2.3 Spectral Elements

2.3.1 The Filter Selection Process

Both WFC3 channels are equipped with a broad selection of spectral elements. These elements were chosen on recommendation of the WFC3 Scientific Oversight Committee (SOC; see Acknowledgements for list of members), 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 in this handbook for details), 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 Integrated Product Team (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.

2.3.2 Filter and Grism Summaries

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, Strömgren, 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.

The UVIS channel has a selectable optical filter assembly (SOFA) that contains a stack of 12 filter wheels housing a total of 48 elements: 42 full-frame filters, 5 quad filters, and 1 UV grism. Each wheel also has an open, or empty, slot. For UVIS observations, the appropriate wheel is rotated to place the chosen filter into the light path, and the other eleven wheels are rotated to place the open slot in the light path. Only a single filter can be used at a time. Since the simultaneous insertion of two filters would result in significant defocus, the ground system does not provide the capability of crossing two filters.

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 2 × 2 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.

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 –35°C, a thermally-isolated enclosure which reduces the thermal loads and background emission onto the detector.

In addition to the wide and medium-band filters, the IR channel includes six narrow-band filters, which likewise sample the most important planetary, stellar, and nebular spectral features in the near-IR.

Finally, wide-band filters with similar wavelength coverage to those of the grism dispersers are available. These allow direct images in the same spectral ranges covered by the grisms. They are used to accurately identify spectroscopic sources and for wavelength calibration. WFC3 contains no ramp filters or polarizers, unlike ACS or WFPC2.

Tables 6.2 and 7.2 provide a complete summary of the filters available for imaging with the UVIS and IR channels, respectively. Filter+WFC3+HST system throughput curves are presented in Section 6.5 (UVIS) and Section 7.5 (IR), as well as in Appendix A. Graphical representations of the UVIS and IR filter wheels are shown in Figures 2.3 and 2.4. Meanwhile, Figure 3.2 shows the overall integrated system throughputs of WFC3 compared to other instruments.

2.3.3 Shutter Mechanism

Integration 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 6.10.4).

There is no mechanical shutter in the IR channel; instead, the detector provides electronic shuttering. Dark-current measurements are obtained by using the BLANK, an opaque aluminum blocker in the filter wheel. The block is also in place whenever the IR channel is not in use, such as during slews, Earth occultations and South Atlantic Anomaly (SAA) passages.