narrow-band passbands in the UVIS channel, consisting of 16 full-field filters and 5 quad filters. Quad filters are 2в2 mosaics occupying a single filter slot; each one provides four different bandpasses, at the cost of each one covering only about 1/6 of the field of view. The narrow-band filters provide the capability for high-resolution emission-line imaging in many of the astrophysically important transitions, as well as the methane absorption bands seen in planets, cool stars, and brown dwarfs.Tables 6.2 and 7.2 provide a complete summary of the filters available for imaging with the UVIS and IR channels, respectively. Individual filter transmission curves as well as filter+WFC3+HST system throughput curves are presented in Appendix A. Graphical representations of the UVIS and IR filter wheels are shown in Figures 2.3, 2.4. and 3.2 shows the overall integrated system throughputs of WFC3 compared to other instruments.Figure 2.3: UVIS Filter WheelsFigure 2.4: IR Filter Wheel2.3.3 Shutter MechanismIntegration times in the UVIS channel are controlled via a mechanical shutter blade very similar in design to the ACS/WFC shutter. Sitting directly behind the SOFA, the WFC3 UVIS shutter is a rotating disk about 12 inches in diameter; it is divided into four 90° quadrants, with alternating quadrants providing the blocking (i.e., there are two open and two closed positions). When the shutter is in the closed position initially, a commanded move of 90° places it into an open configuration; at the end of the exposure, another move of 90° places the shutter back into a closed position. Although the shutter can be operated in either a clockwise or counterclockwise direction, the current flight software always moves the blade in the same direction.For very short exposure times in the UVIS channel, there are minor issues with exposure time non-uniformity (Section 6.7.1) and blurring due to shutter-induced vibration (Section 6.11.4).