Wide Field Camera 3 Instrument Handbook for Cycle 19 - Space ...

12 Chapter 2: WFC3 Instrument Description
(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, 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.
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.
In addition to the wide-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 coverages 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.
Table 6.2 and Table 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
Figure 2.3 and Figure 2.4. Figure 3.2 shows the overall integrated system throughputs
of WFC3 compared to other instruments.